Mulched Rotation Cropland’s Productivity Changes And Characteristics Of Greenhouse Gas Emissions With Soil Microbial Mechanisms

The increasing greenhouse gas concentrations in the atmosphere is one of the main causes of global warming.Agriculture is an important source of greenhouse gas emissions.Various mulching practices have been widely used in arid,semi-arid,and sub-humid regions,owing to their significant yield increasing effect.Based on a field experiment of three-years（2018-2021）conducted on a wheat-maize rotation field in the southern Loess Plateau,China,this study systematically evaluated the economic and environmental benefits,and explored the mechanism of soil greenhouse gas emissions under different mulching practices.Four treatments were evaluated:（1）conventional flat planting without mulching（CK）,（2）flat planting with plastic film mulching（PM）,（3）flat planting with straw mulching（SM）,and（4）plastic film-mulched ridges and straw-mulched furrows（RFPS）.Based on the field experiment observations,the effects of different mulching practices on soil physicochemical properties,crop productivity,water use efficiency（WUE）,and greenhouse gas emissions,and carbon balance were determined.Net ecosystem carbon balance（NECB）and net ecosystem economic benefit（NEEB）were used to comprehensively evaluate the carbon sequestration effect and economic and environmental benefits under different mulching practices.In addition,the effects of two typical mulching measures（PM and SM）on the abundance,community structure,and diversity of soil bacteria,nitrifying bacteria,and denitrifying bacteria were studies based on molecular biology methods.Structural equation model（SEM）was used to explore the relationships among soil physicochemical properties,soil microbial properties,and soil greenhouse gas emissions.Revealing the abiotic and biotic mechanisms of soil greenhouse gas emissions will provide a theoretical basis for the carbon sequestration and greenhouse emissions reduction under different mulching practices,and thereby promote the green agriculture application in arid,semi-arid,and sub-humid regions.The main results are as follows:（1）The positive effect of plastic film-mulched ridges and straw-mulched furrows on crop productivity and water use efficiency was greater than that of plastic film mulching and straw mulching.During the wheat season from sowing to returning green stage,all mulching practices had a warming effect with PM having the highest effect.During the wheat season from returning green stage to harvest,the averaged soil temperatures of PM,SM,and RFPS were all lower than CK.Compared with CK,PM increased while SM and RFPS decreased the soil temperature,in the maize seasons.All mulching practices increased soil water content and soil NO₃^--N content during both wheat and maize seasons.SM and RFPS increased soil NH₄⁺-N content during maize seasons.For the 2021 maize season,as compared with CK,PM and SM decreased the soil p H.Compared with CK,PM,SM,and RFPS increased the annual averaged grain yield by 31.8%,31.8%,and 48.6%,the annual averaged aboveground biomass increased by 27.2%,44.5%,and 52.3%,respectively.All mulching practices（i.e.,PM,SM,and RFPS）improved the water use efficiency（WUE）for wheat and maize in all years.As for wheat,the WUE under PM and RFPS were greater than SM.As for maize,the WUE under SM and RFPS were greater than PM.Therefore,RFPS has the greatest crop productivity and WUE for the wheat-maize rotation systems in the southern Loess Plateau,China.（2）All the mulching practices increased soil CO₂ emissions,with the greatest observed under the straw mulching and plastic film-mulched ridges and straw-mulched furrows treatments.Plastic film mulching decreased soil N₂O emissions,but straw mulching and plastic film-mulched ridges and straw-mulched furrows increased them.PM significantly increased the total CO₂ emission during the wheat seasons,while only significantly increased the total CO₂ emission during the maize season of 2019.SM,and RFPS significantly increased the total CO₂ emission during the wheat and maize seasons.Compared with CK,PM,SM,and RFPS increased CH₄ uptake from soil.The peaks of soil N₂O emissions mainly occurred 3-8 days after fertilization,with PM lower than CK,and SM and RFPS higher than CK.The averaged soil N₂O fluxes showed the order of RFPS>SM>PM>CK during the off-peak emission stage.Compared with CK,the annual averaged soil N₂O emissions under PM was significantly decreased by 30.4%,but significantly increased by 29.7%and36.3%,respectively,under SM and RFPS.（3）Straw mulching and plastic film-mulched ridges and straw-mulched furrows were conducive to increasing carbon sequestration and net ecosystem economic benefit for the wheat-maize rotation systems in the southern Loess Plateau,China.The NECB value of CK and PM were negative,and the NECB of PM was significantly higher than CK.It indicates that the field under CK and PM were carbon sources,while PM can sequestrate carbon when compared with CK.SM and RFPS had positive NECB values,indicating that the fields under SM and RFPS were carbon sinks.Compared with CK,the annual averaged NEEBs under PM,SM,and RFPS were significantly increased by 31.4%,70.5%,and 60.3%,respectively.Therefore,from the perspective of annual economic and environmental benefits,SM and RFPS may be more conducive to increasing carbon sequestration and net ecosystem economic benefit for the wheat-maize rotation system in the southern Loess Plateau,China.（4）Plastic film mulching and straw mulching have different effects on soil bacteria,nitrifying bacteria,and denitrifying bacteriaDuring the 2021 maize season,PM decreased while SM increased the abundance,OTUs,Chao1 index,and Shannon index of soil bacteria.At the phylum level,PM increased the relative abundance of Chloroflexi and Firmicutes,SM significantly increased the relative abundance of Proteobacteria.Compared with CK,PM decreased the averaged AOB-amo A gene abundance by 63.5%,while SM increased by 50.2%.At the genus level,compared with CK,PM increased the relative abundance of Nitrososphaera（AOA）and decreased the relative abundance of Nitrosospira（AOB）,but SM increased the relative abundance of Nitrosospira（AOB）.Compared with CK,SM increased the averaged abundance of nir S,nir K,and nos Z by79.5%,194.4%,and 26.5%,respectively.For nir S-denitrifier,the relative abundance of Rhodanobacter was significantly lower under PM,but was 2.7-7.8 times under SM as compared with CK.For nos Z-denitrifier,SM significantly increased the relative abundance of Bradyrhizobium.（5）Mulching practices influenced soil greenhouse gas emissions by simultaneously affecting soil physicochemical properties and associated microbial communities.Soil NH₄⁺-N,NO₃^--N,bacterial diversity,and community structure had significant positive effects on soil respiration.Soil temperature had an indirect negative effect on soil respiration via influencing soil bacterial community structure.AOB-amo A gene abundance was the main biotic factor affecting soil N₂O emissions.Soil temperature,NH₄⁺-N,and NO₃^--N indirectly affected soil N₂O emissions via influencing AOB-amo A gene abundance.Soil water content had a direct negative effect on soil N₂O emissions.In conclusion,the crop productivity and carbon sequestration ability of SM and RFPS were greater than that of PM.PM decreased the bacterial diversity and changed the bacterial community structure due to the much higher soil temperature in summer,which inhibited the decomposing of soil organic matter,thereby affecting soil CO₂ emissions.The lower AOB-amo A gene abundance may be the reason for the decreased soil N₂O emissions under PM.SM increased soil nutrients,bacterial diversity,and the relative abundance of cultures involved in soil carbon cycling,thereby increasing soil CO₂ emissions.SM significantly increased soil N₂O emissions mainly via increasing the abundance of AOB-amo A and Nitrosospira（AOB）.