The Response Of Winter Wheat Yield,Nutrient Balance,and Soil Carbon Sequestration To Long-term Straw Returning With Nitrogen Fertilizer Application

Returning straw to the fields is an economically effective measure for the straw resource utilization.Properly returning straw to the fields can increase grain yields,improve fertilizer utilization,promote nutrient cycling,and enhance soil carbon sequestration.The positive effects of straw returning can be fully realized when used with nitrogen fertilizer.In2012,a long-term experiment on straw returning with nitrogen fertilizer was conducted in Zhouzhi County,Shaanxi Province,to investigate the effects of different nitrogen fertilizer application with straw returning on winter wheat-summer maize rotations.The main treatment was maize straw returning combined with nitrogen fertilizer（S+N）and straw non-returning with only nitrogen fertilizer application（N）.The sub-treatment included five different nitrogen fertilizer application levels(0,84,168,252,and 336 kg N·hm^-2),amounting to a total of ten treatments.The study investigated the impact of straw returning with these different nitrogen fertilizer application levels on winter wheat yield,nitrogen uptake and utilization,soil nutrient supply capacity,field nutrient balance,and soil carbon sequestration capacity.The following main results were obtained:1)After returning straw to the field,winter wheat shows a trend of increasing yield（up to 12.5%）,showing a pattern of low nitrogen reduction and high nitrogen yield increase.Returning straw to the field requires a nitrogen application of 112.9 kg·hm^-2 or more,otherwise returning straw to the field will reduce winter wheat yield.Returning straw to the field significantly increased the number of wheat ears by 4.3%（P<0.05）,nitrogen application significantly increased the number of grains and ears in wheat ears（P<0.01）,and decreased the thousand grain weight of wheat（P<0.01）.Through path analysis,it can be concluded that the combination of returning straw to the field and applying nitrogen fertilizer increases wheat yield by increasing the number of grains and ears per ear,and the number of ears contributes the most to the formation of wheat yield.2)The combination of returning straw to the field and applying nitrogen fertilizer has an impact on the nitrogen absorption and utilization of wheat.The effect of returning straw to the field on nitrogen accumulation in plants,grains,stems and leaves follows a pattern of"low nitrogen reduction and high nitrogen increase",with a trend of reducing nitrogen accumulation in glumes under high nitrogen.The application of nitrogen fertilizer significantly reduced the nitrogen harvest index of winter wheat（P<0.01）,and straw returning showed a trend of reducing the nitrogen harvest index.The effect of returning straw to the field on nitrogen fertilizer utilization indicators（nitrogen fertilizer partial productivity,nitrogen fertilizer agricultural utilization efficiency,nitrogen fertilizer utilization efficiency,nitrogen fertilizer physiological utilization efficiency）is not significant.With the increase of nitrogen fertilizer input,the nitrogen fertilizer utilization indicators（nitrogen fertilizer productivity,nitrogen fertilizer agronomic utilization efficiency,and nitrogen fertilizer utilization efficiency）of wheat significantly decreased（P<0.05）.3)Returning straw to the field can increase the content of available nutrients in the soil.Returning straw to the field significantly increased the residual amount of nitrate nitrogen in the 0～2m soil layer,ranging from 6.5%to 28.1%（P<0.05）.Straw returning significantly increased the available potassium content in the 0-20 cm and 20-40 cm soil layers by 9.4%and 8%,respectively（P<0.05）.Returning straw to the field reduced the nutrient uptake of N,P,and K by 19.6%,44.3%,and 47.2%（P<0.05）,respectively.The results of Nitrogen balance study showed that under the treatment of N0,N84,S+N0,S+N84,it was in deficit state,and under the treatment of N252,S+N252,N336,S+N336,it was in surplus state.If the annual nitrogen input exceeds 276 kg N·hm^-2,returning straw to the field can fully compensate for the nitrogen deficiency in the system.The phosphorus in the straw returning system is in a surplus state,while the phosphorus in the non returning system is in a deficit state.The potassium balance of the system is in a deficient state,and returning single season corn straw to the field alone cannot fully compensate for the potassium deficiency in the system.4)Returning straw to the field with nitrogen fertilizer can improve soil carbon sequestration capacity.Returning straw to the field and applying nitrogen fertilizer significantly increased carbon accumulation input.The application of nitrogen fertilizer significantly increased the organic carbon storage by 4.73%～8.11%（P<0.05）compared with no nitrogen application,and straw returning could increase the soil organic carbon storage by 2.9%.The application of nitrogen fertilizer significantly improves carbon fixation rate and efficiency,while returning straw to the field significantly reduces carbon fixation efficiency.Returning straw to the field and applying nitrogen fertilizer can increase soil carbon saturation point.5)Returning straw to the field increased soil soluble organic carbon（DOC）by 4.61%;Microbial biomass carbon（MBC）increased by 11.2%;Increase easily oxidizable organic carbon（EO）by 4.51%.With the increase of nitrogen input,DOC shows an increasing trend,soil microbial biomass carbon decreases with the increase of nitrogen application,and EO also shows an increasing trend with the increase of nitrogen application.Returning straw to the field can improve DOC/SOC,microbial entropy,and carbon storage management index.Applying nitrogen fertilizer can increase DOC/SOC,reduce microbial entropy,and improve carbon pool management index.The treatment had no effect on EO/SOC,carbon pool activity,and carbon pool activity index.6)Returning straw to the field increased the content of large aggregates by 4.5%,and applying nitrogen fertilizer increased the content of large aggregates by 11.8%～15.7%compared to not applying nitrogen.The organic carbon content of large aggregates under straw returning significantly increased by 5.2%.The average weight diameter（MWD）and geometric mean diameter（GMD）of aggregates under no return showed a trend of first increasing and then decreasing with the increase of nitrogen level,while under return,they increased with the increase of nitrogen level.Straw return increased the MWD and GMD of aggregates by 8.8%and 7.5%,respectively.Compared with no nitrogen application,the MWD and GMD of aggregates under nitrogen application increased by 14.1%～22.7%and16.8%～23.4%,respectively.In all treatments,the large aggregates contain the majority of organic carbon.Returning straw to the field increased the distribution of organic carbon in the large aggregates,and the application of nitrogen fertilizer increased by 9.7%to 12.9%compared to no fertilization.