Effect Of Biochar Application On Carbon Sequestration And Emission Reduction And Yield Enhancement Effects In Agricultural Soils

A large number of studies have shown that biochar has shown great potential in soil carbon sequestration,greenhouse gas emission reduction,soil improvement,and crop yield enhancement due to its special physicochemical properties.The quantitative evaluation of biochar application and its characteristics,climate factors,soil properties,and field management measures in terms of soil carbon sequestration,greenhouse gas emissions,and crop yield response to biochar is not only beneficial to the application of biochar in low-carbon sustainable agriculture,but also has scientific guidance for the mitigation of global climate change and food security problems.Based on the data from a large number of previous experimental studies,this study combined numerical fitting,random forest importance value and CCA-VPA multivariate statistical analysis with Meta-integration analysis to quantitatively analyze and comprehensively evaluate the greenhouse gas emission reduction effect,soil carbon sequestration effect and crop yield increase effect of biochar application mode on farmland,which led to the following conclusions:（1）Biochar has excellent on-farm GHG emission reduction benefits,especially in terms of significant inhibition of CH₄ and N₂O emissions from rice fields and significant increase in GWP reduction potential;at the same time,biochar application can also increase crop yields to a large extent,thus achieving significant GHGI reduction.Overall,the interaction of biochar application and its own characteristics with natural environmental factors and inorganic fertilizer application contributed to most of the explanation of the differences in GHG emissions,soil carbon sequestration effects and crop yield changes.The structural equation modeling revealed that under the conditions of biochar application,climatic factors,field management measures,biochar application and its own characteristics,and soil properties could all have significant direct effects on GHG emissions,soil carbon sequestration and crop yield on farmland,and the first three could also have significant indirect effects on the soil carbon sequestration emission reduction and yield increase effects of biochar through their effects on soil properties.However,under different crop types and cropping systems,except for the mechanism of biochar on crop yield,which was basically similar,there were some differences in the dominant drivers affecting the emission reduction effect of biochar on CH₄,N₂O,GWP,GHGI,and the relative change rate of soil SOC,carbon sequestration rate,and emission reduction of NGWP.（2）If we consider the carbon sequestration and emission reduction effect of biochar in harmony with the yield increase effect,the high dose（>80t/ha）of biochar application is beneficial to crop yield enhancement and GWP and GHGI mitigation,and does not limit the reduction effect of biochar on farmland CH₄.In addition,when biochar is applied for 1-1.5 years,its reduction of N₂O emissions and GWP on farmland can be maximized,and it can effectively increase the GHGI reduction potential of biochar.The biochar with a C/N of 50-60 and a pH of 8.1-9.4 was selected to achieve the optimal reduction of CH₄ in rice fields and to have a significant reduction potential of GWP and GHGI,and it did not affect the inhibitory effect of biochar on N₂O emissions in agricultural fields.From the viewpoint of preparation raw materials,biochar prepared from rice straw has more obvious advantages in suppressing CH₄emissions and mitigating GHGI in paddy fields,woody biochar is relatively more beneficial to improving crop yields,while biochar prepared from mixed waste has extremely outstanding N₂O and GWP emission reduction potential.Biochar charred at medium and high temperatures（500-600°C）can maximize the reduction benefits of CH₄ and GWP in paddy fields,while biochar pyrolyzed at high temperatures（>600°C）can maximize the suppression of N₂O emissions and increase crop yields in agricultural fields,while biochar charred at 450°C is relatively more favorable for dryland CH4reduction,and biochar pyrolyzed at low temperatures（≤400°C）also has the best GHGI reduction potential and relatively more significant GWP mitigation effect.Therefore,in the selection of biochar carbonization temperature and preparation raw materials,the CH₄,N₂O,GWP,GHGI emission reduction effects and crop yield increase effects of biochar should be traded off accordingly according to the actual farmland economic benefits and low-carbon agricultural production needs.