A pot experiment was conducted in the Net house, Department of Environmental Science, Bangladesh Agricultural University, Mymensingh with BRRI dhan-89 and BINA dhan-10 during boro season rice cultivation (January 2020 to May 2020); with BRRI dhan71 and BRRI dhan 87 during wet season rice cultivation (July 2020 to November 2020). Experimental site is located at 24˚N latitude and 94˚E longitude (AEZ 9). The experimental soils were collected from (0 - 15) cm depth of the top soil of ideal paddy field of experiment filed. The experimental soil was silty loam texture type having a soil pH value of 6.4, SOM 1.78%, available P13.7 ppm, available K 16.3 ppm, low salt content (EC value 2.35 dS/m).

Experimental treatments were T1: Control, T2: (NPKS 100%, No amendments, T3: NPKS (80%) + 20% NPKS from azolla biofertilizer, T4: NPKS (60%) + 40% NPKS from azolla biofertilizer, T5: NPKS (80%) + 20% NPKS from Rice husk biochar, T6: NPKS (60%) + 40% NPKS from Rice husk biochar, T7: NPKS (80%) + 20% NPKS from Vermicompost, T8: NPKS (60%) + 40% NPKS from Vermicompost, T9: NPKS (80%) + 20% NPKS from Sugarcane pressmud (SPM), T10: NPKS (60%) + 40% NPKS from Sugarcane pressmud (SPM), T11: 80% NPKS +20% NPKS from Silicate fertilizer (SF), T12: 60% NPKS +4 0% NPKS from SF. The treatments were arranged with completely randomized design (CRD) and replicated three times for setting the pot experiment. Total number of pots was seventy two (72) where 36 for ambient temperature and 36 for elevated temperature. The treatments were assigned in two different temperature conditions: ambient air temperature condition (outside of greenhouse) and elevated air temperature condition (+1.5˚C higher inside greenhouse). During the experimentation, the average maximum temperature prevailed in the greenhouse condition was 35˚C - 38˚C and outside the greenhouse was below 35˚C.

BRRI dhan 89 and BINA dhan 10 were planted in dry boro season, while BRRI dhan 71 and BRRI dhan 87 were planted during wet Aman season. BRRI dhan 89: BRRI dhan-89 rice cultivar was discovered by Bangladesh rice Research Institute in 2017. The life span of this breed is about 154 - 158 days. BRRI dhan 71: BRRI dhan-71 rice cultivar was invented by Bangladesh rice Research Institute in 2012. The lifetime of this rice cultivar is about 114 to117 days. BINA dhan 10: Bina dhan-10 rice cultivar was discovered by Bangladesh Institute of nuclear agriculture (BINA) in 2012. It was released for boro season. It’s lifespan is about 127 - 132 days. BRRI dhan 87: BRRI dhan-87 rice cultivar was invented by Bangladesh rice Research Institute in 2017. This rice cultivar lifetime is about 122 days.

Soil amendments were mixed up with collected soils five days before rice seedlings transplantation in pot. The soil amendments were applied @ 2.0 t/ha, 8.0 t/ha, 5.0 t/ha, 4.0 t/ha and 0.5 t/ha for azolla biofertilizer, rice husk biochar, vermicompost, sugarcane press mud and silicate fertilizer, respectively. The experimental pots were filled with air dried soil (about 8.0 kg). Chemical fertilizers (N 90 kg/ha, P 15 kg/ha, K 45 kg/ha, S 8.0 kg/ha through urea, triple super phosphate, muriate of potash, gypsum) were added to soils based on integrated plant nutrients system for achieving high potential yield. The composition of Vermicompost was organic C 18.5%, total N 1.5%, cellulose 30%, available P 0.98%, available S 0.6%, potassium 1.2%, Fe 200 ppm and Mn 250 ppm. Enriched SPM compost contains organic C 35.0%, total N 1.6%, cellulose 24%, available P 1.55%, available S 0.95%, potassium 1.41%, Fe 1000 ppm and Mn 310 ppm. Rice husk biochar contains organic C 42.5%, total N 0.9%, cellulose 50%, Fe 250 ppm, S 200 ppm, Mn 500 ppm etc. Silicate fertilizer contains mainly Silicon (35%) with high amount of iron (1500 ppm).

