PolyHIPE Polymer (PHP) was discovered and developed during the 1980s at the Unilever Research Port Sunlight Laboratory, England; the responsible team included the present author. This material was then named PolyHIPE Polymer and acronymed as PHP (Polymerized High Internal Phase Emulsion Polymer, to differentiate PHP from emulsion polymers). PHPs offer unprecedented and unparalleled controllable physical and chemical structures over a very broad range of variables. The key advantage of PHPs is that they can be produced with sizes ranging from ca. 1 μm to 1 cm, with a very narrow pore size distribution and a range of chemical/biochemical functionalizations, including biodegradability [3] [4] [10] - [17] . The pore and interconnecting hole sizes, as well as the surface biochemical functionality, are the most important properties of PHPs in biological applications when the “Confinement Phenomenon” (CP) is the underlying driving force for biochemical transformations involving microorganisms [46] [47] . It is therefore important to understand the PHP structure and control the desired structures through processing.

Based on the availability of such a medium as functionalized PHP (f-PHP), the present author developed the theoretical bases of CP, according to which the behaviour of matter, including the living, is dependent on the size, biochemistry, and the dynamic nature of the environment in which they are confined [3] [46] [47] . This conclusion is based on entropic considerations, but it can in fact be construed as a statement of fact, which has been utilized as an entropic driving force for Process Intensification (PI) [46] in agriculture (A-PI) [12] - [14] , biology, including tissue engineering (B-PI) [48] - [56] , chemistry and environmental remediation (C-PI) [57] - [62] and energy (E-PI) [63] - [66] technologies. However, later studies indicate an intense level of publications on the utilization of PolyHIPE in agriculture [67] - [69] , bioprocess intensification [70] [71] , and in particular in medical applications and tissue engineering as apparent from the recent publications [72] - [80] .

In 1998, Akay et al., [48] published the first ever study on 3D tissue engineering (applied to in vitro cartilage cell growth) using well-characterized PolyHIPE polymers with nanopores on pore walls to provide the uninterrupted passage of messenger molecules, nutrients, and metabolites. PolyHIPEs with uniform pore and interconnecting sizes as well as varying degrees of pore surface bioactivity were used. These surface-functionalized PolyHIPEs (f-PHPs) were used as a flow micro-bioreactor. In some cases, a 2D or 3D network of capillaries (1 mm - 10 µm in diameter) [48] was established within the reactors for local nutrient feeding, oxygenation, and metabolite collection in order to produce artificial organs such as lungs as represented in Figure 3 . The interface between the channels and the bulk can be open or closed by surface modification of the sacrificial capillaries through solubility parameter difference between the capillaries and the bulk PHP [48] [57] . Monoliths with capillaries were also used in the agricultural applications of sn-PHP to allow large roots to branch out within the SRS-medium [12] [13] . These reactors were applied to membrane separations of surfactant molecules or polymers from water [57] [58] . Metallic versions of these reactor systems are also available for catalyst applications in micro-chemical reactors [62] . More recently, Du et al., [81] also used to facilitate cell infiltration and osteogenic differentiation.

In the PHP functionalization, it is necessary to start the process at the emulsification stage because the absorption of chemicals necessary for functionalization is difficult when thick monolithic PHPs are to be processed. Instead, the relevant chemicals are included either in the aqueous phase or the oil phase at the emulsification stage and the polymerization is carried out. In some cases, no further action is required after polymerization, but in other cases, further chemical modifications may be needed.

Irrespective of any post-polymerization action, the inclusion of these additives for f-PHP production resulted in emulsion droplet coalescence during polymerization [48] . Polymerization with coalescence results in enlarged pore size with thicker nano-porous walls dispersed in smaller primary pores. In coalescence pores, the interconnecting pores are numerous per pore, while in the primary pores (which have not undergone coalescence), there are ≈ 12 interconnecting holes per pore. Pickering coalescence is used to obtain large pores when the primary pores are too small for PHP applications [48] [80] .

The above-described technique was used for the surface functionalization of PHP with: (1) phosphoric acid containing hydroxyapatite for bioactivity [48] - [66] , (2) peptide hydrogel for bioactivity [53] ; (3) sulfuric acid, nitric and phosphoric acids for fertilizer generation or microwave-induced sulfonation for enhanced hydrophilicity for agricultural, biological, chemical and energy applications [7] - [14] [48] - [66] ; (4) water-soluble polymers for enhanced water uptake [48] .

Typically, PHPs shown in Figure 4 have an oil phase composition of: Styrene 78%; Divinyl benzene (DVB) 8%; Non-ionic surfactant (Span 80) 14%. The aqueous phase contains 1% potassium persulphate as an aqueous phase initiator. The following examples in Figure 4 illustrate the importance of emulsification and polymerization conditions. Figure 4(a) shows similar-sized pores (≈40 µm) formed at the beginning of HIPE formation (90% void). At this stage, the emulsion can have a double-emulsion form, as apparent from the presence of droplets within the larger pores of the PHP itself. The pores have a large number of interconnecting holes and give the impression of being coalescent pores (see Figure 4(e,f) ). However, unlike coalescent pores, this structure does not have any primary pores between the coalescent pores, as seen in Figures 4(e,f) .

Figure 4(b) illustrates the appearance of the PHP after the emulsion is fully processed to obtain a desired size [82] . Here, the pore size is ≈ 2 µm with a uniform pore size distribution. Polymerization temperature and time were 60˚C and 8 hours, respectively, with the ratio of the interconnect diameter (d) to pore diameter (D) ratio d/D = 0.26. Note that, due to the 3D nature of the PHP, when the fracture surface is produced for SEM, smaller pores appear due to the location of the fracture line. When the aqueous (dispersed) phase volume is increased, d/D also increases, thus reducing the pore wall area for support. This can be overcome when HIPE is polymerized at high pressure in order to increase the polymerization temperature and hence reduce the polymerization time. Figure 4(c) shows the closed pore structure of the 95.5% aqueous phase volume PHP that becomes closed when d/D (ratio of the interconnect diameter to pore diameter) is reduced from 0.26 to 0.12, with fewer holes when compared with atmospheric pressure polymerization [82] . In this case, polymerization temperature was 60˚C and the pressure was 30 bar, with a total polymerization time of 420 min. The reduction of the d/D ratio is due to pressure, as in other experiments carried out at 60˚C, closed pore structures were still evident [82] . It is therefore possible to control the d/D ratio by adjusting the polymerization pressure to reduce the phase volume dependence of d/D while increasing the polymerization temperature.

Depending on the type of cells, large pore size PHPs may be necessary. This can be achieved using coalescence pores, which, however, do not produce uniform pore size, and a large number of primary pores are still present. However, this disadvantage of coalescence pores can be overcome by choosing a suitable additive [48] or by reducing the additive concentration and lowering the polymerization temperature until the gel point is reached [48] . Alternatively, the emulsification temperature can be increased in the absence of any additives [48] . In this case, primary pores with large pore size are obtained, as seen in Figure 4(d) . In this figure, the maximum emulsification temperature was 80˚C, with a pore size of ≈ 140 μm. However, emulsification at higher temperatures requires an oil phase polymerization initiator, as the aqueous phase initiator, potassium persulphate, decomposes when the temperature is above 85˚C. Hence, emulsification above 85˚C can be carried out using an oil phase initiator [48] .

Figure 4(e) describes the primary pores between three large coalescence pores. Figure 4(f) illustrates the structure of a coalescence pore and the mechanism of its formation from the surrounding primary pores. Several examples are present in reference [48] for the formation of coalescence pores using oil phase or aqueous phase “additives” and their effects on the pore and interconnect size.

