Long Term Nitrogen And Phosphorus Fertilizer Application Rates Effects Winter Wheat-Summer Maize Cropping System On Crop Yield And Soil Fertility In Dryland

In the production process of winter wheat and summer maize in the Guanzhong Plain of Shaanxi,farmers usually use a large amount of nitrogen and phosphorus fertilizers for two consecutive seasons to fertilize the fields on the basis of returning winter wheat straw to the field.In actual production,there are often excessive inputs of nitrogen and phosphorus fertilizers and inputs of it in unreasonable distribution ratios.This long-term unreasonable fertilization has exacerbated the unbalanced development of soil nutrients in the farmland ecosystem,leading to the deterioration of the farmland ecological environment,the reduction of resource utilization,and continuously declines the productivity of the farmland.At present,the reduction of fertility of farmland in this production system is a serious problem for the sustainable production of crops,and it is necessary to actively seek solutions.Based on a long-term（2009 to 2019）in situ fertilization experiment in a winter wheat and summer maize cropping system in Guanzhong Plain,this carried out research on the dynamic monitoring and regulation technology of farmland quality in winter wheat-summer maize cropping system under the high-intensity input of nitrogen and phosphorus fertilizers.The experimental design was split-plot design with five nitrogen fertilizer rates(0,75,150,225,and 300 kg N ha^-1,referred to as N₀,N₇₅,N₁₅₀,N₂₂₅,and N₃₀₀),and four phosphate fertilizer rates(0,60,120,180kg P₂O₅ ha^-1,referred to as P₀,P₆₀,P₁₂₀,and P₁₈₀),for a total of twenty combined treatments.This study investigated the effects of long-term combined application of different nitrogen and phosphorus fertilizers on crop yield,soil carbon,nitrogen and phosphorus accumulation,and greenhouse gas emissions in winter wheat-summer maize cropping system,in order to provide nutrient optimization management solutions for this cropping system,and then coordinate the multi-objective development of high crop yield,increasing soil fertility and environmental benefits,and promote the sustainable production of this production system.The result showed that:（1）Based on long-term monitoring of soil moisture and crop productivity levels at depths of 0-300cm under different nitrogen and phosphorus fertilizer application combinations,we found that different combinations can affect the productivity of crops and the level of soil dryness at deeper levels in the soil.This severely affects the sustainable production of winter wheat and summer maize in the system.To control winter wheat yield at 7000 kg ha^-1 and summer maize yield at 8000 kg ha^-1,it is recommended that nitrogen fertilizer application during the winter wheat growing season should not exceed 150 kg N ha^-1 and 100 kg P₂O₅ha^-1,and for summer maize,nitrogen fertilizer application should not exceed 180 kg N ha^-1 and100 kg P ha^-1.These results are helpful in adjusting crop productivity to an appropriate level while preventing excessive depletion of soil moisture at deep depths,thereby achieving sustainable production of crops in the production system.（2）Analyzing the carbon fixation and emission in the winter wheat-summer maize double cropping system under different nitrogen and phosphorus fertilizer combinations,it was found that the carbon budget of the production system in the planting cycle under N0P0,N₀P₆₀,N₇₅P₆₀,N₇₅P₁₂₀,N₁₅₀P₆₀,and N₁₅₀P₁₂₀ treatments were the"sink"of atmospheric carbon dioxide,and was the"source"of atmospheric carbon dioxide under the other combined treatments.Appropriate combined application of nitrogen and phosphorus fertilizers(N₇₅,N₁₅₀,P₆₀,P₁₂₀)significantly increased the amount of carbon sequestration in crops and at the same time significantly reduce soil heterotrophic respiration,which is beneficial to soil carbon fixation.While high nitrogen(N₂₂₅,N₃₀₀)and high phosphorus(P₁₈₀)treatment significantly increased crop carbon sequestration,but significantly increased soil heterotrophic respiration,which was not conducive to soil carbon sequestration.Under different nitrogen fertilizer levels,the soil organic carbon storage in the 0-100cm soil layer was higher than the initial SOC storage under the N0,N75 and N150 treatments;the SOC storage of the plow layer under the N255 and N300 treatments was lower than the initial SOC storage,and the carbon fixation rates were-1.37 and-0.74Mg ha-1yr-1.Under different phosphate rates,the soil carbon sequestration showed a trend of firstly increasing and then decreasing,reaching the maximum value at P60 treatment.It shows that the application of appropriate phosphorus fertilizer can increase the amount and rate of soil carbon sequestration,but the excessive application of nitrogen and phosphorus fertilizers will reduce the amount and rate of soil carbon sequestration and deplete soil carbon pool.