Gas samples were collected by the modified closed-chamber method ( Rolston, 1986 ; Ali et al., 2008 ) during the rice cultivation. The dimension of close chambers was (100 cm). Gas samples were collected once a week started from 21 DAT until rice harvesting to get the average CH₄ emissions during the cropping season. At first, rice planted pot was kept on a water filled tray and then glass chamber was placed on it. Gas samples were collected by a 50 ml air-tight syringe at 0 min, 15 min and 30 min intervals after chamber placement over the rice planted pot. The samples were analyzed to determine the concentration of CH₄ gas by Gas Chromatograph (Shimadzu/GC 2014, Japan) equipped with a Flame Ionization Detector (FID). The analysis column used a stainless-steel column packed with Porapak NQ (Q 80 - 100 mess). The temperatures of column, injector and detector were adjusted at 100˚C, 200˚C, and 200˚C.

CH₄ emission rate was calculated by following equation ( Rolston, 1986 ):

F = ρ·V/A·∆c/∆t·273/T

Here, F = methane emission rate (mg CH₄ m⁻²·hr⁻¹), ρ= gas density (0.714 mg·cm⁻³), V = volume of chamber (m³), A = surface area of chamber (m²), ∆c/∆t = rate of increase of CH₄ concentration in the chamber (mg·m⁻³·hr⁻¹), T (absolute temperature) = 273 + mean temperature in chamber (˚C).

E = Slope (ppm/min) × VC × MW × 60 × 24 × 22.4 (273 + T/273) × Ac × 1000

The emissions as Kg CH₄ (or Kg N₂O)/ha were derived from the slope of the linear regression curve of gas (CH₄ and N₂O) concentrations against the chamber closing time. The slope was referred to as mass per unit area per unit time (mg/m²/h), where VC is the volume of the gas chamber in liters (L), MW is the molecular weight of the respective gas, 60 is minutes per hour and twenty four is hours of the day. The volume of 1 mol of gas in L at standard temperature and pressure is 22.4. T is the temperature inside the chamber (˚C) while 273 is the standard temperature of ˚K. AC is the chamber area (m²) and 1000 is μg/mg.

The seasonal CH₄ flux for the entire cropping period was computed as reported by Singh et al. (1999) :

Seasonal CH₄ flux = ∑n_i = (R_i × D_i)

To estimate the GWP, CO₂ is typically taken as the reference gas, and an increase or reduction in emissions of CH₄ or N₂O is converted into “CO₂-equivalents” by means of their GWPs. In this study, we used the IPCC factors to calculate the combined GWP for 100 years, GWP = 28 × CH₄, kg CO₂-equivalents ha⁻¹) + 265 × N₂O, kg CO₂-equivalents ha⁻¹ ( IPCC, 2014 ). In addition, the greenhouse gas intensity (GHGI) was calculated by dividing GWP by the grain yield for rice ( Mosier et al., 2006 ).