The above-described technique was used for the surface functionalization of PHP with: (a) peptide hydrogel for bioactivity [53] ; (b) phosphoric acid-containing hydroxyapatite for surface coating and bioactivity [48] - [66] ; (c) sulfuric acid for microwave-induced sulfonation for enhanced hydrophilicity and swelling in agricultural, biological, chemical, and energy applications [7] - [14] [48] - [66] , and (d) coalescence pores with highly porous walls [48] .

Figure 5 illustrates the formation of coalescence pores and their wall and pore surface structure. Here, the aqueous phase contains 1% potassium persulphate and 10% nitric acid. The oil phase composition is 70% styrene, 8% divinyl benzene, 14% surfactant (Span 80), and 8% vinyl pyridine. The aqueous phase volume is 85%.

As seen from Figure 5(a) , coalescence pores are formed although the pore size is relatively low. Polymerization is carried out over 24 hours at temperature steps of 40˚C - 50˚C - 60˚C in order to control the coalescence pore size. In Figures 5(b-d) , the pore wall and pore surface structures are shown at high magnifications. These PolyHIPEs have a bimodal interconnectivity; both large interconnects and nano-pores, which are formed through the fusion of ca. 20 nm particles with lines of defects acting as interconnecting pores. This type of polymer is useful in micro-bioreactors and SX-fertilizers, providing rapid transport of messenger and metabolite molecules as well as gas diffusion [48] - [56] .

One of the most functionalised PolyHIPEs is sulfonated s-PHP. It is used in several applications, including agriculture [3] [4] [12] - [14] , water [3] [66] , and gas cleanup [63] [64] [83] , demulsification [56] - [61] , as well as bioreactors for the production of enzymes [55] and antibiotics [56] . A process-intensified method of PHP sulfonation is available, which is also suitable for industrial-scale production [7] [10] . In this method, a small amount of sulphuric acid is incorporated into the aqueous phase, and a stable emulsion is formed and subsequently polymerized. If the concentration of the acid is over ≈15 wt%, the emulsion becomes highly unstable and forms very large (> 5 mm) coalescence pores. Therefore, the amount of (sulphuric) acid in the aqueous phase can be used to control the pore size from micrometres to millimetres. In most applications, acid concentration is kept at 10% to obtain the desired pore size. During polymerization, the material undergoes partial sulfonation (≈4%), which is sufficient for rapid absorption of concentrated sulphuric acid for sulfonation [7] [10] . In order to obtain s-PHP with enhanced water uptake associated with enhanced swelling, surfactant (Span 80) is removed by ethanol extraction after polymerization with slight sulfonation [7] [10] . After the concentrated acid (98%) absorption, it is microwave irradiated for ca. 4 minutes for sulfonation at 1 kW power. Clearly, this is a very fast sulfonation technique compared with classical methods such as those applied to PHP by Haq and Johnson [84] in Unilever patents. During microwave irradiation, polymer is swollen and can be neutralised, typically, with ammonia; a process highly suitable during plasma catalytic ammonia production with in situ reactive product separation using Multi-Reaction Zone Reactors (M-RZR) [3] [4] - [9] .

Figure 6 illustrates the structure formation in coalescence pores when sulphuric acid containing HIPEs are polymerised, followed by sulfonation using microwave irradiation. Here, the aqueous phase contains 1% potassium persulphate and 10% sulphuric acid with a phase volume of 85%. The oil phase composition is: 78% styrene, 8% divinyl benzene, and 14% Span 80 non-ionic surfactant. As seen in Figure 6(a) , the coalescence pores have near-circular interconnectors, similar to the primary pores of the PHPs shown in Figure 4(a-d) . Despite its relatively low aqueous phase volume, it has a high level of openness. This is because, during the microwave irradiation, the surface becomes blistered and some of the blisters drop out, forming the interconnects as seen in Figures 6(b,c) . The blistered surface structure is shown in Figure 6(d) . The surface consists of nanoparticles with hairline cracks. A circular hairline crack separates the region which is a candidate to drop out to form an interconnect, as shown in Figure 6(d) . The hairline cracks form approximately 20 nm channels across the wall, thus providing a large amount of nano-connectivity between pores. These structures are specific to the PHPs prepared through destabilization by acid inclusion in the aqueous phase and subsequent microwave-induced sulfonation.

It is possible to obtain very high surface area PolyHIPE by changing the oil phase composition (inclusion of additives in the oil phase) and by decreasing the oil phase volume [10] [48] [85] - [87] . However, when these polymers are sulfonated using thermal methods (with or without acid present at the emulsification stage, using concentrated or dilute acid), the surface area is reduced drastically. On the other hand, using dilute acid at the emulsification stage followed by polymerization, ethanol washing, and microwave sulfonation appears to enhance the surface area.

High surface area PHP was prepared by using an oil phase composition of: 50% DVB, 25% 2-Chloroethylbenzene, and 25% Chlorobenzene. The aqueous phase was: 1.0% potassium persulfate and 5% H₂SO₄. Phase volume was 85%. The surface area of the resulting PolyHIPE was 129 m²/g. This PHP was thermally sulfonated using the method given by Haq and Johnson [84] . The surface area was reduced drastically to 3.20 m²/g. When the same polymer was sulfonated through microwave irradiation as described above, the surface area increased to 243 m²/g. The results are summarised in Figure 7 .

The scanning electron microscopy study of these PHPs is shown in Figure 7 , which indicates that the unsulfonated and microwave-induced sulfonated s-PHP have the nano-particulate wall surface structure ( Figures 7(a-d) , respectively). However, in thermally sulfonated low surface area s-PHP, the original nano-particulate structure is destroyed, as seen in Figures 7(e,f) .

Articular cartilage is composed of cells, chondrocytes, embedded in a resilient matrix of collagen, proteoglycan, and water. In this example [48] , chondrocytes, which were primary cells obtained from a bovine metacarpophalangeal joint, were used. Chondrocytes were seeded onto two types of PHP. The first was a plain styrene-2-ethylhexyl acrylate PHP that had not been chemically modified. The second PHP was functionalised with a coating of hydroxyapatite (HA-PHP). The penetration of the chondrocytes into the polymer was determined by histology. The production of glycosaminoglycans (GAG), which are found in the extracellular matrix, was determined by a colorimetric assay, and the type of collagen produced in the matrix was determined with immunocytochemistry. The production of glycosaminoglycans (GAG) is an indication of the production of extracellular matrix. This matrix gives cartilage its load-bearing capacity and is crucial to the development of chondrocytes into cartilage tissue.

To make PHP an ideal scaffold it must be biocompatibility and non-toxic. Biocompatibility is the ability to perform with an appropriate host response in a specific application. PolyHIPE is produced from monomers which are toxic and if any of these monomers are released or the polymer has undergone biodegradation through de-polymerization (re-generation of monomers), it can be potentially toxic. Further, the substrate biochemistry needs to enhance cell functions, including proliferation and differentiation. To assess the biocompatibility, in vitro studies were conducted using several cell types but concentrating on macrophages and chondrocytes in PolyHIPE.

Macrophages are responsible for the phagocytosis and digestion of foreign material (both biological and non-biological) in the body. Macrophages are known to have a central role in the inflammatory response to biomaterials and implant failure. Any material that elicits a high response by macrophages would cause the surrounding tissue to become damaged as well as producing an inflammatory response, thereby reducing the biocompatibility of the biomaterial. There are three common ways of assessing the response of macrophages to a biomaterial. They are morphological changes, hydrogen peroxide production and beta glucuronidase production. Morphological changes in the cells are important in cell engineering. Macrophages are spherical in vitro and any deviation from this is a sign of a negative response. The performance of the PHP scaffolds with various pore sizes and surface coatings is summarised in Figure 8 .

