Effect Of Reduced Nitrogen Application And Soybean Intercropping On Soil Greenhouse Gas Emissions And Nitogen Cycle In Sweet Maize Fields

Because of high nutrition and good palatability,sweet maize?Zea mays L.saccharata sturt?is becoming more and more popular.Guangdong Province is the most important region for sweet maize production in China.However,continuous sole sweet maize cropping combined with high chemical nitrogen?N?fertilizer application may lead to serious N pollutions.Those problems become more severe during the growth season and after harvesting when cropping two or three sweet maize seasons in a year,because the N fertilizer application is almost doubled than one maize cropping season.Cereal-legume intercropping is a type of sustainable cropping system for achieving improved crop productivity and mitigated environmental problems simultaneously.But there was few report on sweet maize intercropped with legume.Here,a three-years-and-a-half field experiment was conducted to assess the influence of four different cropping systems?sole sweet maize?SS?,sole soybean?SB?,two rows sweet maize-three rows soybean?S2B3?intercropping,and two rows sweet maize-four rows soybean?S2B4?intercropping?,together with two rates of N fertilizer application to sweet maize(300 and 360 kg N·hm^-2)on crop agronomic trait,crop productivity,soil greenhouse gas?CO₂ and N₂O?,N balance and expression levels of gene related to N cycle in southern China,in order to optimize the sustainable and environmentally friendly production of sweet maize in south China.The main results were as follows:1.It has no negative effect effect on sweet maize agronomic trait to intercrop with soybean.Moreover,there was no significant difference on grain yield and biomass yield of sweet maize per plant.However,due to shading and nutrient competition of sweet maize,gain yield and biomass yield of intercropped soybean per plant declined.Furthermore,compared to conventional-N rate,reduced-N rate had no significantly effect on agronomic trait and productivity of sweet maize,showing that 300 kg N·hm^-2 could completely maintain sweet maize growth.2.Averaged over the seven seasons,grain yield,biomass yield,nitrogen acquisition,phosphorus acquisition,and potassium acquisition of sweet maize-soybean intercrops were17.53 t·hm^-2,9.62 t·hm^-2,226.71 kg·hm^-2,27.75 kg·hm^-2 and 154.03 kg·hm^-2,repectivily.All of them were greater than sole sweet maize.In most case,Total land equivalent ratio for grain yield?TLER=0.87¹.25?was above 1,and actual yield loss?AYL=-0.28⁰.49?was above 0.These indicated that intercrops could increase crop productivity and land use rate,showing the yield advantage.3.Cropping system and N application rate had no significant effect on soil CO₂ flux and cumulative CO₂ emissions.Compared to sole sweet maize,sweet maize-soybean intercrops could reduce soil N₂O flux and cumulative N₂O emissions.Among the N application treatment,sweet maize-soybean 2:4 intercropping with reduced-N rate produced the lowest N₂O emissions(mean value:0.56 kg·hm^-2).Moreover,cropping system and N application rate had no significantly effect on global warming potential.Furthermore,compared to sole sweet maize,sweet maize-soybean intercropping with reduced-N rate could decrease greenhouse gas intensity,which meat emitting lower greenhouse gas per grain yield.Among the N application treatment,sweet maize-soybean2:4 intercropping with reduced-N rate showed the lowest greenhouse gas intensity(mean value:0.11 kg·kg^-1).It was an environmentally-friendly cropping system,which could maintain crop productivity and reduce greenhouse gas emissions simultaneously.4.Averaged over the seven seasons,ammonia volatilization,nitrogen leaching and soil N₂O emissions of sweet maize-soybean intercrops were 23.71 kg·hm^-2,96.83 kg·hm^-2 and1.01 kg·hm^-2,respectively.All of them were lower than sole sweet maize.These indicated intercrops could reduce N loss,thus decreasing environmental pollution.5.The treatment of sole sweet maize and conventional-N rate showed less total N input compared to the treatment of sweet maize-soybean intercrop and reduced-N rate,respectivily.However,the result of total N output was opposite.In every season,the N surplus under sole sweet maize was above 200 kg N·hm^-2,which was significantly greater than sweet maize-soybean intercropping,caused environmental pollution.Compared to sole sweet maize,N balance under sweet maize-soybean intercropping with reduced-N rate was about 100 kg N·hm^-2,which not only could maintain soil fertility and crop yield,but also reduced N pollution.6.Season was the most important factor to effect bacterial community and functional gene expression levels of sweet maize rhizosphere soil.Compared to autumn of 2013,the relative abundance of Actinobacteria and Chloroflexi reduced.Soil pH,ammonium nitrogen,available phosphorus and total potassium have significantly influenced on bacterial community.Moreover,the change of bacterial community has significantly influenced on gene of amoA,nirK,nirS,nosZ and nifH.The expression levels of these genes in autumn of 2016 have significantly increased compared to autumn of 2013.