Winter Wheat Yield Assessment And Optimal Nitrogen Fertilizer Management In The Middle And Lower Reaches Of Yangtze River Under Simulated Carbon Emission Scenarios

Nitrogen fertilizer is one of the essential nutrients for wheat growth and development and plays a vital role in the improvement and stability of wheat yield.China is the world’s largest wheat producer,and N fertilizer applied during wheat cultivation accounts for 20%of global wheat N fertilizer consumption.However,the response of wheat yield to N application is not a simple linear relationship.In addition,the irrational application of N fertilizer（mainly over-application）might reduce wheat production and increase environmental risks.Climate change might affect wheat development and yield and alter the on-farm nitrogen cycle.However,the response of wheat yield and N loss during cultivation to climate change and how to optimize wheat N fertilizer management in climate change still need further research.This study takes the middle and lower reaches of the Yangtze River,one of the significant wheat-producing areas in China,as a case study.We focus on spatialtemporal patterns of the response of wheat yield and N loss to future climate change with the help of a random forest model,using wheat yield records and spatial data on wheat yield,N application,and N loss from agro-meteorological observatories,and proposes different N fertilizer.The study’s results can be used to optimize wheat production under climate change.The study’s results can provide a theoretical basis and reference for optimizing N application,improving yield,and reducing N loss in wheat cultivation under climate change conditions.The main results are as follows.（1）The current average yield of wheat was 5247.9(95%CI:3030.2～6521.3 kg hm^-2);the average N application was 215.8（95%CI:55.9～301.0）kg N hm^-2 in the middle and lower reaches of the Yangtze River.The average annual temperature was 10.2℃,and the average annual rainfall was 1383 mm.The mean values of soil indicators were 21.0 g kg^-1 organic matter,1.1 g kg^-1 total nitrogen,7 pH,1.4 g cm^-3 bulk density,38.6%sand,36.7%silt,and 23.9%clay.The yield estimated model constructed by random forest performed robustly with test sets R² and RMSE of 0.88 and 426.3 kg hm^-2,respectively.The model was used to predict the spatial and temporal distribution of wheat yields in the middle and lower reaches of the Yangtze River region.It was found that the results of the random forest model could capture the temporal variation of wheat yields nicely from 2000 to 2015 in the middle and lower reaches of the Yangtze River region.The results showed that the northern part of the region had a higher wheat yield in 2014,reaching more than 6000 kg hm^-2,while the wheat yield decreases as it goes south.The predicted and actual values have good spatial consistency with differences within 2%.The distribution of differences is relatively uniform without considerable spatial variation,indicating that our random forest model can be better used for spatial prediction.Further analysis of the region’s main factors influencing wheat yield revealed that wheat yield was mainly influenced by mean annual temperature,nitrogen application,latitude and rainfall,and soil factors had less influence on wheat yield.This indicates that wheat yield is more influenced by climate and vulnerable to future climate change.（2）Under two future emission scenarios（SSP126 and SSP585）,there are different degrees of changes in rainfall and temperature in the middle and lower reaches of the Yangtze River region.The spatial distribution pattern of relative changes in annual cumulative rainfall in the middle and lower reaches of the Yangtze River under the SSP126＿2066-2100 scenario is similar to that of the SSP126＿2031-2065 scenario.However,its magnitude of change is relatively more extensive than that of the SSP126＿2031-2065 scenario,which shows a more significant decrease in rainfall in the southwest and a smaller decrease in rainfall in the north.While the regions with less precipitation under the SSP585 scenario have fewer precipitation projections,and the regions with more precipitation projections further increase.The annual mean temperature showed different degrees of increase under both emission scenarios.Wheat yields under different emission scenarios show the same pattern as the historical distribution set high in the north and low in the south.Spatially,wheat yields decreased by about 4%on average in this