In the SEM micrograph in Figure 8(a) , the rounded morphology of the chondrocytes on the surface pores of the polymer can be seen. There is no noticeable effect of the surface bioactivity of the PHP at this point. At later time points, the chondrocytes form a layer on the surface. Figure 8(b) illustrates the effect of the surface biochemistry of various supports. They include the tissue culture plastic (TCP), hydroxyapatite-coated styrene/divinyl crosslinked PolyHIPE (HA-PHP), and the sulfonated-neutralized (with ammonia) hydrophilic PolyHIPE (sn-PHP). As seen sn-PHP is a massively negative control leading to over 10-fold increase in beta-glucuronidase compared to HA-PHP. The tissue culture plastic also provides a positive control with twice the amount of beta-glucuronidase production. As seen in Figure 8(b) , the morphology of the chondrocytes for TCP and PHP are rounded while chondrocyte morphology is highly distorted, associated with very high beta-glucuronidase. These results lead overall to a good response by the macrophages to PHP.

Figure 8(c) shows the total GAG production (cell associated + released into the medium) by chondrocytes after 21 days in culture. Figure 8(d) shows the depth of penetration after 21 days. As seen in Figure 8 , there is an optimum pore size (≈ 24 µm) to maintain the chondrocyte morphology and to optimise the production of GAG. At lower pore sizes, the cells cannot penetrate the polymer (interconnect size, d = 3 µm when pore size D = 8 µm), and at higher pore sizes (d = 7 µm and D = 89 µm), the cell morphology changes from rounded to flat and fibroblastic in appearance. These fibroblastic-like cells proliferate rapidly and form a layer on the surface rather than penetrating the scaffold.

The importance of the above result is that, when sn-PHPs are used for bacterial support in bioprocessing and agriculture, they produce the most productivity with no biocompatibility issues detected in agricultural applications and antibiotic, enzyme, ethanol production [10] - [14] [48] - [56] .

The corresponding experiments with bone (rat osteoblast) cells were also studied using unmodified and HA-modified (HA-PHP) PolyHIPEs with pore sizes of 40, 60 and 100 μm. It was found that the effect of pore size on the phenotype of the cells resulted in few differences, with all polymers supporting osteoblast differentiation and osteogenesis to a similar extent. When comparing all three HA-PHPs by histological analysis, the cells exhibited the same morphology and levels of surface bone formation were not significantly different [52] . However, larger pore-sized polymers promoted more cells to penetrate deeper within the scaffold, as shown in Figure 9(a) . The use of the self-assembling peptide hydrogel (RAD 16-1) further improves osteoblast growth compared with HA-PHP [53] , as seen in Figure 9(b) .

Microreactors made from PolyHIPE were used to support phenol-degrading bacteria, Pseudomonas syringae [51] [65] . Two pore sizes were used in PHP. The large pore size (100 μm) PHP had a phase volume of 90%, and the second PHP had a pore size of 25 μm with a phase volume of 85%. Bacteria within the 25 μm polymer support remained mono-layered without any apparent production of extracellular matrix during the 30-day continuous experimental period, while the 100 μm formed biofilm (with extracellular matrix) within the pores as shown in Figure 10(a) . The monolayer bacterial coverage of the PHP support is shown in Figures 10(b,c) at two magnifications. The bulk of the microbioreactor had ≈1.5 bacteria/μm². The micro-bioreactor performance was characterized in terms of volumetric utilization rate and compared with the published data, including the case where the same bacteria were immobilized on the surface of micro-porous polymer beads and used in a packed bed during continuous degradation of phenol [65] . It is shown that at the similar initial substrate concentration, the volumetric utilization in the microbioreactor is at least 20-fold more efficient than the packed bed depending on the flow rate of the substrate solution. Under optimized reactor conditions, the efficiency was ca 30-fold [51] . The application of the present technique to other bacterial systems also resulted in similar enhancement levels. These include enzyme [55] and antibiotic [56] production.

Both plant leaves and fertilizers in soil are responsible for ammonia emissions. The crop foliage emission of NH₃ occurs during leaf senescence (the last stage of leaf development) when nutrient assimilation is completed and remobilization starts [92] . Although plant nutrient uptake and environmental stresses affect emissions from foliage, the mechanism is not clear. So, the primary research aims to enhance crop yield, rather than address nutrient loss or greenhouse gas emissions. The second source of NH₃ and CO₂ emissions is volatilization from the soil (assuming urea as the primary N-fertilizer) as a result of farming practices and fertilizer hydrolysis, as noted in the chemistry below.

(NH₂)₂ CO + 2 H₂O → (NH₄)₂CO₃ → NH₄⁺ + NH₃ + CO₂ + OH⁻

This means that the microorganisms present in soil need to convert atmospheric nitrogen into ammonium (NH₄⁺) and nitrate (NO₃⁻) ionic forms to become plant-available [93] . In Table 3 of reference [91] a summary of NH₃ and CO₂ emission suppression methods is provided. Pan et al., [91] also suggest practical methods to reduce NH₃ emissions when it is directly used in solution. These methods include fertilization management and the use of bio-fertilizers and slow-release fertilizers with slow-release coatings [94] .

Nitrification is an aerobic process driven by ammonium-oxidizing bacteria or archaea, where ammonium (NH₄⁺) is oxidized to NO₃⁻ for plant availability [93] , including more efficient photosynthesis by plant leaves. The conversion of NH₄⁺ to NO₃⁻ also indicates the energy loss in nitrogen fixation through NH₃ production. However, NO₃⁻ can be further reduced by denitrifying bacteria under anaerobic soil conditions. Both of these ionic species can percolate through soil and contaminate groundwater or undergo further transformation to yield N₂O, NO, and N₂, causing nitrogen loss and emission of the potent greenhouse gas, N₂O, which also causes the depletion of the stratospheric ozone layer [95] - [97] . There are several available methods to prevent nitrogen loss and emission, including the coating of fertilizers for slow release and using synthetic nitrification inhibitors (SNI) [98] [99] .

However, SNIs suffer from disadvantages including cost, availability, activity decay over time, and potential toxicity [96] . Therefore, an alternative has recently been exploited through the plants’ own natural capacity to suppress microbial nitrification, based on the promotion of root exudates and also by biological nitrification inhibitors (BNI) and biological denitrification inhibitors (BDI) [100] [101] . The root-derived fatty acid amides and sterols, and microbe-derived N-acyl-homoserine lactones have been successfully used as biological inhibitions [102] , thus raising expectations regarding the potential of crops to directly control nitrification and denitrification in soils [103] . Nevertheless, the cost of these inhibitors, their constant replenishment and loss in soil, and soil-specific selectivity and efficiency are likely to be their primary disadvantage.

Not all the gas emissions to the atmosphere from soil are harmful and undesirable. As seen in Figure 11 , another nitrous gas emission, nitrous acid (HONO), is generated through NO₂⁻ formation as a result of nitrification and denitrification, and the bacterial decomposition of soil organic matter (SOM) [104] . In air, nitrous acid undergoes photolysis to produce atmospheric hydroxyl radicals, which are vital for the removal of atmospheric pollutants such as CO and CH₄. Nitrous acid is responsible for the production of some 60% of atmospheric hydroxyl radicals, which are also an essential precursor of cloud formation through organic aerosol generation [105] [106] .

Before examining the biochemical pathways as described in Figure 11 for ammonia loss, greenhouse gas emissions, and carbon sequestration in soil, we summarize the emerging nature of agriculture due to three of the environmental stressors; namely eCO₂, eTe, and eDe. Recent research indicates that, to some extent, the mitigative pathways for the destructive consequences of eCO₂ and eTe are inherent in several plants in the form of root exudation. Further understanding of the root exudation process, its composition, its dependence on environmental stressors, and soil phytomicrobiome (microbial community associated with a plant that constitutes the whole root as well as the shoot parts) and its specificity for a given plant, needs to be evaluated. Here we also consider how to best utilize root exudation pathways using, for example, SRS, which allows the evaluation of the interactions between plant roots, water, nutrients, and beneficial bacteria/fungi as well as root exudates as a function of time, concentrated within the SRS particles in soil. These interactions enhance biomass and crop yields, especially under water and nutrient stresses. The SRS-route is considered to be a suitable process to facilitate root plasticity, but without plant genetic modifications, which require the alteration of multiple genes, and thus remains challenging [107] .

