Study On Characteristics And Regulation Of Nitrogen Migration In Direct-seeding Rice Field In Ningxia Yellow River Irrigation Area

In order to solve the problem that the characteristics of nitrogen migration in direct-seeding rice field of Yellow River irrigation area are not clear,this study studied the nitrogen leaching loss,crop absorption and the distribution of inorganic nitrogen content in soil profile after harvest in direct-seeding rice field of Yellow River irrigation area by the method of laboratory experiment and field experiment.Six nitrogen levels were set up:no nitrogen application（CK）,low nitrogen application 120kg/hm²（N1）,medium nitrogen application 210kg/hm²（N₂）,recommended nitrogen application 240kg/hm²（N₃）,high nitrogen application 360kg/hm²（N₄）,and controlled-release nitrogen application 180kg/hm²（OPT）.Based on the analysis of water balance characteristics of direct drought rice field,the nitrogen leaching loss of rice field under different nitrogen application levels was calculated,and the internal relationship between nitrogen leaching loss and non-point source pollution load was analyzed.It was determined that controlled release fertilizer could reduce nitrogen pollution to surrounding water and improve fertilizer utilization rate,providing theoretical basis for rational application of nitrogen fertilizer and control of non-point source pollution in dry direct-seeding rice field in Yellow River irrigation area.The main conclusions are as follows:（1）The characteristics of water balance in direct-seeding paddy fields were monitored.The input was irrigation,and the output included leakage,evaporation and rice absorption.During the growth period of rice in 2021,the irrigation amount was 18 times,and the total irrigation amount was 214,46m3/hm².Lateral infiltration,direct infiltration,evaporation and crop absorption accounted for 25.59%,17.31%,26.57%and 30.53%of the irrigation amount,respectively.The loss of lateral infiltration and direct infiltration was the main form of water loss,accounting for 42.90%of the irrigation amount.（2）The concentrations of TN,NH_4⁺-N and NO_3^--N in surface water under different nitrogen levels fluctuated with fertilization and showed a general downward trend.TN and NH_4⁺-N concentrations in surface water reached the maximum value 3 days after topdressing,then gradually decreased,and decreased to the level before fertilization 8 days later.The concentration of TN in 0～20cm leached water increased with the increase of topdressing urea.The peak value of TN concentration in leached water at 60cm was also observed with two topdressing,but the peak value was much smaller than the peak value at 20cm,and the change of TN concentration at 100cm was not affected by fertilization.The concentration of NH_4⁺-N in leachate water of each soil layer was significantly lower than that of NO_3^--N.The difference of TN leaching loss between different N levels and N treatments was considered as the net leaching loss of N fertilizer.TN leaching loss was 7.62～42.36kg/hm².N1、N₂、N₃、N₄ and OPT accounted for 6.35%、12.97%、13.15%、11.77%and 4.36%of nitrogen application rate,respectively.Nitrogen leakage mainly occurred at the seedling stage,and TN leaching loss could be significantly reduced after OPT was regulated by nitrogen transfer technology compared with N₂ and N₃ treatment.The apparent loss rate of nitrogen increased with the increase of fertilizer application amount The apparent loss rate of nitrogen in the four conventional nitrogen treatments ranged from 39.05%to 51.25%.The loss rate of controlled release fertilizer treatment was 35.13%,which was significantly lower than that of conventional urea treatment（45.12%）.（3）Nitrogen application significantly increased the grain yield of rice,while OPT decreased by 14.28%and 25.00%compared with N₂ and N₃,and there was no significant difference in grain yield among the three treatments.The fertilizer utilization rate of controlled release fertilizer was 40.14%.Nitrogen uptake by straw and grain was significantly increased compared with no nitrogen application,and the total nitrogen uptake ranged from 64.39%to 197.77%.In the study of nitrogen-based fertilizer direction,it was found that when applying conventional nitrogen fertilizer,the recovery rate in crop system decreased with the decrease of nitrogen application rate,and the crop nitrogen recovery rate was 27.78%-71.37%.The recovery rate of absorbed was 69.08%after controlled release fertilizer was applied.（4）In the paddy field soil profile（0-100cm）,the contents of NH_4⁺-N and NO_3^--N were highest in the 0-20cm soil layer after rice harvest,and decreased gradually with the increase of soil depth.With the increase of fertilizer application,the contents of NH_4⁺-N and NO_3^--N increased in the same soil layer under each treatment The NO_3^--N content in the same soil profile was significantly higher than that of NH_4⁺-N.There was no significant difference in the distribution of NH_4⁺+-N and NO_3^--N contents in the soil profile of OPT compared with N₂,N₃ and OPT,while there was a significant difference in the distribution of NO_3^--N contents in the soil profile of N₄ compared with N₂,N₃ and OPT.With the increase of the amount of fertilizer application,the content of NO_3^--N in the soil profile of 0-100cm increased significantly.The results showed that high nitrogen application increased the nitrate accumulation in deep layer of soil profile and increased the risk of leaching loss.