Nitrous oxide (N₂O) is a potent greenhouse gas that contributes to global warming, climate change, and stratospheric ozone depletion [1] . Globally averaged N₂O concentration in the air in 2012 reached 325.1 ppb, which was 120% of the pre-industrial level (270 ppb) [2] . Agriculture has been one of the major sources of global N₂O emissions. Emissions from soil and associated nitrogen (N) inputs, such as synthetic fertilizer, animal manure and crop residue, are the main agricultural N₂O sources, contributing 90% of the total [3] . Mosier et al. [4] estimated that animal manure applied to soils directly contributed 0.3 Gt CO₂-eq/yr (i.e., 10%) to global N₂O emissions. Nitrogen losses in the form of N₂O from land application of animal manure are of global and regional importance to air quality and climate change [5] .

As an alternative to direct land application, composting is one of the widely accepted technologies for recycling organic wastes in agriculture: it can minimize some of the disadvantages associated with direct application of raw wastes, e.g., phytotoxicity, leaching and denitrification of mineralized organic N [6] . Composting consists of the transformation of organic matters (OM) into a relatively well-stabilized product through rapid succession of microbial populations under aerobic conditions. During that process, part of the OM is mineralized to CO₂, whereas the rest is transformed to humic substances, which represent a valuable index of OM stabilization [7] . However, the emission of greenhouse gases from composting of organic wastes is a serious problem. For example, annual global N₂O emissions from composed organic wastes have been estimated at 1.2 × 10⁶ metric tons or approximately 0.4 Gt CO₂-eq [8] . At this scale, composting poses serious environmental risks by contributing to global warming and ozone depletion. When applied to soils, composted manure is also known to increase N₂O emissions by stimulating nitrification and denitrification [9] . Compared with N₂O emissions from chemical fertilizers, N₂O emissions from manure are of greater duration and emission intensity [10] .

The authors believe that one solution to the above challenges is to compost animal manure with biochar and apply this composted biochar-manure (CBM) to soils to enhance crop production and minimize N₂O emission. Biochar is charcoal produced from biomass via pyrolysisor gasification [11] [12] . Biochar can effectively retain NH₃, , and in animal manure [11] . Recent studies demonstrated that bulking manure with biochar reduced N loss while simultaneously enhancing humification, thereby producing mature composts with a high fertilizer value [13] -[16] .

Composting of high organic content wastes has been shown to produce N₂O by the microbial processes of nitrification and denitrification, and N₂O generation is found to depend on the transformation of different nitrogen states in the composting mixture [1] . As shown in Figure 1, under aerobic conditions, -N from manure is rapidly converted into N₂O by incomplete nitrification. But at the conditions of low O₂ contents, -N in the manure emits N₂O through incomplete denitrification process [17] [18] . Considering the shortage of oxygen for most composting piles, denitrifiation is responsible for the most part of N₂O generation in composting, while nitrification makes a substantial contribution to the N₂O emission at the surface of composting pile where O₂ is adequate and temperature is suitable [19] .

Manure properties such as moisture content, NO_x-N content and carbon-to-nitrogen ratio (C:N) along with process management such as aeration, temperature regime, turning, covering and compacting can significantly affect N₂O emissions during composting [20] . For example, it was found that there were large amounts of N₂O emissions during the early stage of composting with high N materials because of NO_x-N denitrifying in the early stage. Conversely, at this period, nitrification that limits N₂O emission is restricted owing to the unsuitable microbial activities at the environment of high temperature and nitrogen/oxygen [21] [22] . The research in dairy

cattle manure and swine waste composting confirmed that the cumulative emissions of N₂O increased significantly by the use of mature compost that contains nitrate and nitrite [17] [23] . In general, materials with a low C:N is desirable for low N₂O emission composing [24] [25] . Moisture content in composting mixtures is another important parameter affecting the quality of the compost because it affects the metabolic and physiological activities of microorganisms. High moisture content enhanced nutrient transport [24] , but too high moisture may cause anaerobic conditions, which provides the beneficial conditions to generate N₂O through the incomplete denitrification pathway of NO_x-N. It also prevents and halts the ongoing composting activities [24] [26] . On the other hand, very low moisture content would cause early dehydration during composting and becomes a limiting factor for the aerobic degradation, thus giving physically stable but biologically unstable composts [24] . In general, 50% - 60% moisture content is identified as suitable for effective composting and inhibiting N₂O emission [24] [27] .