The agricultural practices and technology in the emerging climate marked by eCO₂, eTe, and eWe, have to address several global warming impacts on soil, soil microbiota, root system architecture, and rhizosphere functions. For example, according to Vega-Mas et al., [103] eCO₂ climate increases N₂O emissions due to enhanced nitrification and denitrification, because nitrification bacteria use CO₂ as a carbon source for growth and NH₄⁺ as energy [108] . Further, a more abundant nitrifying population was found in response to eCO₂, alone or in combination with increased temperature [109] [110] . Vega-Mas et al., [103] indicate that, agriculture under future climatic conditions, with eCO₂ and eTe, biological nitrification inhibitor (BNI) production would be promoted since eCO₂ would boost both nitrogen assimilation and production of carbon-rich secondary metabolites. Another important outcome of global warming is the increase in the relative abundance of soil-borne potential fungal plant pathogens (i.e., plant pathogenic bacteria, fungi, oomycetes, viruses, and nematodes) [111] . These pathogens often cause soil-borne disease complexes, which, due to their complexity, frequently remain undiagnosed and unmanaged, leading to chronic yield and quality losses, estimated at 30% globally [112] [113] . As a result, more efficient pest-pathogen control methods are urgently required. Genetic engineering and breeding of pathogen-resistant plant crops involve the alteration of a large number of genes, making this route highly challenging. Furthermore, plant disease resistance could depend on local biotic environmental factors, including plant-associated microbiota, thus confining the genetic engineering route to local applications [114] [115] . This trait is also true for the growth and health of adjacent plants via specific interactions [116] .

The protection against pathogens initially takes place at the root rhizosphere, where the exudate promotes bacteria that reduce pathogen concentration. By way of this dilution, or pathogen-targeting bacteriophages, or by plant-signaling, the state of microbiomes switches from conducive to disease-suppression mode. However, relatively little is known about the underlying plant genes that govern microbiome-mediated disease resistance [117] . Nevertheless, the effect of root exudation is clearly an important route in combating root disease. The synthesis of chemicals mimicking plant exudates, or indeed the development of plant growth promoting rhizobacteria (PGPR)-based inoculants, and their use in soil, is one method of achieving several objectives summarized above [118] . For example, a single small peptide (Thuricin 17) was produced by Bacillus thuringiensis NEB17, isolated from soybean nodules as a bactericide (proteins with antimicrobial activity), and has been shown to be extremely effective in mitigating the effects of abiotic stresses on a wide range of crop plants [119] .

However, similar to the synthesis and use of biological nitrification/denitrification inhibitors, this method can be costly in terms of production and loss during use. It is not universally applicable, since its composition is dependent upon the soil microbial population, notably PGPR. Therefore, the exudate secretion requires the presence of suitable bacterial communities near the rhizosphere, thus requiring soil bacterial supply [120] .

Another method is to promote the in-situ production of plant exudates and keep their concentrations high over the rhizosphere in order to provide quorum signaling to recruit PGPR. In fact, instead of the production of specific components of the root exudates, PGPR-based colonies can be inoculated with a synthetic rhizosphere (SRS) medium and placed in soil as micro-bioreactors, which act as a source of PGPR, nutrients, and water. An SRS medium (synthetic root system) was introduced by the current author [12] [13] , which showed that plant roots are attracted to them through biotropism, chemotropism, and hydrotropism, in which biotropism is based on PGPR–root exudate signaling. The above summary indicates that the colonization of the SRS medium by plant roots is not a random process, but takes place due to a strong entropic driving force. Therefore, the rhizosphere transformations now take place within micro-sized domains, as the SRS medium is an interconnected porous material with a well-defined physical and biochemical structure through which plant roots penetrate and create a new micro-environment. These findings form the basis of a new generation of fertilizers in which the interaction between the rhizosphere and the fertilizer system can be enhanced in order to promote root exudate secretion. This can also protect and confine the rhizosphere, and alter root plasticity and morphology [13] . Therefore, this method has universal applicability, high efficiency, and affordability. Here, we first examine the structure of the rhizosphere and root exudate composition, followed by developments in their effects to suppress emissions from soil and enhance the efficiency of nitrogen use and crop yield.

Root exudates are a complex group of organic substances released in the rhizosphere with the potential to recruit, repel, stimulate, inhibit, or kill other organisms, including detrimental ones. An improved understanding of how root exudates affect interspecies and/or inter-kingdom interactions in the rhizosphere, under ongoing climate change, is a prerequisite to effectively manage plant-associated microbes, including those causing diseases [121] . In soil mass transfer, a biochemical reaction involving water, nutrients, microorganisms, and plant roots takes place in the root rhizosphere. This provides a shelter for the reactants that establish an inter-species relationship between the roots and microorganisms necessary for plant growth and physiology. Factors influencing the soil microbial population include soil quality, soil moisture, soil pH, and rhizospheric root exudates. These are responsible for the plant growth-promoting rhizobacteria (PGPR) and mycorrhizal fungi as well as their chemotactic mobility towards the plant root [120] [122] .

The functions of PGPR, such as direct and indirect mechanisms, metabolism, chemotaxis, secretion, and antibiotic production, are mediated by the gene cluster that triggers host–PGPR interactions. Figure 12 illustrates the location of the rhizosphere in the growing root, its functions with the soil bacteria, and the direct and indirect functions of root hair. In order to achieve efficient quorum sensing for PGPR recruitment, root exudate concentration needs to be high [123] [124] . The majority of root exudation is localized at the root tip, which is the first part of the plant to explore the new soil environment. This plays a crucial role in root responses to environmental stimuli [125] .

In the case of signal compounds, one can use microbe-to-plant signals for direct effects on the plants, or even plant-to-microbe signals to trigger enhanced production of the microbe-to-plant signals in the soil environment. This assumes the presence of the microbes in the soil. One could also use plant-to-microbe signals to control the composition of the phytomicrobiome in ways that are beneficial to the crop plants [118] . Clearly, the presence of PGPR in soil is necessary near the growing root for this mechanism to be effective. The understanding of the mechanisms involved in plant stress signaling and response by phytomicrobiome species, under spatial and temporal climate changes, has been studied for bacterial and fungal biomes. However, the holistic interactions with other species are also needed [126] .

Root system architecture is a fundamental component of plant productivity. It determines the capacity of a plant to forage for and acquire resources in the temporal-spatial soil environment. Root system architecture is genetically determined, but the chemical and physical characteristics of the soil environment ultimately shape and size root growth, especially in response to biotic, abiotic, and edaphic stresses. Roots are heterotrophic organs that are metabolically costly to build and maintain. The production of surplus roots diverts the plant from other useful functions, such as photosynthetic production, to support further soil exploration [107] [127] .

Plants can respond locally to the spatial-temporal soil environment through several mechanisms and perform functions by minimizing the metabolic cost of soil exploration. This can be accomplished by matching plant investment in root biomass and root function with resource availability in the soil [107] . Recent advances in nutrient uptake and root system architecture can respond to nutrient rich patches that can be used to develop strategies to control root and shoot growth. Common metabolites present in root exudates can be sensed by the roots, to continuously modify the root system architecture, which responds to environmental cues such as nitrogen, phosphorus, and water stress [128] [129] .

Here, we confine our attention to the primary functions associated with lateral roots, root hairs, root cluster growth, locally enhanced root proliferation, root tip, rhizosphere, and rhizosheath. The modulation of these functions can be incorporated into symbiotic fertilizers through the synthetic rhizosphere/synthetic root system.

