Risk Of Nitrogen Runoff Loss From Single-season Paddy Fields In The Middle And Lower Reaches Of The Yangtze River Plain

Nitrogen loss by runoff from paddy fields is one of the main sources of agricultural non-point source pollution.Research on methods and techniques to reduce nitrogen loss in rice fields is of great significance to the development of ecological and environment-friendly agriculture.The middle and lower reaches of the Yangtze River Plain is one of the main production areas of single cropping rice in China.Chaohu region,located in the core area of the middle and lower reaches of the Yangtze River Plain,is a typical region of single cropping rice in the middle and lower reaches of the Yangtze River Plain.The study of nitrogen runoff loss in the Chaohu region has certain reference value for reducing agricultural non-point source pollution in the middle and lower reaches of the Yangtze River Plain.SMNRL model is based on precipitation and temperature data from 1957 to 2019,two-year pot experiment,transplanting interval(June 6 to June 25)and water depth at fertilizer stage.The model was used to simulate nitrogen runoff values of single cropping rice at different transplanting dates and surface water depths in Chaohu region.The optimal transplanting and fertilization dates were selected by adjusting fertilization dates to optimize the production mode and water depth control mode,which could reduce the risk of nitrogen runoff loss in single-season paddy fields in the climatic region of the middle and lower reaches of the Yangtze River Plain.The main conclusions are as follows:(1)Precipitation is the most uncertain factor of nitrogen runoff loss from paddy fields in Chaohu region.Single cropping rice precipitation is the main concentration period of annual precipitation in Chaohu region.The average precipitation of rice season from 1957to 2019 accounts for 51.3%of the annual average precipitation.There was a great difference in inter-annual precipitation of rice season.The maximum value of precipitation of rice season was 6.7 times of that of the minimum value,of which the minimum value was 196.8mm and the maximum value was 1323.5 mm.The maximum value was 6.7 times of that of the minimum value.Single cropping rice growing season is the high occurrence period of extreme precipitation in Chaohu region.Extreme precipitation in rice growing season accounts for 38.6%of the precipitation in rice season,and 73.2%of the annual extreme precipitation occurs in rice growing season.(2)The nitrogen concentration in surface water of paddy field decreased exponentially from the first day after fertilization.The decaying period of nitrogen concentration was 9days in the base fertilizer stage,7 days in the tillering fertilizer stage and 3 to 7 days in the ear fertilizer stage.During the fertilizer period,the adjusted water depth had no significant effect on the nitrogen concentration of surface water,but the adjusted water depth had a significant effect on the nitrogen concentration of surface water in the paddy field.(3)Controlling water depth was more effective than adjusting the date of transplanting to reduce the nitrogen runoff loss.Under the same water depth,the simulated value of nitrogen runoff under LW,MW and HW combinations could be reduced by 0.5,0.9 and 1.8kg/hm~2 by adjusting the transplanting date,respectively.With the same transplanting date and different water depth,the simulated runoff value of LW combined with HW water depth decreased by 3.6 to 5.1 kg/hm~2 and 93.1%to 95.1%,respectively.(4)The nitrogen loss in rice field was mainly occurred in the basal and tillering fertilizer stages.The nitrogen loss in rice field could be effectively reduced by rationally adjusting the fertilization dates of basal and tillering fertilizer.Adjusting only tiller application date could reduce the nitrogen runoff simulation value by 29.0%on average,and adjusting the application date of base fertilizer and tiller fertilizer could reduce the nitrogen runoff simulation value by 36.3%on average.The results showed that the optimal transplanting stage was from June 6 to June 15,the optimal basal fertilizer application stage was from June 5 to June 9 and from June 13 to June 14,and the optimal tillering fertilizer application stage was from June 16 to June 20 and from July 3 to July 4.(5)The optimal yield was 718.9-802.5 g/m~2 when the water depth was 3 to 10 cm during the whole growth period.Rice yield would be significantly reduced under high water depth during the whole growth period.Adjusting water depth and flooding days during fertilizer period had no obvious effect on yield.In conclusion,the effective measures to reduce the nitrogen loss from rice runoff in the Chaohu region are as follows:The water depth should be controlled at 3 to 10 cm during the non-fertilizer period.The water depth should be controlled at 3 to 5 cm during fertilization period.The transplanting dates were June 6 to June 9,June 13 to June 14.The basal fertilizer application dates were June 5 to June 14.The tillering fertilizer was from June 16 to June 20and from July 3 to July 4.