| Agricultural production is an important source of greenhouse gas emissions.Agricultural nitrogen（N）fertilizer application is the main driver of reactive N losses.Crop production processes aimed at increasing soil carbon（C）sinks and reducing or offsetting greenhouse gas（GHG）emissions to mitigate climate warming need to be accompanied by a decrease in environmental problems caused by active N losses.However,the direct use of straw incorporation to increase soil C sequestration may increase the risk of greenhouse gas emissions and active N losses,which is particularly significant in arid areas characterized by infertile,sandy soils and poor uptake of atmospheric C dioxide（CO2）,where the stability of straw input C is poor.It is not clear whether soil C sequestration and crop water and N utilization efficiency in arid regions can be improved by optimizing straw-C return methods and N fertilizer application rates to synergistically achieve dual emission reductions of greenhouse gases and reactive N.In order to address the above scientific questions,in this study,two field experiments were conducted for three consecutive years（2019–2021）on maize farmland in the arid area of Northwest China.Experiment set up a 3×4 factorial completely randomized block design experiment with three methods of maize straw incorporation（straw removal（T）,straw incorporation（S）,and straw-derived biochar incorporation（B））and four N application rates(without N fertilizer（N0）,medium N fertilizer rate(N225,225 kg N ha–1),conventional N fertilizer rate(N300,300 kg N ha–1),and high N fertilizer dose(N375,375 kg N ha–1).Based on the life cycle assessment,direct and indirect greenhouse gas and active N emissions from farmland were systematically quantified;the responses of ecosystem C balance,global warming potential and greenhouse gas emission intensity,ecosystem C footprint,N footprint,net income from farmland,and net ecosystem economic benefit were explored in response to the different straw-C additions.The direction and amplitude of changes of spring maize production,environment and economic benefit with N application under different straw-C addition were studied.Z-score method was used to explore the optimal management measures and mechanisms of straw-C and N fertilizer for improving water and N utilization of spring maize,soil C sequestration,and synergistic reduction of greenhouse gas and active N emissions.At the same time,two prediction models were established and validated for the precise management of N fertilization in the field under different straw-C applications by using the soil testing with nitrate threshold in the root layer and the critical N dilution curve for maize.The main results are as follows:（1）due to the increase in C substrate（soil organic C and dissolved organic C）,aboveground biomass,root biomass,and leaf area index,conventional N application increased C emissions through soil respiration（Rs）,heterotrophic respiration（Rh）and autotrophic respiration（Ra）by 23.95%,9.88%,and 62.88%,respectively,and decreased Rh/Rs by 10.41%,as compared with no N application.Under conventional N application,compared with straw removal,straw incorporation further increased Rs,Rh,Ra,and Rh/Rs by 50.94%,70.29%,14.79%and 12.92%,respectively.However,Biochar-applied soil C substrates were decoupled from Rh,with higher soil organic C and soluble organic C content accompanied by lower rates of Rh and resulting in lower Rh/Rs as Ra increased.Therefore,the soil organic C sequestration rate and C emission efficiency of biochar applications were higher than that of straw incorporation under conventional N application.In addition,N application increased the temperature sensitivity（Q10）of Ra and decreased the Q10 of Rh;straw incorporation decreased the Q10 of Rh.Integrated soil and crop factors could accurately predict seasonal variations in Rs.（2）N fertilizer,straw and biochar applications reduced methane（CH4）emissions.Under straw removal,although conventional N application significantly increased N2O emission by114.0%compared with no N application,global warming potential and greenhouse gas emission intensity decreased by 223.0%and 166.8%due to an increase in ecosystem C balance by 285.9%,but they were still demonstrated to be a‘source’of soil C loss and direct greenhouse gas emissions during each maize growing season.Under conventional N application,straw incorporation significantly increased N2O emissions by 34.9%compared with straw removal,while biochar addition decreased by 20.9%.However,they further increased ecosystem C balance by 154.6%and 295.8%,thus straw and biochar incorporation were net sinks of direct greenhouse gas emissions.Based on C balance and direct greenhouse gas emission,biochar addition has better C sequestration and emission reduction effect than straw incorporation.