Lateral roots: The density and length of the lateral roots are probably the two most important architectural phenotypes regulating root microbial associations, given that lateral roots comprise the large majority of root systems. They affect the root exudation zones, attachment surfaces, and the location along both vertical and horizontal gradients. Carbon exudation and the prime rhizosphere soil may initiate functions such as nitrification, ammonification, and soil organic matter degradation, either aerobically or anaerobically. Further, following the recruitment of PGPB by root exudation, root architecture can be modified, but the exploitation of such effects in agriculture is limited [127] .

Root hairs: The functions of root hairs include changing the area, location, and type of attachment surfaces for microbes to the roots, and affect the composition of the root microbial communities. Longer and denser root hairs can have synergistic effects with microbes participating in nitrification and ammonification of organic matter, by extending the surface area for bacterial support, while increasing soil exploration. Although high root hair density implies increased carbon allocation through root exudation, thus increasing root metabolic costs, it is far outweighed by the benefits provided. These benefits can include plant nitrogen capture, biomass growth, competitiveness for nutrients, dictation of the rhizosheath size, and its protection from all types of stressors, and efficient water and nutrient absorption, especially those with limited mobility such as phosphate [127] [130] .

Recently, a significant positive correlation was identified between root hair length and root hair density with relative-water content. This indicates the potential role of root hairs in mediating resilience under water shortages [131] .

Root architecture and anatomy: Under low nutrient supply, root architecture and anatomy interact with rhizosphere microbiota in order to optimize nitrogen capture [132] - [134] . Synergies and trade-offs of microbial associations, as influenced by root anatomy and architecture to optimize nitrogen capture by crops, represent promising avenues for crop breeding and microbiome engineering [127] .

Long roots and perennial crops: The advantages of long and extensive root systems are well known in accessing nutrients and water, and reducing nitrate leaching, which can justify the increasing root metabolic costs as in root hairs [127] [135] - [137] . Differences in biomass allocation are one major distinction, whereby annual plants allocate proportionately more biomass above-ground, while perennial plants allocate more biomass towards extensive root systems. Currently, annual cereals, oil seeds, and legumes are planted on almost 70% of cropland. This combined area supplies a similar portion (~70%) of all human calories. To provide food security for the increasing world population, in a worsening agricultural environment, perennial cereals and energy crops have been proposed to supersede annual plants [137] - [139] . Perennial energy crops were also advocated for soil carbon sequestration through extended root biomass and enhanced above-ground biomass yield, compared with annuals. It has been shown that perennial polycultures significantly outperform annuals in moderate fertility sites [136] [140] .

Despite ongoing critiques that the perceived vision of perennials is unlikely to succeed, and unworthy of expanded investment [141] [142] , recent research progress, theoretical considerations, and advances in breeding and genetic technologies have combined to provide justification for an aggressive expansion in perennial grain research in current and future decades, as reported by Olson et al., [142] and DeHaan et al. [143] .

Annual crops have relatively short growing seasons, shallow rooting depths, and lower root densities. This limits their access to nutrients and water, leaves croplands more vulnerable to degradation, and reduces soil carbon inputs and habitat for wildlife. Perennial crops capture, retain, and utilize more precipitation, have access to soil nutrients deeper within the soil, maintain more plant residues at the soil surface, and have a longer photosynthetic season. All of these benefits increase yearly light interception and enhance productivity. This farming system is resilient to flooding, extreme weather, and storms. In a perennial farming system, some commonly cultivated crops include plantain, banana, wheat, sorghum, and rice. Studies have revealed that axillary buds on perennial rice nodes have the ability to overwinter and subsequently develop into new plants during the following seasons, resulting in lower agricultural inputs [144] - [147] .

Perennials prevent the depletion of soil resources in ecosystem restoration; the residual root systems promote the growth characteristics of ratoon crops such as sugar cane, in which their residues (such as sugar cane bagasse) provide a substantial quantity of biomass for annual bioenergy generation [148] . Over a period of time, perennials ultimately require lower rates of fertilizer, pesticide, and labour inputs; symbiotic/smart fertilizers are particularly suitable for these crops [12] [14] .

It is clear that the rhizosphere is primarily responsible for regulating most of the plant root functions and interfacing them with microbiota, nutrients, and water, while responding to stressors through root exudation. Therefore, it is important to protect the rhizosphere and facilitate its functions so that plants can respond more efficiently to the rapidly evolving stressors due to climate change. In fact, most plants already have an inherent mechanism to protect and facilitate the rhizosphere through the formation of the rhizosheath. It is a soil-rich cover formed by the aggregation of soil and microbial communities. It is facilitated by a network of gel-forming (mucilage) hydrophilic metabolites (rhizodeposits). These are released by microbes (extracellular matrix responsible for biofilm formation), roots, and root tips in the presence of high root hair density and length. The rhizosheath is rich in water, nutrients, labile carbon, and microbial biomass [149] - [151] .

The rhizosheath not only protects the roots and rhizosphere but also provides a protective nutrient-rich micro-environment for the soil microbiome. It is therefore a further manifestation of the symbiotic relationship between plant roots and plant-beneficial microorganisms. This symbiotic platform is particularly important under drought conditions and in deserts where the rhizosphere is an edaphic “mini-oasis” from which other competing microorganisms are excluded [152] . The ingression selectivity is likely to be achieved through the weaponization of antibiotics produced by the beneficial bacteria and mycorrhizal and saprotrophic fungi in the rhizosheath [153] . We can therefore assume that the SX-fertilizers also act as an artificial edaphic barrier for the protection of the mini-oasis generated by the SRS-medium.

Rhizosphere engineering aims to control and manipulate the biophysical properties of the rhizosphere and its components for specific goals, through agronomic, physical, chemical, biological, and genomic approaches [154] [155] . Wang and Kuzyakov [155] provided a recent review of this subject in the enhancement of soil carbon sequestration, and also proposed a number of future research activities to help understand the complex interactions between rhizosphere components and their outcomes. Zhang et al., [156] proposed an entropic approach to rhizosphere engineering in which the nutrient supply administration was localized with improved information exchange, related to root-soil-microbe interactions. This resulted in an increase in nutrient-use efficiency. Localized fertilizer administration (rather than general fertilization of soil) is a known practice. This method is now further refined by on-demand, localized production, fertilizer use, and rhizosphere engineering [110] .

Although the understanding of the root exudate composition, secreted under well-defined abiotic and biotic stressors, and associating each of the compounds with a type of stress so that these chemicals can be manufactured and used in the soil to combat plant diseases and improve crop yield, remains highly challenging. This process is somewhat akin to pharmaceutical manufacture and practice. Further complications include cost, loss of manufactured bio-stimulants in soil, and rapidly changing spatiotemporal soil conditions, especially due to weather events. Therefore, robust and cost-effective routes for fertilizer administration and agricultural practice under eCO₂, eTe, and eWe/eDe need to be devised, which should operate through the function of the root rhizosphere.

The synthetic rhizosphere approach is another route for the protection of rhizosheath functions and for crop yield enhancement, especially under water and nutrient stress.

The performance and mechanism of SRS-medium in agriculture have been evaluated using greenhouse experiments, the details of which are available and closely related to BioProcess Intensification [3] [4] [12] - [14] [46] - [51] [88] . Here, the effects of SRS-medium on biomass and crop yield are demonstrated in three settings: (1) Biomass yield enhancement using ryegrass as a model non-woody, non-leguminous plant and fertilizer containing SRS-medium; (2) Enhancement of fruit (pea pod) yield and its nutritional characteristics using the pea as a model legume and nitrogen-fixing bacteria (Rhizobium leguminosarum) inoculated SRS-medium; (3) Enhancement of biomass yield using lawn grass as a model non-leguminous plant and nitrogen-fixing bacterium (Azospirillum brasilense) inoculated SRS-medium. In each case, the enhancement levels are provided, and the plant root – SRS medium interactions are demonstrated through SEM images of the root-PHP systems. In all cases, biological samples were prepared using the procedure described previously [12] - [14] .