In addition to manure properties, various environmental variables also affect composting and its N₂O emission. Temperature of the composting process is widely considered as a significant factor for composting efficiency and N₂O emission because microbial metabolism and activities are all temperature sensitive and dependent. It was found that temperature of composing below 20˚C or in excess of 60˚C would slow and even stop composting owing to impeded microbial activity [24] , which explains why there was lower or even no generation of N₂O during the thermophilic phase (temperature of higher than 50˚C) in the early stage of composting [28] . Substantial N₂O emissions usually start in the middle stage of composting when the temperature of the composting pile begins to decline [29] . Aeration is another important factor because composting is basically an aerobic transformation of organic matters where O₂ is necessary, and the supply/distribution of O₂ in the composting pile also affects the production and emission of N₂O. Usually, with increasing O₂ the emission of N₂O increases first and then decreases, therefore proper aeration is beneficial for reduction of N₂O emissions [27] [30] . Similarly, controlling the air void of the composting pile through compacting or adding porous materials affects N₂O emission [27] . Besides, other factors such as pile size, pH and available nutrients have also been shown to impact on composting performance and N₂O emission [24] [29] .

The mechanisms of N₂O formation described in Section 2 can help us understand the effect of biochar addition in manure composting. The authors believe that biochar, produced from high carbon content solid biomass, is one of the best bulking agents for reducing N₂O emission in manure composting for the following reasons. First of all, its high porosity results in increased aeration in the composting process, which enhances the supply and distribution of O₂ in the composting pile, and may lead to reduction of N₂O as previously mentioned reasons [16] . Secondly, the high porosity and high surface area of biochar also enables it to absorb and retain large amounts of water which results in decreased N₂O emission by altering redox conditions and denitrifying communities. High moisture content also enhances the metabolic and physiological activities of microorganisms by transporting dissolved nutrients [12] [24] . Thirdly, NH₃ or water-soluble, adsorbed by biochar significantly reduces NH₃ and losses during composting, further to reduce the emission of N₂O, which also offers a mechanism for developing slow release fertilizers [31] . Steiner et al. [11] incorporated biochar to poultry manure and composted them over a 42-d period, and found that NH₃ emissions decreased by 47% - 55% as the rate of biochar incorporation increased, confirmed that biochar was effective to alter N transformation and fate. These beneficial effects may have been caused by the decrease in N availability for denitrification, as biochar can efficiently adsorb and retain ammonia gas and ammonium as well as nitrate ions [32] [33] . Furthermore, other recent studies confirmed that bulking poultry manure with biochar lessened N loss and improved N retention, while simultaneously enhanced humification, thereby produced mature composts with a high fertilizer value [11] [34] [35] . Fourthly, biochar with a higher pH alters the abundance of denitrifying bacteria significantly in manure composting, resulting in less N₂O producing but more N₂O-consuming bacteria communities [12] . Although the benefits regarding the use of biochar as a bulking agent for composting have been demonstrated, research in understanding its role in reducing N₂O emission is still scarce. The mechanism by which biochar impacts N₂O fluxes over the entire composting period is also poorly defined.

In addition to the reported results of biochar reducing N₂O emission in manure composting, biochar also provides benefits on accelerating composting. For example, it not only provides structural support to prevent the physical compaction of the pile and increases air voids allowing the aeration of the pile [36] , but also acts as a biodegradable carbon and energy source for supporting microbial activity and balancing the initial C:N ratio of the mixture [37] . Besides, the addition of biochar to the composting process can reduce the activity of methanogen (CH₄ production) and increase methylotroph (CH₄ oxidation) activity of microbes. Moreover, non-carbon neutral CO₂ can be mitigated by the strong carbon sequestration ability of biochar, which is beneficial for reducing the environmental load of GHG emissions [16] .

The question on how manure interacts with biochar and alters biochar properties is scientifically interesting, though currently little is known. For example, composting may facilitate surface oxidation of biochar by the elevated temperature, especially at the beginning of the composting process. It also changes biochar properties biotically by the high microbial activity or the co-metabolic decay during the degradation of available carbon sources [38] [39] . In addition, the nutrient contents of biochar can be enriched by co-composting with nutrient- rich manure. Biochar absorbs leachate generated during the composting process, resulting in increased moisture content. With the leachate, biochar also absorbs organic matter and nutrients, resulting in increased contents of water-extractable organic carbon, total soluble nitrogen, plant-available phosphorus and plant-available potassium, therefore increasing nutrient retention capability of the composted material. However, it should be noted that the surface area of biochar might decline during the composting process due to the clogging of micropores by adsorbing compost-derived material [40] . Besides, the sorption of organic matter like humic acid from manure could lead to an increase of oxidized functional groups, e.g., carboxylic groups, on the biochar surface, which further increases surface oxidation and absorptivity [32] . Thus, co-composting manure with biochar is considered a promising method that can generate a nutrient- and humus-rich soil amendment agent or slow release fertilizer [35] [41] .

With limited literature available, biochar has demonstrated its potential in enhancing manure composting and reducing associated N₂O emissions. This can be attributed to the high porosity and high surface area of biochar that enables absorption/adsorption and retention of water, NH₃ or water-soluble, as well as nitrate ions, leading to desirable metabolic and physiological activities of microorganisms. The authors believe that co-com- posting manure with biochar is a promising method for both slow-release fertilizer production and greenhouse gas mitigation; however, further research is needed to understand the role of biochar in the composting process and the interaction between manure, biochar, and microbes.

This work was financially supported by the startup fund of North Carolina State University.