eastern region and increased slightly in the northern region.Temporally,wheat yields in this region decreased under the SSP126 scenario,and under the SSP585 scenario,wheat yields in this region showed a westward increase and an eastward decrease.For N losses,N losses during wheat cultivation in the region decreased by 5%to 0 under the SSP126 scenario,while N losses increased by about 5%under the SSP585 scenario.Nitrogen losses varied under different pathways,with the most significant decreases predicted under the NH₃ and NO₃^- pathways and increases predicted in the western region under the NO,N₂O,and Nr pathways.Unit nitrogen losses increased by 0-10%in the southwest region and decreased by 0-5%in most remaining regions.（3）We further analyzed the wheat yield response and N loss to N fertilization under climate change and developed different N application strategies.Under future climate change,yield increased and remained constant（-25%to 5%）with increasing on-farm nitrogen inputs.N loss per unit yield increased with increasing on-farm nitrogen inputs（Figure 5-1:-25%to 40%）.The response curves of nitrogen loss per unit yield to nitrogen use and yield response to nitrogen use in the context of climate change only had an intersection point（-0.3）at -0.5 to 0 times nitrogen application,which indicated that the change in yield in the middle and lower Yangtze River was less than the change in nitrogen loss per unit yield for 0 to 0.3 times reduction in nitrogen use;for>0.3 times reduction in nitrogen use,the change in yield was comparable to the change in N loss per unit yield.In contrast,the yield in the region no longer increased at 0.25 times nitrogen use.The increase was smaller than the nitrogen loss per unit of yield;the nitrogen loss per unit of yield increased linearly with nitrogen increase.This implies that the yield benefits of nitrogen increase may come at a higher environmental cost under future climate change.（4）Finally,we analyzed the effects of different nitrogen application strategies on wheat yield,nitrogen loss,cost-benefit,and environmental cost in the context of climate change.The results showed that wheat yields under both SSP126 and SSP585 scenarios exhibited a 25%N increase（S2）>current N application level（S0）>30%N reduction（S1）,but wheat yields under all three N application levels were lower than those in 2014.At the S0 level,N losses during wheat cultivation in the region would increase by 56.1-56.7 kg N hm^-2 by 2100.S1 reduced N losses compared to the 2014 level,but the unit yield but losses at S1(SSP126:9.9 g kg^-1;SSP585:9.1 g kg^-1)were close to the 2014 unit N losses.It indicates that N application,mainly at the abatement level,significantly impacts wheat yield under future climate change.Although the increase in N loss was higher under the S2 N application level,it was generally consistent with the increase in N loss per unit yield,indicating that the corresponding yield gain was obtained with the S2 N application level.For the cost-benefit,the nitrogen application level（S1）,mainly abatement,had a positive nitrogen fertilizer gain of 233.3 yuan ha-1 because of the reduced nitrogen fertilizer input.However,the nitrogen fertilizer gain was significantly lower than the economic loss due to yield decline,which was 1674.7 and 1627,3 yuan hm^-2 for the SSP126 and SSP585 scenarios,respectively.While the nitrogen application level（S2）,which was mainly yield gain,had a positive nitrogen fertilizer gain of 233.3 yuan hm^-2.We quantified the change in environmental costs for S1 and S2 N application levels relative to the S0 N application level.We quantified the environmental costs of S1 and S2 nitrogen application levels relative to S0.The environmental costs of S1 nitrogen application decreased significantly due to the reduction in nitrogen application,decreasing by 378.7 and 373.7 hm^-2 under SSP126 and SSP585 scenarios,respectively.In contrast,the environmental costs of S2 nitrogen application increased significantly by 278.4 and 269.6 hm^-2,respectively.In summary,this study established a robust random forest model for analyzing changes in wheat yield and N losses under climate change in the middle and lower reaches of the Yangtze River region,proposed different optimal N application strategies,and conducted analyses of yield,N losses,cost-benefit and environmental costs for different strategies.The results help to provide a theoretical basis and technical support for the region to cope with future climate change.