Effects Of Nitrogen Reduction On Annual Yield,utilization Of Nitrogen,radiation,thermal And Water Resources Among Ear/Grain Harvest Summer Maize-winter/Spring Wheat

The North China Plain is the main production area of wheat-maize double cropping system in China.Due to the double limitation of sowing/harvesting period of winter wheat,the maturity and dehydration heat resources of summer maize are tight,resulting in high grain moisture content at maturity stage,which limits the development of mechanical grain harvesting technology and annual mechanization technology of summer maize.At the same time,the groundwater consumption in winter wheat season is serious,and the shortage of water resources has become the first limiting factor for the sustainable development of planting system.The problem of large amount of nitrogen application in highly intensive double cropping production is still prominent,and the utilization rate of nitrogen fertilizer in the current season decreases,which limits the coordinated development of annual yield and resource utilization efficiency.Therefore,it is of great significance to optimize the annual nitrogen and climate resource allocation of maize-wheat double cropping system and explore the efficient planting system of transformational resources to promote the sustainable development of green mechanization of maize-wheat double cropping system in the North China Plain.In this study,winter wheat was transformed into spring wheat to achieve grain harvest of summer maize.The effects of reducing nitrogen fertilizer on the annual yield formation process,nitrogen,radiation,thermal and water use efficiency of summer maize-winter wheat（EHM-WW）and grain harvest summer maize-spring wheat（GHM-SW）were studied.The physiological and ecological mechanisms of reducing nitrogen fertilizer on the annual yield formation,nitrogen utilization,radiation,thermal and water use efficiency of summer maize-winter/spring wheat with ear/grain harvest were elucidated.The suitable nitrogen application rate of annual double cropping system was determined,which provided new technical and theoretical support for the reform of annual mechanized maize-wheat cropping system,nitrogen reduction,stable yield and efficiency increase in North China Plain.The main results are as follows.1.Effects of reducing nitrogen application on the annual yield of ear harvest summer maize-winter wheat and grain harvest summer maize-spring wheat.The grain filling and dehydration time of grain harvest summer maize（GHM）was 23-33days longer than that of ear harvest summer maize（EHM）,and the 1000-grain weight and dry matter accumulation were increased to achieve yield increase,and the grain moisture content was reduced to 22.6%-26.0%,which met the grain moisture content standard of mechanical grain harvest of maize.Appropriate nitrogen reduction significantly improved the photosynthetic performance of maize and wheat,optimized the population structure of wheat,improved the grain filling characteristics,increased the effective spike number,ear/spike grain number and grain weight,and regulated the accumulation and transport of photosynthetic products to increase yield.EHM and GHM were optimized to reduce nitrogen to 201.1-218.3 kg hm^-2and 247.2-248.6 kg hm^-2,respectively,the yield were stable at6.31-10.66×10³kg hm^-2and 7.02-12.00×10³kg hm^-2.Winter wheat（WW）and spring wheat（SW）were optimized to reduce nitrogen to 170.5-173.6 kg hm^-2and 158.8-160.2 kg hm^-2,respectively,the yield were 6.41-6.87×10³kg hm^-2and 4.42-6.23×10³kg hm^-2,respectively.In 2020/21,the yield of summer maize was increased,the yield of spring wheat was stable,and the yield of GHM-SW was 4.6%-26.2%higher than that of EHM-WW（P>0.05）.In2021/22,the yield of summer maize was increased,the yield of spring wheat was decreased,the yield of GHM-SW was 7.6%-15.6%lower than that of EHM-WW,significantly.There were differences in annual yield among nitrogen application levels.The annual yield of EHM-WW and GHM-SW increased by 105.0%-139.7%and 61.8%-99.1%,respectively,with the increase of nitrogen application rate in the two years.There was no significant difference between N_300-240-N_450-360treatments.The linear plus platform model simulation showed that the optimal nitrogen application levels of EHM-WW and GHM-SW were 440.7-461.3 kg hm^-2and 451.4-473.6 kg hm^-2,respectively,and the yield could reach 13.04-17.87×10³kg hm^-2and 11.86-18.42×10³kg hm^-2,respectively.2.Effects of reducing nitrogen application