In all cases, sulfonated-neutralised PHP (sn-PHP) was used after washing and drying the samples to remove any acid/ammonia. The SRS-medium constituted 0.5 wt% of the soil in plant pots placed in greenhouses and subjected to various watering and nutrient programs.

The use of ryegrass as a model perennial plant is useful because abiotic/biotic stresses can be tested over a long period of time and harvests. In these sets of experiments, root growth is directed to the SRS-medium through hydrotropism and chemotropism driving forces. Washed and dried sn-PHP was mixed with certain amounts of nitrogen fertilizer solution. An equivalent amount of fertilizer solution was added directly to the plant pots after seeding the soil. The results, which demonstrate the merits of the SRS-medium, are shown in Table 1 and Figure 13 .

Fresh = Fresh yield; Dry = Dry yield; NS = No significant (p > 0.05) change in yield. For further details (including the commercial fertilizers (MG) and (AN)), see Reference12.

Table 1 clearly shows that sn-PHP provides enhanced yield (both fresh and dry biomass) in the presence or absence of commercial fertilizers. The relative increase in yield increases with increasing harvest number as the initially administered fertilizer depletes. The maximum yield enhancement over the control is 135% (fresh biomass) and 123% (dry biomass). The dry biomass yield enhancement increases substantially under drought conditions (weekly watering) and reaches 240% provided that sn-PHP is administered in powder form (size range 100 μm – 2000 μm) [13] [88] [89] . Under drought conditions, fresh biomass enhancement is only 109%.

Table 1 also differentiates the performance of two different commercial fertilizers, although their effects in biomass enhancement by sn-PHP only become significant at the third harvest. There are two mechanisms for this behaviour: First, sn-PHP acts as a slow-release fertilizer, and second, by the time of the third harvest (63 days after seeding), roots reach all of the sn-PHP particles and associate with them, as seen in Figure 13 .

Figures 13(a,b) illustrate the architecture of the SRS-medium. Here, the sn-PHP pore volume is 90%, and the pore walls are nano-porous, with an average pore size of 150 μm, obtained through the method described previously [7] [13] [14] . The photo image of the SRS-medium and root association is shown in Figure 13(c) , while the SEM images in Figures 13(d-f) illustrate the structure of the roots within the SRS-medium, which itself is modified, compared with Figure 13(a) . The open pores of the original sn-PHP structure ( Figure 13(a) ) are substantially filled, and roots with diameter ca. 200 μm are present (d-f), along with root hairs (diameters ca. 20 μm). The roots are able to branch out ( Figure 13(d) ), and the root hairs penetrate into the pores through sn-PHP interconnecting holes. Furthermore, Figure 13(e) clearly shows the modification of the root architecture.

It was shown that non-biological SRS-medium is only useful under nutrient and water stress [12] [13] . In order to demonstrate the effect of supported bacteria in close proximity to plant roots, sn-PHP was inoculated with a nitrogen-fixing bacterium, Rhizobium leguminosarum and mixed with soil in plant pots. The equivalent number of bacteria was used in the soil when no sn-PHP was present. Sn-PHP was also used by itself (no bacterium) as a baseline for the inoculated sn-PHP. The final set was the control group, which did not have any bacteria or sn-PHP, and the results were compared with this control group. The structure, dimensions, and the amount of sn-PHP used were identical to the previous example (ryegrass growth). After planting one pea seed per pot, the pots were watered with 50 ml of water for the next 42 days, after which the pea crop and the shoots (biomass) were removed separately and weighed immediately, as well as after drying them.

The results are summarized in Table 2 . It was found that the only statistically significant enhancement (p < 0.05) was when sn-PHP was used with Rhizobium leguminosarum. In this case, the dry pea yield was enhanced by 69.6%, whereas dry biomass enhancement was 31.0%.

The reason for this behavior is that as the pea plant roots grow through the Rhizobium inoculated sn-PHP, bacterial infection and nodulation take place. The visual examination of inoculated sn-PHP shows that nodulation only took place if the roots passed through the sn-PHP, indicating that the bacteria dispersed freely in soil did not have sufficient concentration to cause extensive infection and nodulation.

Another important finding was that Rhizobium inoculated sn-PHP had the highest protein content (as apparent from the nitrogen and phosphorus content of the dry pea crop) as well as micronutrient metals. These results are shown in Table 2 . Except for iron, when sn-PHP is used with bacteria, the mineral content of both shoots and pea are higher compared with the case when sn-PHP was not used.

Figure 14 illustrates the stages of micro-environment development within the sn-PHP after its inoculation with Rhizobium leguminosarum and use as the SRS-medium. Figure 14 a shows the establishment of a Rhizobium leguminosarum colony within the sn-PHP after 14 days of incubation. Here, 2 μm long Rhizobium leguminosarum can be identified together with their extracellular matrix building the colony. The inoculated sn-PHP can either be mixed with soil immediately after inoculation or following an incubation period, which ensures extensive colonization within the SRS-medium.

The extensive root coverage in sn-PHP is shown in Figures 14(b-d) . In these figures (with increasing magnification) there is extensive coverage of sn-PHP pores with roots ( Figure 14(a) ), root hairs ( Figure 14(c) ), and bacteria, as well as what appears to be fungi ( Figure 14(d) ). As seen in Figure 14(b) , the root diameter (ca. 200 µm) is significantly greater than the pore and interconnects, which are shown in Figure 14(e) . This figure shows that the surface of the pores is covered with rhizodeposits. Figure 14(f) illustrates a bacterial colony, which however appears to be different from the colony when bacteria are incubated in the laboratory under sterile conditions. Figures 14(g-i) illustrate various types of micro-environments in the SRS-medium. They show the presence of extensive rhizodeposition, which includes secondary metabolites and root exudates [157] [158] . Figure 14(j) shows the presence of two root nodules as well as a third emerging nodule on a small root length. Due to their size, the nodules are always outside the SRS-medium, although the root infection is primarily in the SRS-medium.

Azospirillum are free-living, nitrogen-fixing, plant growth-promoting rhizobacteria which are able to colonize the roots of hundreds of plant species and significantly improve their growth, development, and productivity under field conditions. For example, wheat yield has been increased by approximately 30% by inoculation with Azospirillum brasilense [159] - [161] . Several of their functions have been studied, which indicate that there are multiple mechanisms involved in their ability to promote plant growth [161] . They can be used as a bio-fertilizer as well as in the co-inoculation of legumes with rhizobia, which results in enhanced crop yield in the region of 30%. More recently, their applications have been extended to biotechnology [161] .

Table 3 summarizes the biomass yield enhancement when sn-PHP was inoculated with Azospirillum brasilense and used as an SRS medium, in the same way as described in the previous two sections in greenhouse experiments. There were four groups of soil treatment. 1) Control group: Soil in the plant pots did not contain any bacterium or sn-PHP; 2) Soil contained 0.5 % sn-PHP; 3) Soil contained inoculation medium; 4) sn-PHP had the same amount of bacterial inoculation as in (3) [162] . The ryegrass was watered every day and harvested every 3 weeks when the dry weights were determined. The results were compared with respect to the control group as a percentage of biomass yield enhancement.

