Predicting Yield And Nitrogen Use Efficiency For Wheat-maize System In The Northern China

Nitrogen use efficiency(NUE)is a key indicator to assess the N uptake by crops and can be used to address environmental pollution from mineral N input.The NUE of the major crops in China is low,approximately 30%,and the ubiquitous phenomenon of excessive N application and low NUE leads to a high environmental stress.Furthermore,the IPCC's climate scenarios showed that there would be a significant increase in the frequency of extreme weather anomalies over the next 20 to 50 years.But the impacts of future climate changes on crop growth and nitrogen utilization remain to be undetermined.So indicating the changes of crop yields and NUE under different degrees of climate changes in the future with different fertilization strategies can provide important scientific basis for optimizing use of cropland resources and protecting food safety to cope with the climate change.Thus,in this study,seven long-term(more than 20 years)experiments with typical soil and climate characters in the wheat and maize main producing areas of northern China and four climate scenarios,including baseline,RCP(Representative Concentration Pathway)2.6,RCP4.5 and RCP8.5,were chosen to(1)calibrate and validate the SPACSYS model in terms of grain yield and N offtake,and evaluate the SPACSYS model performance of molding them in northern China;(2)quantitatively assess the effects of various climate change scenarios with fertilization strategies on the yield,N offtake and NUE of wheat and maize by 2100 in this region.The results indicate that:1.SPACSYS can adequately simulate crop yields,N uptake for the wheat and maize rotation of northern China.For simulated and observed yields of wheat and maize,the R2 values for model calibration were from 0.40 to 0.70(P<0.01)with the RMSE from 4.5% to14.6% and RE from 1.2% to 8.5%,respectively,and those for validation were from 0.65 to 0.79(P<0.01)with the RMSE from 3.0% to 13.5% and RE from 1.2% to 4.8%.For simulated and observed N offtake of wheat and maize,the R2 values for model calibration were from 0.32 to 0.80(P<0.01)with the RMSE from 3.9% to 6.2% and RE from 1.3% to 8.9%,respectively,and those for validation were from 0.38 to 0.75(P<0.01)with the RMSE from 3.0% to 7.8% and RE from 3.1% to 9.7%.For the NUE of all the fertilizer treatments,there were no significant differences between simulations and observations,and the simulated NUE was from 16% to 80%,which was similar to the observations(from 15% to 87%).2.For winter wheat-summer maize double cropping areas,including Changping(PL),Zhengzhou(ZZ),Xuzhou(XZ)in the north China plain and Yangling(YL)in northwest China,without considering the impact of cultivar change,future climate changes(RCP scenarios with increasing temperature and precipitation,elevating CO2 concentrate)increased wheat yield(average of 8.5%)and reduced maize yield(3.8%)compared with the baseline,and the maize had a more stable yield than wheat.Under RCP climate scenarios,different fertilizer strategies had no significant difference on wheat yield,while had different influence on maize yield and the magnitude of the increments for maize was ordered as: NPK,NPKM < NPKS < h NPKM(P<0.05).The h NPKM treatment had the highest crop yields with the wheat yield from 571.17 to 627.85 g/m2 and maize yield from 663.10 to 741.38 g/m2.Furthermore,the RCP scenarios reduced the annual NUE by an average of 15% for all fertilization treatments compared with the baseline,and the RCP8.5 led the lowest annual NUE(32.00%-39.81%)among the climate scenarios.For each climate scenario,the highest and most stable annual NUE(38.70%-52.78%)with the highest crop N offtake finding in h NPKM treatment until 2100.And high application of manure and straw result in a more stable crop NUE than other treatments.3.For wheat and maize monoculture areas,including Gongzhuling(GZL)in northeast China and Zhangye(ZY)and Pingliang(PL)in northwest China,without considering the impact of cultivar change,future climate changes decreased the maize yield of northwest China(average of 33.5%),while increased that of northeast China(average of 5.6%)compared with the baseline.Wheat yield decreased under most RCP scenarios(except PL under RCP4.5 and ZY under RCP8.5)with the average relative changes ranging from-49% to 11%.Under RCP climate scenarios,there were no significant differences on both wheat and maize yield under different fertilizer strategies.Manure amendments had a slightly higher crop yields(270~595 g/m2 for wheat and 501~1267 g/m2 for maize)than others,while the N offtake by crops for NPKM was significantly higher than others.In addition,future climate changes(except PL under RCP4.5)reduced the NUE in wheat and maize monoculture system compared with baseline with an average reduction of 45% for wheat and 33% for maize in northwest China and 8% for maize in northeast China.Manure amendment could alleviate the negative influences of future climate change with the lowest relative reduction(or highest at PL under RCP4.5)of NUE(4.65%~46.47% for maize and-55.89%~64.32% for wheat)than other treatments under each climate scenario.Different fertilizer strategies only had significantly different influence on maize NUE in the order of: NPK ? NPKM > NPKS(P<0.05).Maize was more adaptable than wheat under future climate changes with a higher NUE and more stable yields.