Study On Utilization Of Nitrogen Rotation And Nitrogen Fertilizer Input Threshold In Rape Rice Rotation In Chongqing Area

Rape-rice rotation system in the upper reaches of the Yangtze River（about 55%of the Yangtze River basin area）is one of the most widely distributed typical grain farming systems in the Yangtze River basin,and has important strategic significance for China’s food security and green agricultural production.Reasonable fertilizer management has always been a key to significant crop yield.However,in the current agricultural production,the blind use of fertilizer phenomenon is very common.According to the latest research,about 6000 Gg of nitrogen is directly lost after entering the water body in the farmland crop production of the Yangtze River Valley every year,among which rape accounts for about 7.95%,and rice even accounts for22.31%.Therefore,rice and rape are one of the main contributors to the nitrogen load in the Yangtze River basin.Therefore,in Chongqing area,as one of the important components of the upper reaches of the Yangtze River basin,it is very urgent and necessary to determine the input threshold of chemical nitrogen fertilizer of its rape and rice rotation system.At present,most of the research on nitrogen fertilizer input threshold is concentrated on single-season crop rice or rape,while the nitrogen fertilizer input threshold research of rape-rice rotation has not been reported,and the research carried out in Chongqing is rare.In this paper,the random block design was used in the typical rape-rice rotation area in Chongqing from 2018 to 2019.The effects of different chemical nitrogen fertilizer inputs on yield and its constituent factors,nitrogen uptake and utilization,nitrogen residue and nitrogen balance of rape-rice rotation crops in Chongqing area were studied.The input threshold of chemical nitrogen fertilizer in rape-rice rotation system in Chongqing area was constructed based on food security,economic benefit and environmental benefit.In order to provide a scientific basis for the rational application of nitrogen fertilizer in rape and rice rotation in Chongqing area,so as to achieve the purpose of high crop yield,environmental protection and agricultural sustainable development.This not only helps to realize green,high yield and high efficiency of grain and oil crops,but also is very necessary for the maintenance of agricultural ecological environment safety in our country.The main results of this paper are as follows:（1）The appropriate amount of chemical nitrogen fertilizer input is conducive to the accumulation of crop dry matter and nitrogen in the rape-rice rotation system in Chongqing area.During the whole growth period,the dry matter and nitrogen uptake of crops in rape-rice rotation system tended to increase with the growth period.In this study,increase chemical nitrogen fertilizer input increase that aboveground dry matter accumulation by 80.05%-126.22%and 22.30%-66.25%in the rape and rice seasons,respectively,and increased the nitrogen uptake by 39.34-77.67 kg·hm^-2and 31.24-138.87 kg·hm^-2,respectively,reach 94.87%-187.28%and 25%-111.66%.（2）The appropriate chemical nitrogen fertilizer input can optimize the nitrogen utilization efficiency in rape and rice rotation system in Chongqing area.For the rape season,the partial productivity of nitrogen,agronomic utilization rate of nitrogen and apparent utilization rate of nitrogen presented the trend of N300<N225<N150<N75 with the increase of chemical nitrogen fertilizer input.Among them,the partial productivity of nitrogen fertilizer in rape season differed significantly among all treatments;significant differences were found in agronomic and apparent nitrogen use efficiency between nitrogen application and non-nitrogen application.Compared with N75 treatment,the apparent N use efficiency,N partial productivity and agronomic efficiency decreased by 40.70%–50.65%,44.51%–67.43%and 37.06%–57.13%in the rape season,respectively.There was no significant difference in the physiological utilization rate of nitrogen among the treatments of rape.The changing rules of partial productivity and agronomic utilization rate of nitrogen fertilizer in rice season were similar to those in rape season.Among them,the partial productivity of nitrogen fertilizer in rice season varied significantly among different treatments.The agronomic use efficiency of nitrogen fertilizer only under N300 treatment was significantly different from that under N150 and N0 treatments,with decreases of44.81%–70.99%and 10.86%–44.76%respectively as compared with that under N75treatment.There was no significant difference in apparent nitrogen use efficiency between treatments.The change rule of nitrogen physiological utilization rate was N75<N150<N300<N225,and the difference was significant between the nitrogen application treatment and no nitrogen application treatment.Compared with the N75treatment,the increase amplitude was 10.53%–59.51%.（3）The application of chemical nitrogen fertilizer at 0–225 kg N hm^-2can significantly increase the crop yield in the rape and rice rotation system in Chongqing area,but beyond 225 kg N hm^-2,the crop yield does not increase significantly in two seasons.Among the nitrogen treatments,300 kg N hm^-2in the rape season and 225 kg N hm^-2in the rice season obtained the highest yield,which increased by 125.74%in the rape season and 37.74%in the rice season.A fertilizer effect equation based on the relationship between chemical nitrogen input and crop yield in a rape-rice rotation system（using a linear plus platform model）.Taking 95%of the highest crop yield and the highest yield as confidence intervals,the yield thresholds of rape in the rape-rice rotation system in Chongqing area were calculated to be 2238.04–2355.83 kg·hm^-2,and the corresponding chemical nitrogen fertilizer input was 165.35–183.38 kg·hm^-2.The yield threshold of rice was 12058.19～12692.83 kg·hm^-2,and the corresponding chemical nitrogen input was 156.45～193.82 kg·hm^-2.（4）Increasing the amount of chemical nitrogen fertilizer can significantly increase the farmers’income in the rape and rice rotation system in Chongqing area.In this study,the relationship between the amount of chemical nitrogen fertilizer input and the net income of rape and rice（R²=0.914 in the rape season and R²=0.898 in the rice season）was quadratic without considering the labor input.Based on the marginal analysis of farmers’net income,the economic threshold of rape in the rape-rice rotation system in Chongqing area was 11312.22～11907.60 yuan·hm^-2,and the corresponding chemical nitrogen fertilizer input was 172.41～249.30 kg·hm^-2.The economic threshold of rice was 28221.29～29706.62 yuan·hm^-2,and the corresponding chemical nitrogen input was 135.17～246.61 kg·hm^-2.（5）There was a linear correlation between chemical nitrogen fertilizer input and soil nitrogen loss（R²=0.860 in rape season and R²=0.925 in rice season）.Based on the soil nitrogen balance theory,the environmental threshold of the region was obtained.When the rape nitrogen application rate was 83.11～218.57 kg·hm^-2,and the rice nitrogen application rate was 155.47～245.32 kg·hm^-2.It could be used as the chemical nitrogen input threshold to balance the relationship between plant nitrogen absorption and environmental benefits in the rape-rice rotation system in Chongqing area.（6）Through comprehensive analysis of target yield,economic benefit and environmental benefit in rape-rice rotation system of the Chongqing area,the chemical nitrogen input threshold of rape in rape-rice rotation system of the upper reaches of the Yangtze River is 165.35～183.38 kg·hm^-2.Under this condition,the obtained yield is 2238.04～2355.83 kg·hm^-2,the applied amount of soil nitrogen apparent loss is 33.07～40.33 kg·hm^-2,and the benefit range of farmers is 1197.82～11469.95 yuan·hm^-2;the input threshold of chemical nitrogen fertilizer for rice was156.45～193.82 kg·hm^-2.Under this condition,the target yield of rice was12058.50～12692.83 kg·hm^-2,the apparent loss of soil nitrogen was 0.69～27.15kg·hm^-2,and the benefit range of farmers was 28734.43～29373.33 yuan·hm^-2.