Table 3 indicates that sn-PHP provides good biomass growth enhancement after 6 weeks when the root penetration into the SRS-medium is completed. However, when the soil nutrients are depleted (12 weeks of growth period), its performance is reduced, but still above that of the control. When Azospirillum brasilense was placed in soil, the best performance of ca. 30% is similar to that observed previously in field trials [160] [161] . Further, when sn-PHP was used as the support for Azospirillum brasilense, the highest enhancement was 145% after 12 weeks. Initially, the enhancement was low due to the fact that the grass roots needed to penetrate into the SRS-medium and the bacteria needed to proliferate within the pores. The SEM study of the root association with sn-PHP indicates that the root architecture within the SRS-medium is very similar to those illustrated in Figure 13 and Figure 14 .

The effect of sn-PHP containing the bacteria (Bradyrhizobium japonicum) was investigated on soybean growth, under both normal and reduced water conditions, but in the presence of reduced soil nutrients. B. japonicum is a gram-negative, nitrogen-fixing bacteria that forms a symbiotic relationship with soybean plants. The effect of the biologically active SX-fertilizers in fixing nitrogen from air is not immediately obvious because, in the presence of readily available soil nitrogen, legumes prefer to use the available nitrogen from the soil. In order to accelerate the bacterial effects, the nutrient content of the soil used in the present greenhouse experiments was reduced by successive washings using tap water. Before and after washing, the soil nutrient content was: Nitrogen = 0.22 wt% and 0.14 wt%; Phosphorus = 8.66 µg/ml and 5.87 µg/ml; Potassium = 203 µg/ml and 109 µg/ml, respectively. Sulfonated neutralised PolyHIPE, sn-PHP, was either used without any bacterium or with B. japonicum. Each pot contained 500 g soil and sn-PHP use was 2.5 g/pot. B. japonicum inoculation with 2.5 g sn-PHP was carried out using Hoagland's nitrogen-free nutrient solution [12] . Two watering regimes were used: (1) Normal watering with 100 ml water per pot twice weekly; (2) Droughted soybean was given 50 ml water per pot, twice weekly.

A summary of the results is shown in Figure 15 . Dry shoot and root weights obtained under normal and drought watering are presented for: (1) Control (contains no sn-PHP or B. japonicu); (2) Same as the “control” with sn-PHP; (3) Same as the “control” with B. japonicu; (4) Same as (2) but sn-PHP contains B. japonicu. The presence of sn-PHP (without B. japonicu) results in a 97% enhancement in shoot dry weight under normal watering. Under drought conditions, the corresponding enhancement was 87%. The enhancement in dry root weights for normal and droughted watering was 54% and −9.9% (which is, however, statistically not significant), indicating that the biomass enhancement in the presence of sn-PHP has been achieved with a lesser amount of root, indicating that sn-PHP does act as an artificial root, especially under drought conditions. These results also indicate that under drought conditions, root dry weight increases, while the reverse is true in the presence of sn-PHP.

When B. japonicum is administered into the soil, there is no statistically significant change in the dry shoot and root weights. When B. japonicum is administered within sn-PHP, the dry shoot and root weight increases in normally watered plants are 121% and 25.6%, respectively. The corresponding shoot and root dry weights for droughted watering are 25.6% and −18%, respectively. These trends are the same as in the previous cases studied here. It is interesting to note that under droughted watering, root weight decreases (or increases slightly) in the presence of sn-PHP compared with the control (no sn-PHP). The increase in shoot weight due to bacteria within sn-PHP is 25% under normal watering. With droughted watering, this increase is 5%. However, the enhancement of SB-fertilizers on biomass and crop yield is more pronounced over a longer time scale as the soil becomes more depleted in nutrients.

Fertiliser levels fall during plant growth as a result of wash-out and plant metabolism. As shown earlier, for example, in grass growth, the effect of the SRS-medium started to show after the second or third harvest under normal watering. As nutrients were already depleted, the beneficial effects of the SRS-medium became clear immediately even with normal watering. Under water stress, the observed effects are similar to those observed in our previous studies [12] - [14] [162] . Under water stress, although there is substantial enhancement in biomass yield, the increase in root weight in the presence of polymer is not significant, indicating that the available root mass is sufficient to sustain enhanced biomass growth.

The advantages of long and extensive root systems are well known in accessing nutrients and water, and reducing nitrate leaching, which can justify the increasing root metabolic costs as in root hairs [127] [135] - [137] [163] [164] . Differences in biomass allocation are one major distinction, whereby annual plants allocate proportionately more biomass aboveground, while perennial plants allocate more biomass towards extensive root systems. The soil carbon sequestration potential [164] is also affected by the root characteristics [163] . Plants also adapt to environmental stresses through alterations in root morphology and plasticity [164] . It is possible that SRS-medium facilitates such alterations (such as longer root hairs) so that the root system becomes more efficient.

According to DeLisi et al., [165] , the difference between atmospheric GHG levels that will be reached if current trends continue, and levels that would be achieved if the goals of the US decarbonization plan are met—even with worldwide implementation—is inconsequential. Therefore, DeLisi et al., [166] and Gidding [167] argue that any mitigation strategy must include the drawdown of atmospheric greenhouse gases and advocate a particular drawdown strategy, agrigenomics, offering the opportunity for a revolutionary trifecta: climate change mitigation, food security, and medical advances. DeLisi et al., [166] thus propose to use synthetic and systems biology techniques to reduce greenhouse gases. These techniques include, among other possibilities, engineering plants to convert CO₂ produced by respiration into a stable carbonate, designing plants with an increased root-to-shoot ratio, and creating plants with the ability to self-fertilize. The foregoing analyses for the AR-system indicate that the feasibility of these aims has already been achieved. For example, the inclusion of nitrogen-fixing bacteria both for legumes (Section 5.3, using Rhizobium leguminosarum) and non-legumes (Section 5.4, using Azospirillum brasilense) achieves a constant supply of nitrogen and micronutrients. Also, by placing SX-fertilizers at different levels in the soil, roots can grow deeper as a result of hydro-, chemo-, and bio-tropisms [3] . In any case, a switch to perennial crops will achieve this objective. However, the enhancement of the root-to-shoot ratio also means reduced biomass and crop productivity. The long-term mineralization of CO₂ by using the AR-system has also been discussed previously (Section 4, and reference [3] ).

One of the important advantages of the proposed AR-system based on bacterial SB-fertilizer is that several different SB-fertilizer grades can be prepared and used in the soil, each grade producing different metabolites in the rhizosphere in order to enhance the root exudate functions, including photosynthesis, plant resource-use efficiency, and facilitating the crosstalk between plant and soil microbes to remedy stresses [168] . For example, the generation of specific metabolites (enzymes) can significantly enhance photosynthesis as shown in artificial photosynthesis [169] [170] .

Calcium oxalate, Ca(COO)₂, (denoted as CaO_x), a mineral of low solubility, is found widely in plants, but its fate in soils has been neglected until recently. A comprehensive review by Uren [171] considers the likely forms and reactions in soils that may be significant in the recycling of both calcium (Ca) and carbon (C). In soils, calcium oxalate is both formed by fungi and utilized by saprotrophic microbes and by some mesofauna as a source of energy and C. In acidic soils, the oxidation releases soluble products, the Ca as Ca²⁺ and the C as bicarbonate, whereas in alkaline soils some Ca is released as Ca²⁺ but some C and Ca²⁺ form calcium carbonate, which may contribute to the formation of calcrete, usually in soils found in arid regions. This latter reaction has been considered as a possible major pathway, which may be utilized in the sequestration of carbon dioxide [171] . Calcium carbonate formation is favored by degradation of CaOx by oxalotrophic bacteria accompanied by an increase in soil pH in a process known as the oxalate-carbonate pathway [171] [172] . In their recent review, He et al., [173] conclude that CaOx has far-reaching implications in plant physiology, ecology, and human health. Physiological roles include calcium regulation, defense against herbivory, and potential contributions to carbon sequestration, while they have ecological significance in nutrient cycling and plant-soil interactions. However, concerns over human health are due to kidney stone formation and the potential hazards posed by these crystals in human nutrition, particularly in the context of oxalate-rich diets and kidney stone formation [173] .

