Effects Of Reduced Nitrogen Application On Soil Water And Fertilizer Utilization,yield And Nitrogen Balance Of Winter Wheat In Dryland

Weibei dryland is one of the main grain producing areas in China.The shortage of water resources caused by low precipitation and uneven seasonal distribution and low soil fertility caused by long-term soil erosion are important factors that limit the sustainable development of agriculture in the region.Nitrogen is an important factor in the growth and development of dryland crops that affects the formation of yield and quality.Rational application of nitrogen fertilizer is conducive to alleviate adverse effects of drought and promoting crop yield and quality.However,excessive nitrogen application is very common in agricultural production.According to the survey results,49.1%of farmers in this area had excessive application of nitrogen fertilizer,and the proportion was still increasing year by year.Excessive application of nitrogen fertilizer did not bring sustained high yield and efficiency,but caused excessive consumption of limited soil moisture in the early growth stage,exacerbated drought stress in the middle and late growth stages of growth,adversely affected yield and quality,and reduced nitrogen use efficiency in farmland.In addition,excessive application of nitrogen fertilizer can also have a negative impact on the environment.The accumulation of nitrate in soil can pollute surface water and groundwater,and increase greenhouse gas emissions from dryland farmland,which is not conducive to the green development of agriculture.The phenomenon of excessive nitrogen application in dryland farmland needs general attention,and making farmers in the region reduce the amount of nitrogen fertilizer application through reasonable guidance is the focus of future fertilization in dryland fields.Therefore,from 2017 to 2021,a field-positioned nitrogen application experiment was carried out in Heyang County,Shaanxi Province for four consecutive years.Jinmai 47 and Chang 6359 were used as test materials,and the local conventional high nitrogen application rate of 240 kg·hm^-2was used as the control.There are four nitrogen reduction gradients with nitrogen reduction by 25%,conventional nitrogen reduction by 50%,conventional nitrogen reduction by 75%and no nitrogen fertilizer application,represented by N₁,N_0.75,N_0.5,N_0.25 and N₀,respectively.Effects of nitrogen application on soil water and fertilizer utilization,yield performance,soil nitrogen balance,and to clarify the optimal nitrogen application amount for the sustainable development of wheat fields in Weibei dryland.The results show that:（1）Nitrogen reduction had a significant effect on the water consumption of different varieties of winter wheat in the 0-200 cm soil depth.A moderate increase in nitrogen fertilizer application can promote the growth of wheat,promote the absorption of soil water by the root system,and increase the water consumption during the growth period.Compared with N₀treatment,the total water consumption under N_0.5,N_0.75 and N₁treatment of Jinmai 47 and Chang 6359 increased by 3.6%,7.2%,6.3%（Jinmai 47）and 10.3%,11.4%,13.2%,respectively.The water use efficiency（WUE）and precipitation use efficiency（PUE）increased first and then decreased with the increase of nitrogen fertilizer application rate.N_0.75 was beneficial to improve the water use efficiency and precipitation use efficiency of Jinmai 47,and its water use efficiency and precipitation use efficiency were increased by 14.5%,7.7%（WUE）and 21.6%,6.5%（PUE）,respectively,compared with N₀ and N₁ treatments.However,N_0.5was conducive to improving the water use efficiency and precipitation use efficiency of Chang 6359,and its water use efficiency and precipitation use efficiency were 14.7%,9.3%（WUE）and 33.9%,9.3%（PUE）higher than those of N₀ and N₁.