（3）Under the strongly acidic（pH<5.5）soil environment,biochar has the most significant effect on CH₄ emission reduction in paddy fields and crop rotation,and is more beneficial to the suppression of N₂O emission and mitigation of GWP,and also has the best potential for crop yield increase and GHGI emission reduction;while for neutral and weakly alkaline（6.5≤pH<7.5）dryland soils,the CH₄ emission reduction effect of biochar is for dryland soils with neutral and weak alkalinity（6.5≤pH<5）.Under low organic carbon content（<7g/kg）,biochar has a significant effect on CH₄reduction in dryland and rotational cropland soils,and also has a relatively good potential for GWP and GHGI reduction.In addition,biochar can better reduce N2O under carbon-rich soil conditions（SOC≥20 g/kg）.Biochar had higher CH4 and GWP reduction potential for paddy and rotational cropland soils with high total N content（≥1.3 g/kg）,but biochar stimulated positive effects on CH₄ emissions and N₂O emissions from dryland soils under high N soil environment.Under the soil environment with high total phosphorus content（≥1.5 g/kg）,biochar has a highly significant N₂O emission reduction potential on farmland,while under low phosphorus soil conditions（TP<0.5 g/kg）,biochar can suppress CH₄ emissions from rice fields to a greater extent,while maximizing GWP and GHGI emission reduction potential and crop yield increase effects.When the total potassium content of soil was relatively low（<10 g/kg）,the N₂O reduction effect,GHGI mitigation effect and crop yield increase effect of biochar were relatively more significant;while under the soil environment with higher total potassium content（≥20 g/kg）,the suppression effect of biochar on CH₄ emission from paddy field was more significant,and the reduction potential of GHGI was greatly increased.Biochar had a relatively better CH₄reduction effect on paddy soils with≥15%clay content,while it had the most significant CH₄ reduction effect on sandy loam soils with low clay content from the whole crop rotation cycle;biochar had an extremely outstanding on-farm N₂O reduction effect on loamy soils,while the GWP reduction potential on sandy clay loam soils was slightly higher than that of loamy soils.The yield enhancement effect of biochar on crops under powdered clay loam soil conditions was particularly outstanding,while biochar application in loamy,sandy clay loam and loamy clay loam soils was able to increase the mitigation magnitude of GHGI to a significant level.（4）The mixture of biochar and inorganic fertilizers contributed to the suppression of CH₄ and N₂O emissions,the mitigation of GWP and GHGI,and the improvement of crop yield,so the ratio of N,P,and K elements in inorganic fertilizers largely influenced the reduction of CH₄ and N₂O emissions,the mitigation of GWP and GHGI,and the effect of crop yield increase by biochar.In order to maximize the CH₄ emission reduction benefits of biochar in paddy fields and drylands,the ratios of nitrogen and phosphorus fertilizers should be controlled within the interval of 6-8 and 0-3,respectively.In general,biochar has a relatively strong inhibitory effect on soil CH4emission when the ratio of N and K in inorganic fertilizer is in the range of 1.4-3.In order to achieve the optimal effect of biochar in mitigating N₂O emissions and GWP and GHGI,the ratios of N,K and P in inorganic fertilizers should be regulated in the intervals of 0-2,≥5 and≥2,respectively.（5）From a regional scale,biochar has the most prominent effect on CH4 emission reduction in rice fields in central China,and can correspondingly suppress dryland CH₄emissions and farmland N₂O emissions in northern regions（northeast and northwest）and maximize GHGI emission reduction potential,and minimize GWP in central China and northeast China,while significantly increasing crop yields in northern China.This shows that low temperature,low precipitation and high sunshine hours are favorable for biochar to reduce N₂O emissions,GWP and GHGI,while high temperature,high precipitation and low light conditions are relatively more favorable for biochar to increase crop yield and create favorable conditions for biochar to reduce CH₄emissions from rice fields and crop rotation.（6）In terms of soil carbon sequestration potential,the sustainable life of biochar that can effectively enhance soil organic carbon pool is about 24.95 years,and once it exceeds 25 years,the biochar remaining in the soil may accelerate soil SOC mineralization to some extent,but its decomposition rate（<0.61 t/ha/year）is extremely low,much smaller than the rate of atmospheric CO₂ fixation by plants and soil microorganisms through photosynthesis and chemoenergetic synthesis.The rate of CO₂fixation by plants and soil microorganisms through photosynthesis and chemoenergetic synthesis is very low,and therefore does not cause significant loss of SOC from the soil carbon pool in the short term.Among all the key influencing factors,biochar application rate was the most critical factor affecting the relative rate of change in soil SOC content,carbon sequestration efficiency,and NGWP,while biochar application time and application rate were the most important drivers of soil carbon sequestration rate;besides,the influence of soil factors on the soil carbon sequestration effect of biochar was also not to be underestimated.Among them,the responses of soil carbon sequestration rate to biochar application time,soil carbon sequestration efficiency to biochar application amount and relative change rate of soil SOC to initial SOC,TN and TP contents showed significant decreasing payoffs,which were mainly caused by the imbalance between input and output of exogenous organic carbon and the oversaturation of endogenous organic carbon in soil.