| Agricultural activities are one of the dominant sources for nitrous oxide(N2O)and methane(CH4).The exploration of strategies to mitigate greenhouse gas emissions from agricultural soil has important theoretical and realistic significance for abating climate change.The addition of biochar to agricultural soil has been recognized as an appropriate strategy because it can effectively reduce the greenhouse gas(GHG)emissions while increase long-term carbon(C)sequestration and crop productivity.However,its mechanism and effective duration in agricultural field are not well understood.The grain crop play a central role in agricultural production,and rice-wheat rotation system is an important cropping system.Therefore,it is of great significance to evaluate if biochar can achieve carbon sequestration and GHG mitigation in long-term,which can provide theoretical basic for the rational applications of biochar in agriculture ecosystem.Meta-analysis,field and incubation experiments were employed simultaneously to examine the effects of biochar on N2O and CH4 emission in rice-wheat rotation system,which the following four sections:(1)Comprehensive meta-analysis investigation on the biochar application effects to soil was carried out to evaluate the productivity enhancement and greenhouse gas mitigation potential of biochar amendment in grain crop ecosystems,with a special focus on field condition in grain crop ecosystems,on the yield,N2O,CH4 emission and yield-scaled N2O or CH4 emission.(2)A three-year field experiment was conducted in the rice-wheat rotation system including 6 experimental treatments with two nitrogen levels(0,2 50 kg N ha-1)and three biochar amendment rates(0,20 and 40 t ha-1),to evaluate how yield and greenhouse gas(N2O and CH4)emission were affected by long-term nitrogen application amended with biochar.Net global warming potential(NGWP)and greenhouse gas intensity(GHGI)were also calculated.The objective of this study was to identify the long-term effect of biochar on the yield and GHG emissions and determine their key controlling factors.(3)The 15N-18O labeling technique combined with inhibition method were applied to investigate the pathways of N2O production(nitrifier nitrification,nitrifier denitrification,nitrification-coupled denitrification and heterotrophic denitrification)as affected by biochar,and the relative contribution of ammonia-oxidizing bacteria(AOB)and ammonia-oxidizing archaea(AOA)to potential ammonia oxidation(PAO)and N2O production.The duration effective and mechanism of biochar was explicated by N2O related microbial gene abundance.(4)We quantified the methanogen and methanotrophy abundance in paddy soils,and a microcosm experiment was carried out to examine the effects of different mineral N levels and forms on methane oxidation potential(MOP)and whether biochar amendment stimulate methanotrophs and methane oxidation for prolonged period in fertilized paddy soils.The main results are listed as following:1.Meta-analysis indicated that biochar amendment in the field strongly decreased CH4 and N2O emissions by 17.1%and 8.9%,respectively,and increased the yield by 9.4%,while the productivity enhancement and mitigating effects showed greater potentials under high N inputs(>100 kg N ha-1).The yield enhancement and reduction in N2O emissions in biochar-amended soil persisted over the studied years after a single amendment,while biochar did not change soil CH4 fluxes for a shorter period(<0.5 year).Meanwhile,biochar decreased yield-scaled N2O and CH4 emission by 47.3%and 71.8%under high biochar inputs(>20 t ha-1),the long-term effects of biochar amendment on the yield and GHG fluxes were confirmed by our meta-analysis.2.Over the three-year observation period,biochar amendment years earlier resulted in significant decreases in N2O and CH4 emission by 32.2%and 18.3%,respectively,with increases in yield and SOCSR by 9.6%and 220%,on average.In addition,considering the GHG emission,the soil organic carbon sequestration rate and yield,biochar significantly decreased the NGWP and GHGI by 41.2%and 46.7%.As a result,it could be concluded that the application of biochar to cropland is a viable option to abate climate change in the long term by soil carbon sequestration and GHG mitigation.3.By using the 15N-18O labeling technique and inhibition method,nitrifier-induced pathways(nitrifier nitrification,nitrifier denitrification and nitrification-coupled denitrification)were the dominant source of N2O.The contribution of nitrifier nitrification to N2O production exceeded 2/3,the AOB-driven PAO and N2O production was largely enhanced by biochar amendment.Biochar amendment decreased nitrifier nitrification-driven N2O production,and no difference was found in both biochar treatment.Increased nosZ Ⅰ and nosZ Ⅱ abundance indicating that biochar mitigated N2O emission for prolonged period.4.The abundance of methanotrophy was enhanced by biochar amendment,while methanogen was not after six years of field application.Both ammonium and nitrate were essential for CH4 oxidation,and that nitrate was better in promoting MOP than ammonium.Moreover,ammonium-amendment promoted type I pmoA,and nitrate-amendment enhanced type Ⅱ pmoA abundance.Absence of differences in abundance of methanotrophs and MOP between fresh biochr and six-year aged biochar indicated the long-term mitigation potential on soil CH4 emissions:Not only the fresh biochar resulted in an increase of PMO transiently after amendment,but also produced positive influence after six-year amendment.In conclusion,meta-analysis indicated that biochar amendment could decreased soil greenhouse gas emission under the premise of ensuring crop yields.Based on the three-year in-situ observation,this study revealed the mitigating effect and mechanisms of biochar aged in soil on N2O and CH4 emission,abundance of microbial functional genes controlling N2O emission and soil N2O production pathways were affected by biochar,and biochar increased soil methane oxidation potential after six-year amendment.Biochar amendment years earlier resulted in significant decreases in N2O and CH4 emission,which had long-term effect on GHG mitigation and crop production in the rice-wheat rotation system. |
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