Greenhouse gas emissions vary in response to different biochar amendments: an assessment based on two consecutive rice growth cycles

Effect of different types of biochar on soil properties and functional microbial communities in rhizosphere and bulk soils and their relationship with CH4 and N2O emissions

Biochar as an agricultural soil amendment plays vital roles in mediating methane (CH4) and nitrous oxide (N2O) emissions in soils. The link between different types of biochar, bulk soil, and rhizosphere microbial communities in relation to CH4 and N2O emissions is being investigated in this study. The rice pot experiment was conducted using biochar at two temperatures (300°C and 500°C) in combination with three biochar levels (0, 2, 10% w/w). Soil properties and the abundance of genes associated with CH4 and N2O emissions from both rhizosphere and bulk soils were investigated. The study also aimed to examine the structure of microbial communities (pmoA, nosZ) in rhizosphere and bulk soils whereas CH4 and N2O emissions were monitored while growing rice. Results showed that biochar at 300°C and 10% incorporation significantly increased the CH4 emissions by up to 59% rise compared to the control group. Random Forest analysis revealed that the ratio of mcrA/pmoA along with the abundance of mcrA from both rhizosphere and bulk soils, the abundance of AOA, TN, DOC, and the community composition of pmoA-harboring microorganisms from both bulk and rhizosphere soils were important predictors of CH4 emissions. Therefore, the ratio of mcrA/pmoA in rhizosphere soil and the abundance of AOA in bulk soil were the main factors influencing CH4 emissions. Variation Partitioning Analysis (VPA) results indicated that the effects of these factors on bulk soil were 9% of CH4 emissions variations in different treatments, which contributed more than rhizosphere soils' factors. Moreover, random forest analysis results indicated that the abundance of AOB in bulk soil was the most important predictor influencing N2O emissions. The VPA result revealed that the factors in rhizosphere soil could explain more than 28% of the variations in N2O emissions. Our study highlights that rhizosphere soil has a more significant effect than bulk soil on N2O production. Our findings further the understanding of the link between bulk and rhizosphere attributes, and their impact on CH4 and N2O emissions in paddy soils. In summary, we recommend the application of biochar at 500°C and 2% incorporation rate for agricultural production in the area.

Being applied at rice or wheat season impacts biochar's effect on gaseous nitrogen pollutants from the wheat growth cycle

Biochar (BC) application to agricultural soil can impact two nitrogen (N) gases pollutants, i.e., the ammonia (NH₃) and nitrous oxide (N₂O) losses to atmospheric environment. Under rice-wheat rotation, applied at which growth cycle may influence the aforementioned effects of BC. We conducted a soil column (35 cm in inner diameter and 70 cm in height) experiment to evaluate the responses of wheat N use efficiency (NUE), NH₃ volatilization, and N₂O emission from wheat season to biochar applied at rice (R) or wheat (W) growth cycle, meanwhile regarding the effect of inorganic fertilizer N input rate, i.e., 72, 90, and 108 kg ha^-1 (named N72, N90, and N108, respectively). The results showed that BC application influenced the wheat growth and grain yield. In particular, BC applied at rice season increased the wheat grain yield when receiving 90 and 108 kg N ha^-1. The improved wheat grain yield was attributed to that N90 + BC(R) and N108 + BC(R) enhanced the wheat NUE by 53.8% and 52.8% over N90 and N108, respectively. More N input led to higher NH₃ volatilization and its emission factor. Interestingly, 19.7%-34.0% lower NH₃ vitalizations were recorded under treatments with BC applied in rice season, compared with the treatments only with fertilizer N. BC applied at rice season exerted higher efficiency on mitigating N₂O emission than that applied at wheat season under three N input rates, i.e., 60.5%-77.6% vs 29.8%-34.8%. Overall, considering the crop yield and global warming potential resulting from NH₃ volatilization and N₂O emission of wheat season, N90 + BC(R) is recommended. In conclusion, farmers should consider the application time and reduce inorganic fertilizer N rate when using BC.