（2）After a 4-year experiment,with the increase of nitrogen fertilizer application rate,the soil organic matter content of dryland wheat field increased first and then decreased.The soil organic matter content of Jinmai 47at 0-60 cm soil depth was higher under N_0.75 treatment,which was 9.1%and 4.0%higher than that of N₀ and N₁,respectively.The soil organic matter content of Chang 6359 at 0-60 cm soil depth was higher under N_0.5 treatment,which was 5.9%and 2.4%higher than that of N₀ and N₁,respectively.With the increase of nitrogen fertilizer application rate,the soil total nitrogen content showed an increased trend.The soil total nitrogen content at 0-60 cm soil depth under N_0.5,N_0.75 and N₁ treatments in Jinmai 47 and Chang 6359 increased by 0.1%,1.2%,2.5%（Jinmai 47）and 0.6%,0.9%,2.2%（Chang 6359）,respectively,compared with pre.The contents of total phosphorus,total potassium,available phosphorus and available potassium showed a decreased first and then increased with the increase of nitrogen application rate.（3）Reducing nitrogen application had a significant effect on grain yield of different wheat varieties.N_0.75 was beneficial to improve grain yield of Jinmai 47,which was 21.6%and 6.5%higher than that of N₀ and N₁,respectively.The average increase rate of each treatment was N_0.75>N_0.5>N₁>N_0.25,which was 9.2%(N_0.25),21.3%(N_0.5),21.6%(N_0.75),and 14.2%（N₁）higher than that of N₀,respectively.N_0.5 was beneficial to increase grain yield of Chang 6359,which was 33.9%and 9.3%higher than N₀ and N₁,respectively.The average increase rate of each treatment was N_0.5>N_0.75>N₁>N_0.25,which was 16.8%(N_0.25),33.9%(N_0.5),32.7%(N_0.75)and 22.5%（N₁）higher than N₀,respectively.By analyzing the effect of reduced nitrogen application on dryland wheat yield,it was found that N_0.75 was conducive to improving the yield stability and sustainability of Jinmai 47,while N_0.5 was conducive to improving the yield stability and sustainability of Chang 6359.（4）Reduced nitrogen application had a significant effect on nitrogen absorption and utilization of different varieties of winter wheat.Recovery efficiency of nitrogen（REN）,agronomic efficiency of nitrogen（AEN）and nitrogen use efficiency（NUE）showed a trend of’first increase and then decrease’with the increase of nitrogen fertilizer application rate,while nitrogen partial productivity（PFPN）decreased with the increase of nitrogen fertilizer application rate,and nitrogen demand for 100 kg grain（NRG）increased with the increase of nitrogen fertilizer application rate.By analyzing the effect of reduced nitrogen application on nitrogen uptake of dryland winter wheat,it was found that N_0.75 was beneficial to improving nitrogen uptake and utilization of Jinmai 47.Compared with N₁ treatment,nitrogen recovery efficiency,REN,AEN,PFPN,NUE and nitrogen uptake increased by 65.7%,184.6%,42.8%,24.6%and 2.4%,respectively.N_0.5 was beneficial to improving nitrogen uptake and utilization of Chang 6359.Compared with N₁ treatment,REN,AEN,PFPN,NUE and nitrogen uptake increased by 68.8%,259.1%,122.5%,20.9%and 9.5%,respectively.（5）With the extension of the experiment years,the content of nitrate nitrogen in dryland field increased year by yea and moved to the deep soil.Compared with N₀ treatment,the content of nitrate nitrogen in N_0.25,N_0.5,N_0.75 and N₁under Jinmai 47 was increased by 12.5-81.0 kg·ha^-1,56.6-145.0 kg·ha^-1,30.6-107.9 kg·ha^-1 and 43.4-165.9 kg·ha^-1.The content of nitrate nitrogen in N_0.25,N_0.5,N_0.75 and N₁ under Chang 6359 increased by 13.2-85.3 kg·ha^-1,37.2-104.6 kg·ha^-1,34.0-108.3 kg·ha^-1 and 56.4-223.5 kg·ha^-1than the N₀ treatment.Except N₁ treatment,the accumulation of soil nitrate nitrogen in 0-100 cm soil layer of other treatments met the standard of soil nitrate nitrogen residue after harvest.Soil nitrogen surplus of Jinmai 47 and Chang 6359 changed from negative to positive with the increase of nitrogen application rate,that is,soil nitrogen pool changed from loss to surplus.In winter wheat growing season,when nitrogen fertilizer application rate was 0-60 kg·ha^-1,soil nitrogen pool was deficit,when nitrogen fertilizer application rate was 120-240 kg·ha^-1,soil nitrogen pool was surplus.Comprehensive consideration of winter wheat grain yield,economic benefits and environmental benefits,the best nitrogen fertilizer application range is 118.2-163.5 kg·ha^-1.In conclusion,N_0.75(180 kg·ha^-1)treatment under Jinmai 47 and N_0.5(120 kg·ha^-1)treatment under Chang 6359 were beneficial to promoting water use efficiency and precipitation use efficiency,improving nitrogen absorption and utilization in fields.Considering the high yield,high efficiency and low nitrogen loss of fields of different varieties,the optimal nitrogen fertilizer application range in the Weibei dryland was 118.2-163.5 kg·ha^-1.