Effect Of Cover Cropping On Water And Nitrogen Balances And Crop Yield In Dryland Cropping Systems

In the recent decades,cover cropping has been widely adopted to enhance ecosystem services,such as increased soil aggregation and carbon sequestration,reduced nutrient leaching and erosion,and suppressed weeds and pests compared to no cover cropping.However,little is known about the effect of cover crops on soil water storage and water productivity.Especially in arid and semiarid regions with limited precipitation,soil water content was usually the main factor limiting crop production.Therefore,by combining data from peer-reviewed publications,two Meta-analysis were carried out to assess the effect of cover crop on precipitation storage efficiency（PSE,the percent of precipitation that is stored in the soil during the fallow period）during the fallow period,soil water storage at succeeding crop planting（SWSP）,succeeding crop yields,evapotranspiration（ET）,and water use efficiency（WUE）.Additionally,we examined the effect of summer cover crop and N fertilization rates on water and nitrogen balances and crop yield from 2017-2018 to 2020-2021 at two sites:Chang’an and Changwu in the Loess Plateau of China.Cover crops were soybean（SB）,sudangrass（SG）,mixture of soybean and sudangrass（SS）,and no cover crop（CK）.Nitrogen fertilizer rates were 0,60,and 120 kg N ha^-1 that were applied to winter wheat,respectively.The main results are as followed:（1）Through synthesizing data from 99 peer-reviewed publications across the world,cover crop decreased PSE by 33.4%and soil water storage for the whole profile（SWSP）at soil depth by 13.2%,but increased soil water storage to a depth of 30 cm（SWSP30）by 6.00%（P<0.05）compared to no cover crop.However,cover crop did not affect succeeding crop yield,but decreased ET by 6.20%and increased WUE by 5.01%（P<0.05）compared to no cover crop.Cover crop decreased PSE by 9.61%（P<0.05）compared to no cover crop in the management of residue removal.Leaving cover crop residue at the soil surface or incorporating into the soil reduced PSE,SWSP,and ET,but increased SWSP30 and WUE compared to residue removal.Additionally,maintaining cover crop aboveground biomass at5000 kg ha^-1 and leaving a 20-d interval between cover crop termination and succeeding crop planting,also enhanced PSE and SWSP30.Although cover crop had minimal impact on succeeding crop yield,WUE of succeeding crops can be increased with cover cropping by decreasing evapotranspiration.（2）Through synthesizing data from 46 peer-reviewed publications in the Loess Plateau of China,compared to fallow without green manure,green manure decreased PSE by28.28%,SWSP by 4.93%and ET by 2.51%（P<0.05）,but increased succeeding crop yield by 2.37%and WUE by 8.97%（P<0.05）,respectively.Strong polynomial relationship was found between PSE,SWSP and the interval between legume green manure termination and succeeding crop planting,and both PSE and SWSP maximized at the interval of 13 d.Succeeding crop yield and WUE were also correlated with legume green manure aboveground biomass in a polynomial way.Succeeding crop yield and WUE maximized when the legume green manure aboveground biomass was 2.2 and 3.1 Mg ha^-1,respectively.Overall,introducing green manure during fallow period reduced soil water but enhanced succeeding crop production and water use in the Loess Plateau.Leaving a 13-d interval between legume green manure termination and succeeding crop planting and terminating legume green manure at a biomass of 2.2-3.1 Mg ha^-1can be reliable practices to mitigate soil water consumption in such dryland areas.（3）Cover crop aboveground biomass were 15.9%and 26.5%（P<0.05）greater with SG and SS than SB at both sites,respectively.Compared to no cover crop,cover crop decreased PSE by 21.7%（P<0.05）and SWSP by 15.0%（P<0.05）at Chang’an and decreased PSE by15.7%（P<0.05）and SWSP by 5.43%（P<0.05）at Changwu,with higher in SB and SG than SS at Chang’an,and higher in SG and SS than SB at Changwu.However,soil water storage at winter wheat harvest was different among cover crop treatments at both sites.Average across years,winter wheat aboveground biomass,grain yield and harvest index were 12.9%,17.7%and 4.88%（P<0.05）averagely greater with SB and SS than SG at both sites,and higher in SB and SS than SG.Cover crop averaged decreased soil water storage during the fallow period by 19.1%（P<0.05）,soil water uptake during the winter wheat growing period by 21.2%,and ET by 9.53%compared to no cover crop at both sites,but averaged increased WUE by 27.2%（P<0.05）.The annual soil water balance was not different among cover crop at Chang’an,and was greater for SG than SB at Changwu.Additionally,winter wheat grain yield and WUE increased with increased N fertilization rates,but no effect on the annual soil water balance.Overall,SB and SS and 120 kg N ha^-1 effectively maintain water productivity.（4）Cover crop N accumulation were 65.7%and 95.7%（P<0.05）greater with SB and SS than SG at both sites,respectively.Compared to no cover crop,cover crop increased wheat aboveground biomass N uptake by 25.7%（P<0.05）and grain N uptake by 29.9%（P<0.05）at Chang’an and wheat aboveground biomass N uptake by 23.2%（P<0.05）and grain N uptake by 25.6%（P<0.05）at Changwu,with higher in SB and SS than SG at both sites.The relative N contribution from SB and SS was greater than that for SG,and SB and SS improved the mean relative N contribution from N fertilizer.SB and SS averagely increased wheat grain N recovery rate of cover crop by 101%and 142%（P<0.05）than SG at both sites,and SS averagely increased wheat grain N use efficiency of cover crop by 22.1%（P<0.05）than SG at both sites,respectively.Additionally,the nitrogen fertilizer replacement value of the cover crop was greater by 77.1%and 139%（P<0.05）for SB and SS than SG at Chang’an,and by 38.7%and 45.1%（P<0.05）at Changwu,respectively.And cover crop averagely increased total N input,total N output and annual N balance by 65.1%,19.9%and137%than CK at both sites,with higher in SB and SS than SG.Additionally,wheat aboveground biomass N uptake,grain N uptake and annual N balance increased with increased N fertilization rates in all years at both sites.SB and SS had more efficient on promoting annual nitrogen balance than SG,and SB and application nitrogen fertilizer,which could increase the risk of N losses and pollution during next fallow period.Therefore,SS and 120 kg N ha^-1 restrain serious environmental risks due to lower N surplus and reduce N fertilization application to winter wheat on the Loess Plateau of China.