Biostimulants decreased nitrogen leaching and NH3 volatilization but increased N2O emission from plastic-shed greenhouse vegetable soil

Biostimulant application is an effective strategy to enhance soil fertility and plant growth. However, its comprehensive impacts on nitrogen (N) uptake and reactive N (Nr) losses via leaching, ammonia (NH₃) volatilization, and nitrous oxide (N₂O) emission from plastic-shed greenhouse vegetable system are still little known. Therefore, a field experiment was conducted with cauliflower-tomato growth rotation (from September 6, 2018, to July 17, 2019) receiving three biostimulants, i.e., humic acid (HA), algae extract (AE), and chitosan (CT), as well as a control without stimulant. The cumulative Nr losses over the cauliflower-tomato growth cycle via leaching, NH₃ volatilization, and N₂O emission were 104-175 kg N ha^-1, 2.32-3.85 kg N ha^-1, and 0.70-0.85 kg N ha^-1, respectively. Biostimulant application significantly (P < 0.05) retarded the total N leaching by 17-44% in tomato season, while suppressed the NH₃ volatilization by 18-38% in cauliflower season. Overall, AE showed the best inhibition efficiency on Nr losses by significantly (P < 0.05) decreasing total N leaching and NH₃ volatilization by 36-44% and 38-52% in both vegetable seasons, compare to the control. However, all three biostimulants stimulated the N₂O emission under both vegetable cycles. Interestingly, all biostimulant-added treatments promote the cauliflower and tomato yield, particularly following the HA and AE amendments, which bring local farmers approximately 4,384-10,035 yuan RMB ha^-1 more income. Enhanced yield under biostimulant treatments was due to higher N uptake capacity and enhanced root morphology. In summary, biostimulants have a contrasting influence on three major Nr lost pathways in greenhouse vegetable production. We recommend that AE is the most optimal biostimulant as it increases vegetable yield and decreases total N leaching and NH₃ volatilization while not dramatically increase the N₂O emission.

Effect of biofertilizer and wheat straw biochar application on nitrous oxide emission and ammonia volatilization from paddy soil

Biofertilizer can improve soil quality, especially the microbiome composition, which potentially affect soil nitrogen (N) cycling. However, little is known about the responses of nitrous oxide (N₂O) emission and ammonia (NH₃) volatilization from biochar-amended paddy soil to the biofertilizer application. Therefore, we conducted a soil column experiment using four 240 kg N ha^-1 (equivalent to 1.7 g N pot^-1) treatments consisting of biofertilizer (3 t ha^-1, equivalent to 21.2 g pot^-1), biochar (7.5 t ha^-1, equivalent to 63.6 g pot^-1), and a mixture of biofertilizer and biochar at the same rate and a control (CK). The results showed that the N₂O emissions and NH₃ volatilizations were equivalent to 0.15-0.28% and 18.0-31.5% of rice seasonal N applied to the four treatments, respectively. Two treatments with biofertilizer and biochar individual amendment significantly increased (P < 0.05) the N₂O emissions to same degree by 30.2%, while co-application of biochar and biofertilizer further increased the N₂O emission by 74.4% compared to the control. The higher N₂O emission was likely attributed to the increased gene copies of AOA, nirK, and nirS. Applying biofertilizer significantly increased (P < 0.05) NH₃ volatilization by 24.7% relative to the control, while applying biochar had no influence on NH₃ volatilization. Co-application of biofertilizer and biochar significantly decreased (P < 0.05) NH₃ volatilization by 12.3% compared to the control. Overall, the net global warming potential based on NH₃ and N₂O in current study increased by 13.0-26.0% in both the individual- and co-application of biofertilizer and biochar. Interestingly, both individual- and co-applications of biofertilizer and biochar increased the rice grain yield by 16.5-38.3%. Therefore, applications of biofertilizer and biochar did not increase the GHGI. Particularly, the co-applying of them significantly lowered (P < 0.05) the GHGI by 15.2%. In conclusion, biofertilizer and biochar should be co-applied to achieve the goals of environment protection and food security.

