Study On Runoff Loss Risk And Nutrient Balance Of Nitrogen And Phosphorus In Paddy Field In Western Chongqing Hilly Area Under Optimized Mode Of Chemical Fertilizer Reduction

The problem of agricultural non-point source pollution caused by unreasonable use of fertilizers in agricultural planting industry has been paid more and more attention.Rice,as one of the main food crops in China,has the problems of excessive fertilization and inadequate water storage protection in the process of planting.If runoff occurs,it will aggravate the eutrophication pollution of the surrounding water body to a certain extent.Therefore,how to reduce the amount of fertilizer applied to reduce pollution and not reduce yield,and further improve nutrient recovery and utilization is a major challenge to balance economic benefits and ecological security of rice fields.In this study,a two-year field plot experiment was conducted in Hechuan District,Chongqing City,a typical hilly area in western Chongqing.Two fertilization modes（farmers’conventional fertilization mode,fertilizer reduction optimization mode:rice special fertilizers combined with silicon fertilizer）were set to treat with different fertilization levels,namely,no fertilization（CK）,low gradient of farmers’conventional fertilization（FP-33%）,gradient of farmers’conventional fertilization（FP）,high gradient of farmers’conventional fertilization（FP+50%）,low gradient of rice special fertilizer（OPT-50%）,gradient of rice special fertilizer（OPT）,high gradient of rice special fertilizer（OPT+50%）and equal amount of conventional fertilizer（Con）.The dynamic changes of nitrogen and phosphorus in different forms of paddy surface water under two fertilization modes under different fertilization levels were studied,and the optimal fitting equation of nitrogen and phosphorus concentration in paddy surface water after fertilization was explored.On the basis of the plot experiment,the paddy field water balance model suitable for this study area was established by referring to relevant research and verification optimization parameters.The nitrogen and phosphorus loss load model of rice season runoff was constructed by combining the principle of’The rain-runoff process of paddy field’.The nitrogen and phosphorus loss load of runoff corresponding to different drainage outlet heights was estimated and scenario analysis was set.The field experiment results were used to compare the effects of farmers’conventional fertilization mode and fertilizer reduction optimization mode on rice yield,composition factors and nutrient absorption and utilization,and to explore the best fertilization scheme to achieve weight loss and pollution reduction without yield reduction.It aims to provide theoretical basis and scientific basis for reducing fertilizer application and controlling total agricultural non-point source pollution in Sichuan-Chongqing rice region.The main research results are as follows:1.After fertilizer application,except for nitrate nitrogen（NO₃^--N）peaked at 5 d after fertilization,other nitrogen and phosphorus forms peaked at 1-3 d after fertilization.In different forms of nitrogen and phosphorus,total nitrogen（TN）and total phosphorus（TP）were mainly dissolved,and dissolved total nitrogen（TDN）and dissolved total phosphorus（TDP）accounted for71.0%and 75.8%,respectively.There was a significant linear correlation between nitrogen application level and TN in surface water and between phosphorus application level and TP in surface water,respectively.NH₄⁺-N/TN indicated that the nitrogen conversion level of fertilizer reduction optimization mode was lower than that of farmers’conventional fertilization mode.Taking the amount of fertilizer and days after fertilization as independent variables,the fitting effect of TN and TP in surface water of the two fertilization models was the best in exponential form.2.Nitrogen and phosphorus loss load of paddy field runoff was mainly affected by the height of drainage outlet,rainfall and fertilization.In the same year,the nitrogen and phosphorus loss load of rice season runoff gradually decreased with the increase of drainage height.The runoff nitrogen and phosphorus loss load of fertilizer reduction optimization mode under different drainage heights was lower than that of farmers’conventional fertilization mode.Under the same fertilization rate,compared with farmers’conventional fertilization mode,fertilizer reduction optimization mode can reduce the nitrogen loss load by 10.4%and the phosphorus loss load by 7.9%on average.The estimation of nitrogen and phosphorus loss in paddy runoff under conventional fertilization mode in each township of Hechuan District showed that the load range of nitrogen runoff loss in each township was 1.99–2.05 kg hm^-2 at 6 cm drainage outlet.The high-risk areas of nitrogen runoff loss were concentrated in the middle of the region,and the low-risk areas were the towns in the eastern region and the streets of Heyang City.The number of towns with high and above risk of nitrogen runoff loss in Hechuan District accounted for half,and the high-risk towns accounted for 43.3%.When the drainage outlet increased by 2 cm,the number of risk towns with high nitrogen runoff loss and above decreased by 33.3%.The runoff loss of phosphorus in townships of Hechuan District was much smaller than that of nitrogen.The high and low risk areas of phosphorus runoff loss were similar to the distribution of nitrogen runoff loss.The high risk townships accounted for 40.0%,and the high risk townships accounted for 20.0%.However,when the drainage outlet increased by 2 cm,the phosphorus runoff loss load township decreased by about 37.0%.Longshi Town has the highest risk of nitrogen and phosphorus loss in paddy runoff under farmers’conventional fertilization mode.The simulation of nitrogen and phosphorus loss in runoff under fertilizer reduction optimization mode showed that the model reduced the risk of nitrogen and phosphorus loss in each region by reducing the nitrogen and phosphorus loss load,thus reducing the risk of nitrogen and phosphorus loss in regional paddy runoff.3.The results showed that OPT treatment could not reduce yield in the short term on the basis of46.5%reduction of nitrogen fertilizer and 22.4%reduction of phosphorus fertilizer.The change of soil nutrients showed that the change of paddy soil was in a state of consumption during the experiment.Except for available phosphorus,there was little difference in soil nutrients among the fertilization treatments after the experiment.The results of nutrient absorption and utilization showed that the agronomic efficiency（AE）of total nutrient fertilizers was 6.77–23.42 kg kg^-1,the partial fertilizer productivity（PFP）of total nutrient fertilizers was 24.82-112.49 kg kg^-1,the nitrogen recovery efficiency（NRE）was 16.4%–45.2%,and the phosphorus recovery efficiency（PRE）was36.8%–141.5%.Under the same fertilizer rate,AE and PFP of OPT+50%treatment increased by22.3%and 7.0%respectively compared with Con treatment.NRE of OPT treatment was higher than FP treatment,and NRE of OPT+50%treatment was increased by 6.2%compared with Con treatment.In 2020,the PRE of OPT-50%treatment was significantly higher than that of FP treatment,and the PRE of OPT treatment was increased by 87.1%compared with FP treatment.The nitrogen balance of paddy field system under different treatments was in surplus state,so some nitrogen was lost in different ways.The nitrogen storage of paddy soil changed from positive to negative,indicating that the paddy soil was in a state of consumption as a whole during the experiment,and the soil nitrogen fixation ability was weak.The apparent surpluses of soil phosphorus in different treatments were in a loss state,and the loss decreased with the increase of phosphorus application rate.Compared with FP treatment,OPT treatment could increase the aboveground phosphorus accumulation on the basis of phosphorus reduction.