Soil redox potential (Eh), leachate water pH, EC, TDS, and total dissolved iron (TDFe) conc. were measured at every week interval during rice cultivation. Soil organic carbon (SOC) ( Allison, 1965 ), total-N% ( Keeney & Nelson, 1982 ), available P (Colorimetric method, Olsen Sommers), available S (by the calcium chloride 0.15% extraction method), available Si (1M Na –acetate, pH 4.0, UV Spectrometer) were determined following standard methods. Exchangeable calcium (Ca), sodium (Na) and potassium (K) were extracted from soil using 1 M CH₃COONH₄ solution and their concentrations in the extract were directly determined by Flame Photometer (Model: FP 902 PG Instrument). Free iron oxides in soil were extracted by Diethylene Tri amine Penta Acetate (DTPA) solution and its concentration in the extract was determined directly by an Atomic Absorption Spectrophotometer ( Loeppert & Inskeep, 1996 ). The concentrations of total dissolved iron and ferrous iron in the leached water samples were determined by 1,10-phenanthroline method. Ammonium (NH₄) concentration in water samples were determined by Indophenol blue method at 640 nm wave length using a UV spectrophotometer (UV–VI Mini 1240, Shimadzu Corporation, Kyoto, Japan). ${\text{NO}}_3^{-}$ concentration in water samples were determined at 410 nm using a UV spectrophotometer (Brucine-sulfanilic acid method). After rice harvest, soil bulk density was determined using cores (volume 100 cm³, inner diameter 5 cm), filled with fresh moisture soils. The collected soil core samples were oven dried at 105˚C for 24 h and then measured the weight of dried core samples. Soil porosity was calculated using the bulk density (BD) and particle density (PD, 2.65 Mg m³) according to the equation: Porosity (%) = (1 – BD/PD) * 100.

Data on plant height, productive tiller number per hill, leaf area, SPAD value, and CO₂ assimilation rates were measured at flowering to heading stage. The CO₂ assimilation rate was measured with Portable Photosynthesis system (LI-COR, LI 6400) and SPAD readings were measured by Chlorophyll meter (SPAD-502, Minolta Camera Co. Ltd., Osaka, Japan). A fully matured leaf from the top of the plant was selected for recording the SPAD values and the mean of five readings per plant was taken. During the maturity to harvesting stage, ripened grains (%) per hill, grain yield per hill, and other yield attributes were also recorded. Harvest index (HI) was also calculated.

At first experimental data were entered into Microsoft Excel. Then analysis of variance (ANOVA) was performed through using R software (R-4.3.3, 2024 version). Duncan’s multiple range test (DMRT) was conducted to identify statistically significant differences between group means at a 5% significance level.

CH₄ emission rate within the first two weeks after rice was transplanted in potted soil was low under ambient temperature condition Figure 1(a) , which increased significantly at 35 DAT onwards (active tillering stage) and peaked at 77 DAT-80 DAT (flowering to heading stage). Among the treatments, higher CH₄ emission rates were recorded in rice husk biochar amended pots (T5, T6) and sugarcane pressmud (SPM) amended (T9 and T10) rice planted pots, while lower CH₄ emissions were observed in T1 (control), NPKS (100%) fertilization (T2), Azolla biofertilizer (T3, T4), vermicompost (T7, T8) and silicate fertilization (T11, T12) treated rice planted pots. The first CH₄ peak (20 - 30 mg/m²/hr) was observed at 35 days after rice transplanting (active tillering stage) followed by the highest peak (35 - 40 mg·m⁻²·hr⁻¹) at 77 - 80 DAT (flowering stage). The maximum decrease in CH₄emission rate (15% - 30%) was obtained by silicate fertilization and azolla biofertilizer amendments and improved soil redox status (Eh) also observed in the amended potted soils ( Figure 1(a) ). After that, CH₄–C emission rate sharply dropped probably due to improved soil redox status at rice grain maturation, thereby least CH₄ emission was recorded around 120 DAT before rice harvest.

On the other hand, higher CH₄ emission rate along with more intense redox potential status were observed under elevated temperature condition compared to ambient temperature grown rice planted pots, which may be due to rapid microbial activity and faster decomposition of soil organic matter. The first CH₄peak (35 mg/m²/hr) was recorded at 35 DAT in 100% NPKS fertilized rice planted pots, while the 2^nd CH₄ peak (48 mg/m²/hr) was observed at 77 - 80 DAT ( Figure 1(b) ). After three weeks of rice transplanting, intensive soil redox status developed (Eh value around −100 mV), which reached towards most intensive reduced condition (Eh value −200 to −250 mV) by 70 - 84 DAT ( Figure 1(b) ). At flowering to heading stage (70 DAT-84 DAT) the rice rhizosphere showed the most reduced conditions in almost all treatments, although silicate fertilization and azolla biofertilization amendments significantly (p < 0.05) improved (increased the soil Eh value) soil redox status, probably due to their high content of electron acceptors such as free iron oxides, Mn and S.