Wood vinegar and biochar co-application mitigates nitrous oxide and methane emissions from rice paddy soil: A two-year experiment

Both biochar (BC) and wood vinegar (WV) influence the nitrous oxide (N₂O) and methane (CH₄) emissions from agricultural systems. However, the impacts of BC and WV co-application on rice production, N₂O and CH₄ emissions are not well documented. We here conducted a two-year soil columns experiment with four treatments: WV (5 t WV ha^-1), BC (7.5 t BC ha^-1), WV + BC (5 t WV ha^-1 +7.5 t BC ha^-1) and a control (no treatment). The results showed that BC and WV + BC produced higher rice grain yield than the control by 14.1-15.9% in 2016 and by 4.1-5.2% in 2017, respectively. While WV increased rice grain yield by 11.2% in 2016, it had no significant influence on yield in 2017. Both WV and BC significantly mitigated N₂O emissions by 22.4-41.8% in 2016 and 22.4-36.9% in 2017, respectively. Interestingly, WV + BC treatment showed the highest N₂O mitigation efficiency, with a 52.9-62.8% mitigations in 2016 and 2017. Furthermore, the co-application of WV and BC significantly mitigated CH₄ emissions by 42.6% in 2016 and 35.3% in 2017, respectively, while applying WV or BC alone had no annually-consistent mitigation effect on CH₄ emissions. Overall, GWPt of rice growth cycle was most significantly suppressed by WV + BC with a 48.7-56.1% reduction, followed by WV and BC with 20.4-28.0% and 19.7-35.7% reductions, respectively. Consequently, the WV + BC treatment had the highest GHGI mitigation effect, averaging with 56.7% over two consecutive rice growth cycles. In conclusion, co-application of WV and BC is recommended for rice cultivation, which can both improve rice yield and minimize GHG emissions.

Rice nitrogen use efficiency does not link to ammonia volatilization in paddy fields

Ammonia (NH₃) volatilization is a major pathway of nitrogen (N) losses from paddy fields, and could be potentially mitigated by cultivation of high nitrogen use efficiency (high-NUE) rice cultivars. However, the relationship between NUE and NH₃ volatilization has not been validated under field conditions. A field experiment was conducted to evaluate the impact of four rice cultivars with different NUE [Wuyunjing 23 (W23), Zhendao 11 (Z11), Wuyujing 3 (W3), and Aoyusi 386 (A386)] on NH₃ volatilization, as well as the related mechanisms. Two high-NUE rice cultivars W23 and Z11 was not more effective in reducing total NH₃ volatilization from the paddy field compared to cultivar A386 with the lowest NUE. Cultivar A386 had 12.7-17.8% and 35.7-54.1% lower NH₃ volatilization than other three rice cultivars at tillering fertilization stage (TFS) and panicle fertilization stage (PFS), respectively, mainly due to its greater shoot N accumulation, root biomass and volume at TFS and its greater shoot biomass, leaf area index and shoot N accumulation at PFS. There was no significant difference in NH₃ volatilization among W23, Z11 and W3 at TFS. However, premature senescence phenomenon at later growth stages of A386 eventually led to its lowest NUE among the four rice cultivars. Our results suggest that NUE of rice does not link to NH₃ volatilization from paddy fields. In order to make high-NUE rice cultivars also effective in mitigating NH₃ volatilization, future breeding works should aim to improve N uptake capability and canopy structure at early tillering and panicle development stages while prevent premature senescence of rice plants to maintain high yields.

Rice straw biochar mitigated more N2O emissions from fertilized paddy soil with higher water content than that derived from ex situ biowaste

Biochar could mitigate greenhouse gas emissions, especially nitrous oxide (N₂O). Effects of interactions between different biochar and water content on N₂O emissions from rice (Oryza sativa L.) paddy soils have not been thoroughly understood. We evaluated effects of different biochar (derived from Camellia oleifera fruit shell, FS; spent mushroom substrate made of Camellia oleifera fruit shell, MS; rice straw, RS; at the rate of 40 g kg^-1) and water contents (70% and 120% water holding capacity, WHC) on N₂O emissions from rice paddy soil fertilized with nitrogen (N, 0.2 g kg^-1), and examined microbial functional genes associated with N₂O emissions to understand the underlining mechanisms. The results showed that RS biochar was higher in pH, available N, dissolved organic N, and decreased more N₂O emissions from soils with N and 120% WHC treatment relative to MS and FS biochar (by 363% and 200%, respectively). Although RS biochar potentially increased the abundance of ammonia-oxidizing archaea amoA gene (AOA), changes in functional gene abundance did not concur with decreases in N₂O emissions. Instead of changes in microbial communities, the relatively higher pH as well as lower available N and dissolved organic C and N of RS biochar could have contributed to the decrease in N₂O emissions compared with MS and FS biochar. Thereby, the in situ application of rice straw via biochar could be considered in the mitigation of N₂O emissions from fertilized rice paddy soil instead of biochar derived from ex situ feedstock.