（3）Analyzing the accumulation of aboveground nitrogen,annual N₂O emissions,nitrate nitrogen in the 0-300 cm soil layer,and total nitrogen in the 0-100 cm soil layer of crops in winter wheat summer maize double cropping system,it was found that appropriate application of nitrogen and phosphorus fertilizers significantly increased the aboveground nitrogen accumulation and NHI of winter wheat and summer maize.Long-term combined application of different nitrogen-phosphorus fertilization has resulted in diverse soil environments with accumulated nitrate nitrogen.Soil nitrogen storage and nitrate accumulation decreased when no nitrogen was applied or when 75 kg ha^-1 of nitrogen was applied.Soil nitrogen and nitrate reached a basic equilibrium state when 150 kg ha^-1 and 225 kg ha^-1 of nitrogen were applied.Under the N₃₀₀ treatment,nitrate accumulation significantly increased,with severe nitrate accumulation in deep soil layers.When the accumulated nitrate nitrogen in the 0-100 cm layer was controlled within 100 kg ha^-1,soil N₂O emissions and accumulated nitrate nitrogen in the100-300cm layer were relatively low.Based on the residual nitrate nitrogen threshold,it is recommended to apply 156 kg ha^-1 of nitrogen and 110 kg ha^-1 of phosphorus for winter wheat and 165 kg ha^-1 of nitrogen and 100 kg ha^-1of phosphorus for summer maize was basically meet the nitrogen absorption of crops,the N₂O emissions was lower,and maintain the nitrogen supply capacity of the root zone（0-100 cm soil layer）,prevent the accumulation of nitrate nitrogen in the deep soil,and still maintain a high grain yield.（4）Analyzing the aboveground phosphorus accumulation of crops,soil total phosphorus in the 0-100cm soil layer,and available phosphorus in the 0-300cm soil layer in the winter wheat-summer maize double cropping system,it was found that long-term different applications of nitrogen and phosphorus fertilizers have resulted in diverse soil environments for total and available phosphorus accumulation.Without phosphorus fertilizer,both total and available phosphorus in the soil was in a state of depletion.Soil total phosphorus accumulation gradually increases with phosphorus application rates of 60,120,and 180kg ha^-1 in the 0-100cm soil layer.Due to the easy adsorption and fixation of phosphate ions in calcareous soils,the overall available phosphorus in the 0-300cm soil layer decreases with increasing years,but the overall available phosphorus content in the soil profile under P₁₂₀ and P₁₈₀ phosphorus application levels is higher than that under P₀ and P₆₀ levels.The activation coefficient of available phosphorus in the soil can be significantly increased by applying 0,75,and 150 kg ha^-1 of nitrogen fertilizer at the same phosphorus fertilizer level,but when the nitrogen application rate exceeds 225 kg ha^-1,the soil phosphorus activation coefficient decreases.Maintaining available phosphorus in the soil at 13.40 mg kg^-1 after winter wheat harvest and16.26 mg kg^-1 after summer maize harvest can reach the higher crop yields.On this basis,continuing to increase the available phosphorus content in the soil has no significant effect on the increase in yield for both winter wheat and summer maize.（5）Analyzing the pollutant emissions throughout the lifecycle of fertilizer production and use under different nitrogen-phosphorus fertilizer combinations in winter wheat-summer maize double cropping system,we found that nitrogen application is the main cause of increased carbon footprint,with an average increase in carbon emissions of-123%-343%.While phosphorus application can significantly reduce the1 carbon emissions,the reduction magnitude was 76%-+14%.The combined application of nitrogen and phosphorus fertilizer significantly increase crop yields（by 30%-33.5%）,leading to a greater decrease in carbon emission per unit yield（decreasing83%-16%）.Therefore,we believe that appropriate nitrogen and phosphorus fertilizer combinations are helpful for low-carbon production in this ecosystem.Our results highlight the importance of appropriate nitrogen and phosphorus fertilizer combinations in improving productivity and reducing environmental harm in this intensive management system,providing theoretical foundations and practical insights for a low-carbon,profitable,and environmentally sustainable production system of winter wheat-summer maize double cropping system.