At ambient air temperature condition, the maximum seasonal cumulative CH₄ flux (15.67 - 15.90 g/m²) with mean GWP 391.0 kg CO₂ eq·ha⁻¹ was found in rice husk biochar followed by sugarcane pressmud (15.30 - 15.70 g/m²; mean GWP 387.0 kg CO₂ eq·ha⁻¹), vermicompost amendment (13.7 - 14.30 g/m²; mean GWP 350 kg CO₂ eq·ha⁻¹), NPKS fertilized (13.50 - 13.70 g/m², mean GWP 340 kg CO₂ eq·ha⁻¹), Azolla biofertilization (11.60 - 12.80 g/m², mean GWP 305 kg CO₂ eq·ha⁻¹), silicate fertilized (10.50 - 12.30 g/m², mean GWP 285 kg CO₂ eq·ha⁻¹) and control treatment (8.6 - 10.63 g/m², mean GWP 240 kg CO₂ eq·ha⁻¹) during wet aman season BRRI dhan 87 cultivation. On the other hand, elevated air temperature accelerated the total cumulative methane flux in all amended rice plants. The maximum seasonal cumulative CH₄ flux 15.20 - 16.50 g/m² was found in rice husk biochar amendment (mean GWP 396 kg CO₂ eq·ha⁻¹) followed by sugarcane pressmud 15.50 - 16.30 g/m² (mean GWP 398 kg CO₂ eq·ha⁻¹), vermicompost amendment 14.90 - 15.60 g/m² (mean GWP 381 kg CO₂ eq·ha⁻¹), NPKS fertilized 15.10, g/m², (mean GWP 378 kg CO₂ eq·ha⁻¹), Azolla biofertilization 13.30 - 14.50 g/m² (GWP 348 kg CO₂ eq·ha⁻¹), silicate fertilized 11.30 - 12.50 g/m² (GWP 298 kg CO₂ eq·ha⁻¹) and control treatment (12.90 g/m², GWP 323 kg CO₂ eq·ha⁻¹).

In case of BRRI dhan 71, the least cumulative CH₄ flux 7.60 g/m² (mean GWP 190 kg CO₂ eq·ha⁻¹) was found in control (T1) treatment with maximum cumulative CH₄ flux 14.10 - 15.50 g/m² (mean GWP 372.0 kg CO₂ eq·ha⁻¹) was found in SPM amended soil under ambient air temperature. On the other hand, elevated air temperature significantly increased cumulative CH₄ flux in all treatments. The maximum increase in cumulative CH₄ fluxes (14.5% - 16.0%) and GWPs (10.7% - 11.6%) were observed in rice husk biochar and SPM amended soils.

