Microbial Mechanisms Of N-fertilizer Postponing Application On The Reduction Of Soil GHG Emissions In Pea/maize Intercropping

How to reduce greenhouse gas emissions from farmland under the premise of ensuring crop yield is one of the major challenges facing sustainable agricultural development.It has been demonstrated that integrated the delayed application of N fertilizer technology to pea/maize intercropping has the characteristics of high yield and low emission.However,the research on the emission reduction mechanism of this integrated system just altered the soil physical and chemical properties,which revealed little information is available about the microbial mechanisms of intercropping emission reduction.Strengthening the advantage of intercropping emission reduction by optimizing N management lacks theoretical and practical bases.In 2019-2021,a field experiment was conducted relying on the long-term positioning test in the Hexi oasis irrigation area.Under the premise of the same total nitrogen application amount,with the traditional nitrogen application（N3）as the control,20%and 10%of the traditional nitrogen application were transferred from the jointing stage to 15 days after silking,respectively,thus formed the delayed application 20%（N1）and delayed application 10%（N2）.The response of crop yield,greenhouse gas emission characteristics,and greenhouse gas influencing factors were investigated under different delayed applications of N fertilizer,so as to reveal the regulatory mechanism on greenhouse gas emissions of pea/maize intercropping.The aim was to provide a theoretical base for further optimization of delayed applications of N fertilizer technology.The main results are as follows:（1）The delayed application 20%N fertilizer significantly increased the crop yield of pea/maize intercropping and simultaneously reduced the greenhouse gas emission during the crop growing season.Compared to the weighted mean value of two sole crops,the intercropping increased the total grain yield by 31.6%,and delayed application 20%N fertilizer increased it by 22.9%over the traditional application.Soil CO2 emissions and N₂O emissions of pea/maize intercropping were 30.8%and 62.1%lower than that of sole maize,and that of the delayed application 20%N fertilizer was 23.1%and 38.0%lower than traditional application.The warming potential of pea/maize intercropping was decreased by 31.9%in comparison to the sole maize,and the delayed application 20%N fertilizer decreased it by 23.6%compared to traditional application.（2）The delayed application 20%N fertilizer affected soil greenhouse gas emissions by regulating soil total nitrogen,organic matter and labile organic matter at crop different growth periods.Before pea harvest,compared with traditional nitrogen application,the delayed application 20%N fertilizer decreased the content of soil total nitrogen,organic matter and labile organic matter of pea strips by 4.0%,17.8%,and 21.5%,respectively,and decreased that of maize strips by 4.3%,13.2%,and23.7%,which is beneficial to soil CO₂ emissions reduction.After pea harvest,the delayed application 20%N fertilizer decreased the soil total nitrogen of pea strips by1.5%compared with traditional nitrogen application,whereas increased the labile organic matter of maize strips by 14.0%.Changes in these indicators were conducive to N₂O emission reduction.（3）The delayed application 20%N fertilizer decreased soil microbial biomass and enzyme activity,which was contributed to soil greenhouse gas reduction.Before pea harvest,there was no significant difference in the soil microbial biomass carbon of the delayed application 20%N fertilizer in pea strips compared with traditional nitrogen application,whereas maize strips were decreased by 23.5%.The soil catalase activities of the delayed application 20%N fertilizer in pea strips were 30.9%lower than traditional nitrogen application and maize strips were 30.7%.Changes in these indicators were beneficial to CO₂ emission reduction.After pea harvest,the delayed application 20%N fertilizer increased the soil microbial biomass nitrogen of pea strips and maize strips by 16.4%and 16.1%compared with traditional nitrogen application.Meantime,the urease activity of pea strips and maize strips were decreased by 15.9%and 16.9%compared to traditional nitrogen application.The variation of these indicators was beneficial to N₂O emission reduction.（4）The delayed application 20%N fertilizer reduced the soil bacterial diversity of pea strips before pea harvest,which is beneficial to the soil CO₂ emission mitigation.The delayed application 20%N fertilizer reduced the relative abundance of Gammaproteobacteria before pea harvest and Bacteroidia after pea harvest,thereby decreased the CO₂ emission.At the pea flowering stage,the delayed application 20%N fertilizer decreased the abundance of soil archaea amo A gene,bacteria amo A gene,nir K gene,and nir S gene of pea strips,which contributed to soil N₂O emission mitigation.Similarly,at the maize silking stage,it decreased the abundance of these four genes of maize strips and simultaneously increased the abundance of nos Z gene,which also contributed to soil N₂O emission mitigation.（5）The key influencing factors of greenhouse gas emissions in intercropped pea and maize strips are different.The results of canonical correlation analysis showed that the soil microbial biomass carbon and N₂O emission fluxes were ranked the first among the influencing factors and emission indicators of pea strips,respectively.Furthermore,the correlation coefficient of the content of soil organic matter,the abundance of soil bacteria amo A gene and nos Z gene is large,and soil N₂O emission flux is significantly positively correlated with organic matter content and bacteria amo A gene abundance,and significantly negatively correlated with nos Z gene abundance.The soil total nitrogen and soil CO₂ emission fluxes were ranked the first and the soil labile organic matter and N₂O emission fluxes ranked second among the influencing factors and emission indicators of maize strips,respectively.The delayed application 20%N fertilizer decreased the content of soil organic matter and bacterial amo A gene abundance and simultaneously increased the abundance of the nos Z gene,which contributed to the soil N₂O emission mitigation of pea strips.The delayed application 20%N fertilizer decreased the content of the soil labile organic matter,which contributed to the soil CO₂ emission of maize strips.In conclusion,the delayed application 20%N fertilizer decreased the relative abundance of soil dominant bacteria classes and the abundance of nitrogen cycle functional genes,which was conducive to the reduction of greenhouse gas emissions of cereal-legume intercropping in the Hexi oasis irrigation area.