Figure 16 shows that CaO_x is also produced within the sn-PHP either by ingrown fungi or plant roots. It is likely that crystal growth within the pores will have a different crystal structure from that grown from solution or with the soil.

Crop yields and nutritional security are extremely dependent on the climatic conditions projected for the future, and consequently, most of the food produced for human consumption is under its menace [174] [175] . Elevated CO₂ is closely related to increased demands for nutrients and water for plant growth [176] . The artificial root system is primarily designed for efficient delivery of the essential (H, C, O) and primary macronutrients (N, P, K) to the plant while promoting the plant’s own defence mechanisms against biotic and abiotic stresses. Figure 17 illustrates the macro- and micro-nutrients, which indicate the hierarchy of the macro- and micro-nutrients. Figure 17 also indicates that Group 3d transition metals as catalysts ranging from Sc to Zn are also included in the list of micronutrients, including Mn, Fe, Ni, Cu, Zn (primary micronutrients), and Co as the secondary micronutrient [177] .

The other primary micronutrients include B, Cl, and Mo (Group 4d catalyst), while the emerging secondary micronutrients include Na, V, Se, Si, and Al. Some of these non-transition metals used as primary/secondary macro/micro-nutrients (K, Ca, Mg, S, B) are also used as catalyst promoters in the QCs [178] . Therefore, in choosing the composition of the QCs, their post-service function as micronutrients should be considered to provide full circular economy.

Compared with the essential and primary macronutrients, the effect of climate change on micronutrients has not been investigated until recently. It is known that when agricultural yields are reduced because of inadequate soil micronutrient concentration, malnutrition may result. Deficits in soil micronutrients can result from a number of variables, such as the use of intensive cropping practices, micronutrient leaching, soil characteristics, and decreased use of farmyard waste [177] . Figure 18 shows some of the primary micronutrient requirements at various stages of plant growth [179] .

The function of these micronutrients was summarised by Marschner [181] as: B: Sugar metabolism, flowering, fruit and seed development; Mn: Photosynthesis; Fe: Chlorophyll synthesis; Cu: Enzyme activation and nitrogen metabolism; Zn: Early growth and development. This summary clearly indicates the root-leaf connectivity as the availability of B, Mn, and Fe directly affects leaf function. Therefore, micronutrient limitation has particular importance because of their role in key biochemical pathways, which in turn has an impact on the susceptibility of plants to biotic and abiotic stresses. However, the response largely depends on plant genotype, and each mineral element has complex interactions with several changing climate variables [182] .

Numerous studies have been carried out to investigate soil micronutrient absorption and bioavailability. In order to improve crop utilization of soil nutrients, Tiwari et al., [179] provided a recent analysis of the relationships between plant functions and soil nutrients. The internalization of micronutrients through the transcellular transport mechanism by plant or mammalian cells is dependent on the biological and dynamic barriers involved, as well as particle characteristics such as size, shape, charge, and surface chemistry [183] [184] . The particle physicochemical properties also determine the biological function of the internalized particles. Further, in plants, there appears to be a defensive biochemical pathway to exclude toxic particles (including nano- and micro-plastics) through the thickening of the cell wall via lignin synthesis, carbohydrate metabolism, and lipid transport [185] .

Although the silica-supported QCs can deliver both silica and metals in the micronutrient category (such as B, Mg, V, Mn, Fe, Co, Ni, Cu, Zn, Mo, Si, S, Se) either as a catalyst or catalyst support/promoter, their pore structure (pore and interconnect size) is not suitable for root penetration and their association with the plant. They can be used in porous sachets as particles. Alternatively, Hypercrosslinked PolyHIPE Polymer (HX-PHP) or Sulfonated Hypercrosslinked PolyHIPE (sHX-PHP) supported nano-catalysts can be used as AR at the end of their service life. Hypercrosslinked Polystyrene (HXPs) or Sulfonated Hypercrosslinked Polystyrene (sHXPs) have a very high surface area by virtue of their extensive pores in the nano-to-subnano scale. Due to increased crosslinking density, the start of polymer decomposition increases by about 100˚C [186] . Furthermore, as shown by Mikkelsen et al., [187] , the dielectric constant of polymers is similar to that of silica but the dielectric constant of sHXPs increases with the increasing degree of sulfonation by an order of magnitude while its electrical conductivity enhancement reaches three orders of magnitude. Other applications and preparation methods of HXPs and sHXPs are available [188] [189] .

Therefore, both HX-PHP or sHX-PHP can be used as catalyst supports instead of SiO₂ or Al₂O₃. Recently, Markova et al., [190] using XHPs, inserted Fe and Ru co-catalysts for Fischer-Tropsch synthesis, obtaining a high yield of high hydrocarbons. However, the synthesis of HX-PHP or sHX-PHP or their use as catalyst supports is not currently available. Catalytic HX-PHP and sHX-PHP can be used as fertilizers at the end of their service life for the delivery of micronutrients. Alternatively, Hypercrosslinked PolyHIPE (HX-PHP) or Sulfonated Hypercrosslinked PolyHIPE (sHX-PHP) supported nano-catalysts can be utilized as AR at the end of their service life, thus providing a circular economy and environmental management.

The preparation of such HX-PHP or sHX-PHP supported catalysts has been conducted recently [82] . Figures 19(a,b) illustrate the scanning electron microscopy image of the HX-PHP after synthesis at two magnifications. It can be seen from Figure 19(a) that the usual PHP structure is still present, but it is partly obscured by the presence of Friedel-Crafts catalyst (FeCl₂) particles, but otherwise, the usual PHP structure is maintained. Figure 19(b) illustrates high wall porosity, which is also reflected in its very high surface area (ca. 1000 m²/g). Figure 19(c) illustrates the transmission electron microscopy image of a HX-PHP after the removal of FeCl₂ by 1,2 dichloroethane solvent extraction followed by Co(NO₃)₂ catalyst precursor insertion and the subsequent decomposition of Co(NO₃)₂ under plasma in a H₂ stream. The XRD spectrum of the resulting HX-PHP with cobalt catalysts is shown in Figure 19(d) , which shows that Co(NO₃)₂ decomposed to CoO. The catalyst particles mainly appear to be located around the nano-sized pores in the form of rings. Therefore, the nano-sized pores are not blocked up with the catalyst.

The catalyst precursor Co(NO₃)₂ is inserted into the pores of HX-PHP after dissolving it in ethanol. Following drying, HX-PHP particles were dispersed into 3 mm diameter BaTiO₃ spheres, which act as a Plasma Catalysis Promoter (PCP), and placed into the Dielectric Barrier Discharge (DBD) reactor as shown in Figure 20 .

This reactor unit was then inserted into the general purpose DBD-equipment described previously [5] - [7] . A constant stream of H₂ was passed through the reactor at 50 ml/min at 250˚C. The packed bed of PCP and HX-PHP particles was contained between two perforated stainless-steel electrodes, and plasma was generated between them [5] - [7] [21] - [23] . The composition of the gases from the outlet was periodically checked until no nitrogen oxide was present. The BET surface area of the original HX-PHP (after solvent extraction to remove FeCl₂) was reduced from 1020 m²/g to 433 m²/g after catalyst precursor insertion and its decomposition and reduction in H₂ atmosphere, which is partly due to the presence of a high density CoO catalyst.

The use of HX-PHP or sHX-PHP as a support for catalysts has been demonstrated. These catalysts can be used in plasma as well as in thermochemical reactions, followed by use as XA-fertilizers in artificial root applications and the delivery of micronutrients. If the catalyst precursor within the pores of HX-PHP can also act as a Friedel-Crafts catalyst, then the second stage of the catalyst insertion will not be required, thus simplifying the process.