Effects of biochar content on gas diffusion coefficient of soil with different compactness and air contents

Biochar has been found to be a potentially suitable amendment for landfill cover material and agricultural soil. The addition of biochar can improve the physical (e.g., adsorption capacity) and hydrological properties (e.g., water/gas permeability) of soil. However, no experimental study is available about the effect of biochar content (BC) on the gas diffusion coefficient (D_P) of soil. The present study investigated the effect of BC on D_P under different degree of compaction (DOC; 85%, 90%, and 95%) and soil air contents (SAC; 5%, 10%, and 15%). It was found that DOC and BC had negligible effects on D_P when SAC was low (~ 5%). In contrast, when the SAC was relatively high (~ 15%), soil with DOC of 85% had the largest D_P for BC ranging from 0 to 15% (w/w). Only when the SAC was large (~ 15%), the addition of biochar generally increased D_P.

Continuous Sub-Irrigation with Treated Municipal Wastewater for Protein-Rich Rice Production with Reduced Emissions of CH4 and N2O

Herein, we introduce continuous sub-irrigation with treated municipal wastewater (TWW) as a novel cultivation system to promote resource recycling and cost-effective forage rice production in Japan. However, both TWW irrigation and forage rice cultivation were previously considered to intensify CH₄ and N₂O emissions. In the present study, therefore, we evaluate the emissions of greenhouse gases (GHGs) and yielding capacity of forage rice between conventional cultivation and continuous sub-irrigation systems employing different water supply rates. Results indicated that continuous sub-irrigation with TWW resulted in high rice yields (10.4–11 t ha⁻¹) with superior protein content (11.3–12.8%) compared with conventional cultivation (8.6 t ha⁻¹ and 9.2%, respectively). All TWW irrigation systems considerably reduced CH₄ emissions, while higher continuous supply rates significantly increased N₂O emissions compared with the conventional cultivation. Only the continuous irrigation regime employing suitable supply rates at appropriate timings to meet the N demand of rice plants decreased both CH₄ and N₂O emissions by 84% and 28%, respectively. Overall, continuous sub-irrigation with TWW provides high yields of protein-rich forage rice without the need for synthetic fertilisers and effectively mitigated GHG emissions from paddy fields.

Responses of ammonia volatilization from rice paddy soil to application of wood vinegar alone or combined with biochar

Wood vinegar (WV) was applied alone or combined with biochar (BC) to observe their efficiency on suppressing the ammonia (NH₃) volatilization from rice paddy soil. Five treatments, i.e., control (240 kg N ha^-1 applied in urea), WV-5 and WV-10 (240 kg N ha^-1 plus 5 and 10 t WV ha^-1, respectively), and their counterparts WV-5-BC and WV-10-BC (WV-5 and WV-10 plus 7 t BC ha^-1), were evaluated by a soil columns experiment. The N fertilizer was split applied as basal and two supplementary fertilizations (named BF, SF1 and SF2, respectively). The results showed that WV-5 treatment increased rice grain yield up to 11.2% compared to the control. Compared with the control, four WV-amended treatments, exhibited lower pH values of the floodwater (7.94-8.18 vs 8.47 and 7.85-7.91 vs 7.98) and the topsoil (6.52-6.76 vs 6.82 and 6.82-6.92 vs 6.99) during the BF and SF1 periods. Both WV-5 and WV-10 increased the NH₄⁺-N contents of topsoil by 10.9-17.8% and 16.1-36.2% after BF and SF1, respectively, than control treatment. Additionally, the floodwater of the WV-amended treatments had higher NH₄⁺-N concentration than control during the first three days after N fertilization, which can be attributed to the stimulating effect of WV on soil urease enzyme activity. WV did not effectively reduce NH₃ volatilization as hypothesized. Interestingly, four WV-amended had relatively reduced the yield-scale NH₃ volatilization by 13.6% than the control. It is suggested that WV needs to be applied with BC at a moderate rate to achieve optimum rice yield and mitigate NH₃ volatilization.