Effect Of Straw Return And Nitrogen Reduction On Soil Carbon Sequestration,Fertility And Sustainable Production

Maintaining a sufficient amount of arable land and continuously improving the fertility quality of these farms is an important strategic issue for the state to guarantee food security.The intensive wheat(Triticum aestivum L.)/maize(Zea mays L.)–cropping system(wheat/maize rotation system)is the the major agricultural practice in in the Guanzhong Plain.It is of great significance to maintain the grain production sustainability in this system for ensuring the food security.However,there are still some problems to be solved in the following three aspects.Firstly,large amounts of crop straw,produced in this system every year,can be utilized by straw return,feed,industrial raw materials,and other resource utilization.It is well known that straw return can increase the sequestration of soil organic carbon(SOC)and improve soil fertility,but there exists resource competition between straw return and other resource utilizations,and it is not clear that the lowest amount of straw needed to maintain soil basic fertility.Secondly,as for straw return,unreasonable returning ways lead to vast release of greenhouse gases in the decomposition process,exacerbate the global climate change,and express non-significant effect on SOC sequestration and soil fertility.Thirdly,the excessive N application causes N loss by volatilization,groundwater pollution by nitrate,and other environmental problems.It needs scientific support for reasonable N reduction under straw return.For these three issues,although lots of work has been conducted,a systematic and in-depth study is still needed to select the most effective straw-return way for improving SOC sequestration and soil fertility,to scientifically answer the C inputs and balance point for maintaining the favourable balance of SOC in this system,to evaluate the effect of single or double straw return on SOC sequestration and soil fertility and the effect of N reduction on sustainable grain production under straw return.In view of this,based on the long-term straw-returning experiment,this research has systematically explored the effects of different combinations of wheat and maize straw on SOC sequestration,soil fertility and sustainable grain production,under the intensive wheat/maize rotation system in Guanzhong Plain during the period of 2008-2014,and the effects of N reduction on crop growth and the changes of SOC and N under the return of wheat and maize straw.Through these studies,we want to explore the optimal combinations of wheat and maize straw,and the N application rate to maintain the sustainable grain production under the intensive wheat/maize rotation system.The main results are as follows:(1)A 4-year-long field experiment established in 2008 was conducted to investigate the effects of the combined return of maize and wheat straw on grain production and carbon sequestration at a soil depth of 0–20 cm in an intensive wheat/maize rotation system in the Guanzhong Plain.The study involved four treatments with four replicates: low return of maize and wheat straw(ML–WL),low return of maize straw and high return of wheat straw(ML–WH),high return of maize straw and low return of wheat straw(MH–WL),and high return of maize and wheat straw(MH–WH).The results showed that compared with the SOC stock in the pre-experimental soil,the SOC stock in the 0–20 cm soil layer decreased by 2.6% in ML–WL,and in contrast,increased by 1.9% in ML–WH,4.8% in MH–WL and 14.4% in MH–WH.A minimum C input of 4.07 Mg/hm2/yr was required to maintain the initial level of SOC.The ranking of the treatments with respect to yield and the sustainable yield index(SYI)of maize and wheat was ML–WL < ML–WH < MH–WL < MH–WH.The average SYI of maize and wheat reached a maximum value of 0.651 when the plant-derived C input was 10.51 Mg/hm2/yr,which was higher than the maximum value(SYI of 0.64,plant-derived C input of 9.76 Mg/hm2/yr)found in MH–WH,indicating that the yields and SYI of wheat and maize may be further improved by adopting modified field management practices.(2)A 6-year-long field experiment established in 2008 was conducted to investigate the effects of the combined return of maize and wheat straw on soil bulk density(BD),SOC concentration and stock at a soil depth of 0–40 cm in an intensive wheat/maize rotation system in the Guanzhong Plain.The returning ways of wheat straw included high stubble straw mulching and returning to field(WH),straw chopping and direct returning to field(WC),and no straw returning(WN);the returning ways of maize straw included straw chopping and direct returning to field(MC),straw chopping and returning to field with subsoiling(MM),and no straw returning(MN).Therefore,there were 9 combinations of wheat and maize straw in every rotation cycle.The results showed that compared with WN–MN,the return of wheat and/or maize straw decreased the BD,but increased SOC concentration and stock in every layer of 0–40 cm.Compared with the pre-experimental soil,the return of wheat and/or maize straw increased SOC sotck on an equivalent depth of 0–20 cm and equivalent masses of 2590 and 5730 Mg/hm2,and the increase rates reached maximum value in WH–MM by 0.6%,13.5% and 9.1%,but decreased by 5.5%,5.3% and 5.0% in WN–MN.However,SOC stocks on an equivalent depth of 0–40 cm increase in the return of both wheat and maize straw,and decreased in WN–MN and the single return of wheat or maize straw.Therefore,the WH–MM was the most conducive to SOC sequestration under the intensive wheat/maize rotation system.