（3）Based on life cycle assessment,the indirect greenhouse gas and active N emission factors before biochar addition into the field were quantified as–0.72 t CO2-eq t–1 and–1.26kg N-eq t–1,respectively.Under conventional N application,integrated utilization of straw biochar（indirect greenhouse gas emission abatement from biochar before application to the field+direct greenhouse gas emission abatement from field application）showed a strong C‘sink’(C footprint=–2861.2 kg CO2-eq ha–1 yr–1),while straw incorporation exhibited a weak C‘source’(C footprint=1143.6 kg CO2-eq ha–1 yr–1).In addition,under conventional N application,the integrated utilization of straw-derived biochar significantly reduced active N loss(–12.7 kg N-eq ha–1 yr–1 N footprint)and increased net ecosystem economic benefit by4293.3￥ha–1 yr–1 compared with straw incorporation,respectively.This approach proved to be an effective method for enhancing the C sinks and economic benefits and decreasing greenhouse gas and active N emissions of maize agroecosystems.（4）Transpiration of maize was estimated using the modified dual crop coefficient approach.Compared with straw removal,straw incorporation and biochar addition reduced pre-tasseling evapotranspiration,increased post-tasseling evapotranspiration and the proportion of maize transpiration to evapotranspiration,and biochar application was more effective than straw.Maximum values for grain yield and water use efficiency were achieved at N300,N300,and N225 for straw removal,straw addition and biochar addition,respectively.The main reason for increased grain yield and water use efficiency under BN225 treatment was the longer,thinner,and deeper root architecture.This avoided redundant root growth,improved root N efficiency,and synergized shoot-root relationships for maize,facilitating greater absorption of soil water and nutrients during the reproductive stage.In contrast,the root system in the SN300 treatment was short,thick and shallow,which was mainly due to the higher available N content on the soil surface（0–40 cm）.（5）Biochar application significantly increased aboveground N accumulation and N recovery efficiency in maize compared with straw incorporation,especially under N225,which was mainly due to increased available N content during the reproductive stage.Due to the decreased N harvest index and increased accumulation of nitrate N,the grain N requirement of BN300 and BN375 was significantly higher than that of BN225.As a result,it suggests that the rate of N application under biochar management should not be too high.In addition,after maize harvest,the critical threshold values of nitrate in the root zone（0–100cm）under straw removal,straw and biochar incorporation were determined to be 49 kg ha–1,104 kg ha–1,and 67 kg ha–1,respectively,when the N supply capacity in the same soil depth was the greatest while ensuring that no nitrate was leached from the deeper soils（100–200cm）.The optimized N application（TONR）prediction model based on soil testing and nitrate threshold under different straw treatments was modified by integrating maize target yield and soil nitrate residue.In addition,in-season optimized N applications were accurately assessed and validated for straw removal,straw and biochar incorporation using three readily available variables（N nutrient index determined by critical N dilution curve of maize,days after sowing,and N recovery efficiency）.（6）The responses of ecosystem C balance,ecosystem C footprint and net ecosystem economic benefit to N application rates well fitted the quadratic model,whereas N footprint increased as an exponential function with increasing N application rate.N300,N300 and N225were the optimized N rates for maximizing grain yield and ecosystem C balance under straw removal,straw incorporation and biochar addition,respectively.Compared with SN300,BN225 treatment steadily increased soil C sequestration(ecosystem C budget=1191.3 kg C ha–1yr–1),maize productivity（in terms of grain yield by 5.81%,water use efficiency by 9.93%and N recovery efficiency by 6.20%）,and economic efficiency(by 492.51￥ha–1 net income and 4036.96￥ha–1 net ecosystem economic benefit),while reducing environmental risks(by1602.6 kg CO2-eq ha–1greenhouse gas emissions and 36.1 kg N-eq ha–1 active N losses).Therefore,integrated utilization of straw-derived biochar（direct abatement in the field+indirect abatement during biochar production）combined with reduced N application of N225is a productive,environmentally-and economically-sustainable method of optimal straw management in low fertile maize farmland in arid areas.This system also offers the most suitable solution to solve the problems of direct straw incorporation into the field with greenhouse gas and active N emissions and the low economic benefits of biochar addition to the farmland.It is important for guaranteeing food security and mitigating climate warming and environmental degradation.The results and conclusions of the study provided a new idea and theoretical basis for the optimal utilization of straw resources. |
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