During boro season rice cultivation, BRRI dhan 89 and BINA dhan 10 revealed maximum cumulative CH₄ fluxes, yield scale methane emissions and GWPs with rice husk biochar and SPM amendments in soils under both ambient and elevated air temperature conditions. At ambient air temperature, the maximum GWPs for BRRI dhan 89 cultivation were found 345 - 373 kg CO₂ eq·ha⁻¹ in SPM and 365 - 370 kg CO₂ eq·ha⁻¹ in biochar amendments, while least GWP was recorded 272 - 307 kg CO₂ eq·ha⁻¹ in silicate amended rice planted soils. Elevated air temperature, resulted maximum GWPs with SPM (420 - 438 kg CO₂ eq·ha⁻¹), biochar (418 - 434 kg CO₂ eq·ha⁻¹) amendments, while least GWP 302 - 337 kg CO₂ eq·ha⁻¹ was found in silicate amendment soil. In case of BINA dhan 10, the maximum GWP was found 382 - 391 kg CO₂ eq·ha⁻¹ with biochar amendment (ambient temperature), which increased towards 383 - 402 kg CO₂. eq·ha⁻¹ at elevated temperature. SPM amendment in soil caused maximum GWP 373 - 383 kg CO₂ eq·ha⁻¹ (ambient temperature) which increased to 390 - 403 kg CO₂ eq·ha⁻¹ at elevated air temperature. Silicate fertilizer amendment resulted minimum GWP 282 - 312 kg CO₂ eq·ha⁻¹ (at ambient temperature) and 283 - 333 kg CO₂ eq·ha⁻¹ at elevated temperature. Among the amendments, silicate fertilizer, azolla biofertilizer and vermicompost were found effective to decrease cumulative CH₄ fluxes, yield scaled CH₄ emission and GWPs. Rice cultivars also showed variation in regards to cumulative CH₄ fluxes, yield scaled CH₄ flux and GWPs. BRRI dhan 87 revealed comparatively higher seasonal cumulative CH₄ fluxes, yield scaled CH₄ flux and GWPs than BRRI dhan 71 during wet aman rice growing season; while higher seasonal cumulative CH₄ fluxes, yield scaled CH₄ flux and GWPs were found in BRRI dhan 89 compared to BINA dhan 10 cultivation during irrigated boro rice cultivation.

Soil amendments with vermicompost, sugarcane pressmud, azolla biofertilizer, silicate fertilization and rice husk biochar significantly (p < 0.05) increased rice yield compared to inorganic chemicals (T2:100%NPKS) treated rice planted pot. During wet aman season rice grain yield was increased by 3.0% - 16% with soil amendments compared to chemical fertilization (100% NPKS) treated rice plants (600 g/m²) under ambient temperature condition. The maximum grain yield was recorded with vermicompost (660 - 690.0 g/m², T7, T8) followed by enriched SPM (650 - 680.0, T9, T10), azolla biofertilation (640 - 670.0, T3, T4), silicate fertilization (620 - 650.0, T11, T12), rice husk biochar (610.0 - 630.0, T5, T6), inorganic NPKS (510 - 600 g/m², T2) and T1 (160 - 180 g/m² Control) treatments. However, rice grain yield was decreased sharply by 35.0% - 45.0% under elevated temperature irrespective of soil amendments and rice cultivars ( Table 1 ).

Note: T1: Control, T2: NPKS 100%, No amendments, T3: NPKS (80%) + 20% NPKS from azolla biofertilizer, T4: NPKS (60%) + 40% NPKS from azolla biofertilizer, T5: (80%) NPKS + 20% NPKS from Rice husk biochar, T6: (60%) NPKS + 40% NPKS from Rice husk biochar, T7: (80%) NPKS + 20% NPKS from Vermicompost, T8: 60%) NPKS + 40% NPKS from Vermicompost, T9: (80%) NPKS + 20% NPKS from Sugarcane Pressmud, T10: (60%) NPKS + 40% NPKS from SPM, T11:80% NPKS + 20% NPKS from SF, T12: 60% NPKS + 40% NPKS from SF.

During dry boro season rice cultivation, the maximum grain yield 800 - 830 g/m², 770 - 800 g/m², 760 - 790 g/m², 760 - 780 g/m² were recorded with vermicompost (T7, T8), enriched SPM (T9, T10), Azolla biofertilization (T3, T4) and rice husk biochar (T5, T6) amendments at ambient temperature, which decreased drastically at elevated temperature condition in the greenhouse ( Table 2 ).