on annual nitrogen use efficiency of ear harvest summer maize-winter wheat and grain harvest summer maize-spring wheat.Summer maize still maintained a certain nitrogen absorption capacity after delayed harvest.Compared with EHM,the total nitrogen（TN）of leaves at GHM harvest stage decreased by 32.4%,and the TN of other organs increased by 8.5%-40.7%,which had an important contribution to the increase of GHM yield.The total nitrogen distribution（TNd）of maize and wheat grains at harvest stage reached 22.8%-34.2%and 57.2%-71.8%,respectively.There was no significant difference in TN distribution between different cropping systems.Compared with WW,the dry matter accumulation and nitrogen uptake and transport capacity of SW were significantly increased.The 20-40 cm soil layer TN of GHM-SW was lower than that of EHM-WW,but there was no significant difference in 0-20 cm soil TN.Compared with EHM-WW,the nitrogen partial factor productivity（PFPN）,nitrogen agronomic use efficiency（ANUE）,nitrogen use efficiency for grain production（NUEg）and nitrogen harvest index（NHI）of GHM-SW decreased by 4.6%-13.5%,6.2%-32.2%,13.8%-31.9%and 2.0%-11.5%,respectively,while total nitrogen accumulation（TNa）and grain nitrogen accumulation（GNa）increased by 8.0%-31.6%and 1.9%-28.2%,respectively.Compared with N_180-120-N_240-180,PFPN,ANUE,NUEg and nitrogen use efficiency for biomass production（NUEb）of N_300-240and N_360-300decreased by 13.3%-47.3%,6.5%-38.8%,4.8%-29.6%and 2.3%-30.1%,respectively,while TNa and GNa increased by 8.6%-92.6%and 10.1%-84.0%,respectively.There was no significant difference in all indexes except PFPN.3.Effects of reducing nitrogen application on the utilization of radiation,thermal and water resources of ear harvest summer maize-winter wheat and grain harvest summer maize-spring wheat.Compared with EHM-WW,the annual climate resources of GHM-SW are inclined to the maize season with high photosynthetic efficiency,which promotes the rational allocation of radiation and thermal resources in the two seasons and creates conditions for high efficiency and high yield.The radiation and accumulated temperature distribution rate of GHM were significantly higher than those of EHM by 33.4%-46.0%and 6.7%-15.7%,respectively.The distribution ratios of maize season and wheat season were increased by 59.9%-84.8%and16.6%-39.2%,respectively.The precipitation and distribution rate of WW in 2020/21 were18.7%and 15.7%higher than those of SW,and decreased by 10.0%and 9.1%in 2021/22.In2020/21,the production efficiency of radiation(PE_Ra)of GHM is 6.4%-20.8%lower than that of EHM,and the production efficiency of water（PE_W）is 10.9%-18.5%higher than that of EHM.The PE_Ra,production efficiency of accumulated temperature(PE_AT)and PE_Wof SW are20.1%-117.1%,7.5%-80.8%and 1.7%-83.8%higher than WW,respectively.In 2021/2022,PE_Wof GHM increased by 3.5%-13.9%,PE_ATand PE_Wof SW decreased by 19.8%-30.7%and5.9%-18.7%.PE_Ra,PE_ATand PE_Win maize season,wheat season and annual increased significantly after nitrogen application.The annual PE_Ra,PE_ATand PE_Wof N_240-180-N_450-360treatment were 12.4%-32.3%,12.2%-32.6%and 12.3%-32.5%higher than that of N_180-120,and there was no significant difference between N_240-180-N_450-360treatments.In conclusion,the traditional planting system in the heat resource-limited area of the North China Plain can achieve stable yield of 13.04-17.87×10³kg hm^-2by reducing nitrogen to 201.1-218.3 kg hm^-2in the summer maize season,170.5-173.6 kg hm^-2in the winter wheat season,and 440.7-461.3 kg hm^-2in the whole year of ear harvest summer maize-winter wheat system.The yield was stabled at 11.86-18.42×10³kg hm^-2by changing the planting system to grain harvest summer maize-spring wheat,with the nitrogen was reduced to 247.2-248.6 kg hm^-2in the summer maize season,158.8-160.2 kg hm^-2in spring wheat season,and451.4-473.6 kg hm^-2in the whole year.At the same time,it can optimize the allocation of annual climate resources,improve the utilization efficiency of nitrogen,radiation,thermal and water resources of annual and single-season crops,reduce the grain moisture content of summer maize,and reduce the water consumption of winter wheat season.It provides theoretical support for the realization of mechanized harvesting of summer maize grains and the annual mechanization of maize-wheat planting system in the North China Plain,annual nitrogen reduction,stable yield and efficiency increase,and efficient and sustainable utilization of annual climate resources.