Effects Of Mulching,plant Density And Nitrogen On Grain Yield,water And Nitrogen Use Efficiency,and Greenhouse Gas Emissions Of Rainfed Summer Maize In Northwest China

The lack of irrigation water,the loss of nitrogen(N)resource and low N use efficieny in semi-arid and semi-humid drught-prone regions in Northwest China,soil mulching,planting density and nitrogen management are used to improve crop productivity in this region.In this paper,a 5-year(2015-2019)field experiment was conducted in a typical semi-humid but drught-prone region to determine whether agronomic management has potential of improving maize productivity but mitigating adverse effects on enviroment.The first experiment(2015-2017)contained three mulching methods(NM:flat cultivation with non-mulching;SM:flat cultication with straw mulching;RP:ridge-furrow cultivation with film mulching on ridge)and three planting densities(LD:low density,plant spacing of60×37 cm;MD:medium density,plant spacing of 60×25 cm;HD:high density,plant spacing of 60×18.5 cm).The second experiment(2018-2019)contained three mulching methods(NM,SM and RP)and three nitrogen rates(U1:100 kg N/ha,U2:200 kg N/ha and U3:300 kg N/ha).In addition,on the basis of 200 kg N/ha rate,we set four N types(U:urea,U+CRU,urea+slow-release fertilizer=3:7,U+DMPP:urea combined with nitrification inhibitor and U+NBPT:urea combined with urease inhibitor)under three mulching methods(NM,SM and RP).The effects of different agronomi practices on rainfall partitioning,maize growth,WUE,NUE use efficiency,economic benefit,NH₃ and greenhouse gas emissions of summer maize were explored.The main results were shown as below:(1)Soil temperature was mainly affected by mulching methods.The lower soil temperature under SM had no inhibition effect on maize growth,while the higher soil temperature under RP had a promotion effect on maize growth.The soil water storage effects of different practices were mainly in 0-60 cm soil layer and the differences among treatments decreased with increasing soil depth.In the early stage of maize,the water storage in 0-60cm soil layer increased with SM and RP.After maize tasseling,the water storage in 0-60 cm soil layer showed different trends due to different soil water status and growth status in different years.With the increase of planting density,soil water storage decreased due to the intensification of soil water competition.Nitrogen management practies had no obvious effect on soil water storage.(2)Straw and plastic mulching could improve maize plant height,stem diameter,LAI,leaf chlorophyll content,photosynthetic rate,aboveground dry matter and final grain yield.There was no significant difference on these growth indices between SM and RP in most cases,especially in wet years(2018 and 2019).LAI increased significantly with increasing planting density.The maximum maize height also increased with the increase of density,while the stem diameter,chlorophyll content and photosynthetic rate were in the order of LD>MD>HD.MD produced the highest biomass and grain yield.Maize growth and grain yield generally increased with the increasing N rate.Under 200 kg/ha N rate,the application of CRU,DMPP and NBPT improved maize growth in different degrees,but there was no significant difference among the three enhanced-efficieny fertilizers.The U3+RP treatment produced the highest grain yield(9183.2 kg/ha)over 2018 and 2019 seasons,but the treaments of SM+CRU,SM+DMPP,RP+CRU,RP+DMPP under U2 all obtained the comparable grain yield compared to U3+RP.The benefit of SM treatment is comparable to that of RP.With the increase of N application rate,the net income still increased,but the increase range decreased.Under different N types,the net income was ranked as U+DMPP>U+NBPT>U+CRU because the increase of grain yield could not compensate the increase of fertilizer costs.(3)There were positive relationships between the throughfall,stemflow,canopy interception and rainfall amount and intensity at different growing stages.The relative stemflow and canopy interception increased with leaf area index for different rainfall classes,while relative throughfall showing a decreasing trend with increasing LAI.The relative stemflow and canopy interception generally decreased with increasing rainfall intensity,while the relative throughfall increased with increasing rainfall intensity.Mulching had no significant effect on canopy interception and net rainfall,whereas planting density significantly affected all the components of rainfall redistribution.SM and RP significantly increased the productive water loss ratio(Tp/ET),and thus significantly increased crop WUE.The highest WUE was usually obtained under MD.In terms of different N application rates,U2 and U3 increased WUE by 15.1%and 14.8%,respectively,compared with U1 on average.WUE was also improved under enhanced-efficieny fertilizers,which was increased by 8.1%,8.4%and 5.2%under U+CRU,U+DMPP and U+NBPT,respectively,and there was no significant difference among the three enhanced-efficieny fertilizers.(4)The residual NO₃^--N,NH₄⁺-N and total mineral N in the soil was mainly affected by nitrogen application,which increased significantly with the increasing N rate.Residual soil NO₃^--N was significantly reduced under DMPP,the NBPT-treated soil showed the lowest residual NH₄⁺-N amount over the two seasons.Mulching methods signiciantly promoted nitrogen uptake efficiency(NUPE)and partial fertilzier productivity(PFP).There was no significant difference in NUE between SM and RP in most cases.NUPE and PFP were the highest at medium planting density.Excessive N rate led to the decrease of NUPE and PFP.The application of CRU,DMPP and NBPT at the same N rate could improve NUE.(5)Soil mulching had no significant effect on NH₃ volatilization and CH₄ absorption.RP significantly increased CO₂ emissions,while there was no significant difference in CO₂between SM and NM.Seasonal N₂O emission is generally decreased under SM,but increased under RP.Overall,global warming potential(GWP)and greenhouse gas intensity(GHGI)were significantly increased under RP,but decreased under SM.Nitrogen application rate had little effect on CO₂ and CH₄ emissions,but significantly increased N₂O emissions.With the increase of nitrogen rate,GWP and GHGI increased significantly.Compared with U,U+NBPT and U+DMPP produced much lower NH₃ volatilization and N₂O emission,respectively.U+CRU also reduced NH₃ volatilization and N₂O emission.Both U+DMPP and U+NBPT exhibited inhibitory effect on methane(CH₄)oxidation,but the CH₄ pattern under U+CRU was inconsistent in the two years.The smallest GWP and GHGI were generally observed under U+DMPP among various N types.After comprehensively evaluating the indice of growth,WUE,NUE,grain yield,net inome and environmental benefits such as NH₃ and greenhouse gas emissions,we concluded that under the condition of not obviously decrease crop growth,wheat straw mulching with nitrification inhibitor application at application rate of 200 kg/ha with nitrification inhibitor DMPP achieved better balance among agronomic,economic and environmental benefits for dryland maize in northwest China.