(3)Based on the 6-year-long field experiment,the research investigated the effects of 9 combinations of wheat and maize straw on the concentrations and stocks of SOC and labile fractions(ie.,LBC,including total,highly,moderately and lowly labile),and their corresponding carbon pool management index(TCPMI,HCPMI,MCPMI,and LCPMI).The results showed that compared with WN–MN,the return of wheat and/or maize straw increased the concentrations and stocks of SOC and LBC with the same trend: both return > single return > no return.In 0–40 cm soil layer,the highest stocks of SOC,total and moderately LBC were found in WH–MM with significant increase by 12.4%,32.7% and 80.2%,respectively,over WN–MN,while the highest stock of highly LBC was found in WC–MM with an significant increase by 16.1% over WN–MN,but no significant difference was found with the WH-MM.The trend of TCPMI,HCPMI,MCPMI,and LCPMI was consistent with the corresponding LBC stock,and the TCPMI and MCPMI in WH–MM increased significantly by 37.0% and 86.1%,respectively,over WN–MN.(4)Based on the 6-year-long field experiment,the research investigated the effects of 9 combinations of wheat and maize straw on Grain yields,substance inputs,and labour inputs,estimate amounts of C inputs,C outputs,C efficiency and economic benefits during the production of wheat and maize.The results showed that the C input of the field was 2765~3161 kg C/hm2 in the wheat and maize rotation period,the lowest and largest C input were found in WN–MN and WC–MM,respectively,and the C input derived from irrigation had the greatest proportion,followed by mechanical and fertilizer carbon inputs.The WH–MM had the largest C outputs,C production efficiency,C economy efficiency,C ecological efficiency,net income,return on substance and labor.Therefore,the WH–MM had the highest C efficiency and economic benefits,and achieved the win-win between environmental and economic benefits.(5)Based on the 6-year-long field experiment,the research evaluated the sustainability of the intensive wheat/maize rotation system.The results showed that compared with WN–MN,the return of wheat and/or maize straw increased the stocks of SOC,LBC,available N,available P,and available K,the concentrations of microbial biomass carbon,the activities of sucrase,protease,alkaline phosphatase,dehydrogenase activity and grain yields of wheat and maize.The WH–MM had the highest nutrient index and crop index,while the WC–MC had the highest microbial index,but the sustainability index in WH–MM was significantly higher than the WC–MC.Therefore,the WH–MM was the best straw-returning way for the sustainable grain production.(6)Based on the 6-year-long field experiment,the research evaluated the effects of 15% and 30% N reduction based on conventional N(ie,187.5 kg/hm2 for maize and 150 kg/hm2 for wheat)on crop growth and the changes of SOC and N under the return of wheat and maize straw.The results showed that significant decrease of grain yield was only found in 30% N reduction for maize,and the rate was 9.4%.SOC stock in every treatment increased compared with the pre-experimental soil,but SOC stock in 15% and 30% N reduction were significantly decreased by 6.5% and 10.3%,respectively,compared with conventional N,LBC stocks were significantly decreased by 15.8% and 19.7%,respectively,andthe CPMI in 30% N reduction were significantly decreased by 20.9%.What's more,soil mineralized N accumulation in 0–200 cm soil layer in 15% and 30% N reduction were decreased by 12.9% and 22.8%,respectively,after wheat harvest,and by 11.1% and 22.9%,respectively,after maize harvest,compared with conventional N.Furthermore,30% N reduction could also decrease N and K content and uptake in crop grain and straw.Finally,the apparent soil N balance and N balance rate were lower in 15% and 30% N reduction than conventional N,but the apparent soil N balance could also be maintained in 15% and 30% N reduction.Therefore,considering grain yields,SOC sequestration,nutrient uptake,and apparent soil N balance,15% N reduction(ie,160 kg/hm2 for maize and 127.5 kg/hm2 for wheat)was the best N application rate.In general,the initial SOC level could be maintained in the single high return of wheat and maize straw,and the high return of both wheat and maize was best for SOC sequestration and the sustainable grain production.The return of wheat and/or maize straw increased the stocks of SOC and its labile fractions,and the optimal improvement was found in the combination straw-returning way of high wheat stubble straw mulching and returning to field and maize straw chopping and returning to field with subsoiling(WH–MM),and this way has the best advantage on soil fertility,sustainability,C efficiency and economic efficiency.In addition,no significant decrease was found for grain yields,SOC sequestration,nutrient uptake,and apparent soil N balance in 15% N reduction based on conventional N.Therefore,the combination straw-returning way of high wheat stubble straw mulching and returning to field and maize straw chopping and returning to field with subsoiling was optimal straw-returning way for SOC sequestration soil fertility,and the sustainable grain production.,and15% N reduction(ie,160 kg/hm2 for maize and 127.5 kg/hm2 for wheat)was the best N application rate under the return of both wheat and maize straw.