In general, grain yield was decreased by 44.0% - 55.0% ( Table 2 ) under elevated temperature condition. Akter et al. (2017) reported that rice grain yield was decreased by 30% - 52% under 2˚C - 3˚C higher than ambient temperature during dry boro season rice cultivation. Net photosynthetic CO₂ assimilation rate was 15.6 - 16.3 (µmol·m⁻²·S⁻¹) at ambient temperature (29˚C - 31˚C), which increased significantly (p < 0.05) with azolla biofertilization, vermicompost, silicate and enriched SPM amendments, however, decreased sharply under elevated temperature (3˚C higher than ambient temperature). Combined application of organic and inorganic fertilizers (IPNS) significantly influenced N, P, K uptake by boro rice and aman rice cultivars under ambient and elevated temperature condition. Under ambient temperature condition, the increase in N uptake by rice plant over NPKS (100%) fertilized rice plant ranged 5% - 15%, where maximum (15%) increase was found by Vermicompost and SPM (14%) amendments. In case of P and K, uptake were increased by 6% - 16% and 8% - 15% compared to NPKS (100%) fertilized rice plant. However, under elevated temperature condition N, P.K uptake by rice plant drastically decreased, which affected rice grain yield to decrease significantly (p < 0.01).

Note: T1: Control, T2: NPKS 100%, No amendments, T3: NPKS (80%) + 20% NPKS from azolla biofertilizer, T4: NPKS (60%) + 40% NPKS from azolla biofertilizer, T5: (80%) NPKS + 20% NPKS from Rice husk biochar, T6: (60%) NPKS + 40% NPKS from Rice husk biochar, T7: (80%) NPKS + 20% NPKS from Vermicompost, T8: 60%) NPKS + 40% NPKS from Vermicompost, T9: (80%) NPKS + 20% NPKS from Sugarcane Pressmud, T10: (60%) NPKS + 40% NPKS from SPM, T11:80% NPKS + 20% NPKS from SF, T12: 60% NPKS + 40% NPKS from SF.

A significant (p < 0.05) improvement in soil physico-chemical properties with combined application of Azolla biofertilizer, rice husk biochar, vermicompost, sugarcane press mud compost and silicate fertilization alongwith NPKS fertilizer were found just before rice harvesting stage ( Table 3 ).

Note: T1: Control, T2: NPKS 100%, No amendments, T3: NPKS (80%) + 20% NPKS from azolla biofertilizer, T4: NPKS (60%) + 40% NPKS from azolla biofertilizer, T5: (80%) NPKS + 20% NPKS from Rice husk biochar, T6: (60%) NPKS + 40% NPKS from Rice husk biochar, T7: (80%) NPKS + 20% NPKS from Vermicompost, T8: 60%) NPKS + 40% NPKS from Vermicompost, T9: (80%) NPKS + 20% NPKS from Sugarcane Pressmud, T10: (60%) NPKS + 40% NPKS from SPM, T11: 80% NPKS + 20% NPKS from SF, T12: 60% NPKS + 40% NPKS from SF.

Azolla biofertilizer, rice husk biochar, vermicompost and sugarcane press mud increased soil porosity, SOC, T-N, soil pH, available phosphate, available silica (SiO₂), sulphate, and free iron oxides in the post-harvest soils. Soil redox status also increased with azolla biofertilization, sugarcane press mud and silicate amendments, probably due to the cumulative effects of free iron oxides and sulphate which acted as electron acceptors, thereby, decreased methane production and eventually reduced CH₄ emission ( Jackel & Schnell, 2000; Ali et al., 2008 ). Increased SOC with azolla biofertilizer, biochar and silicate amendments could be due to the slow decomposition of added materials and enhanced rice growth such as shoot and root biomass production, which contributed to the accumulation of SOC at rice harvesting stage. The maximum concentration of available silica, free iron oxide, available Fe, available Mn, S, P and K were found in the amended soils, which controlled CH₄ emissions. Jagadeesh Babu et al. (2006) reported that K induced higher oxidizing conditions in the rhizosphere of the rice plants, thereby inhibited CH₄ formation and its subsequent release to the atmosphere.

There were positive correlations of seasonal cumulative methane flux with the rice plant productive tillers/hill, aboveground biomass, grain yield and LOC ( Table 4 ), being supported by Gogoi et al., (2005) , while negative correlations were found with soil porosity, exchangeable K, soil Eh, available SiO₂, available P₂O₅, sulphate and free iron oxides content in soil, being supported by our previous research studies Ali et al. (2008, 2015) .