Effects Of Water And Nitrogen Regulaion On The Migration, Transformation And Utilization Of Nitrogen And Phosphorus In Paddy Field

The impact of agricultural non-point source pollution on the environment is increasing year by year,seriously threatening the water quality safety of groundwater,rivers,and lakes.The loss of nitrogen（N）and phosphorus（P）driven by precipitation and irrigation from rice fields contributes most to agricultural non-point source pollution.Optimizing water and nitrogen management strategies is imperative in order to reduce the threat of N and P losses in the northern paddy fields to environmental pollution of surrounding water bodies,while ensuring the safe of rice production.The present study was conducted from 2018 to 2020 at the Irrigation New Technology Experiment Station of Dandong City,Liaoning Province.The field experiment was established in a split-plot design.The main plots were two irrigation regimes:continuously flooded irrigation（CF）and alternate wetting and drying irrigation（AWD）;the sub-plots were four N managements:no N fertilizer（N0）,conventional N fertilizer（NU）;zeolite application（NZ）;controlled-release N fertilizer application（NP）,eight treatments with three replicates.In this study,we compared the dynamics and loss characteristics of N and P in the surface water,leachate,and runoff,and measured N and P uptake and transformation in plants,rice yield,and the water and nitrogen use efficiencies.This study aims to explore the loss laws and of N and P of northern paddy fields,clarify the different effects of water-nitrogen managements on the loss,uptake,and transformation mechanisms of N and P,and reveal the regulatory mechanism of nutrient-controlled N managements on N and P migration and transformation pathways in water-saving irrigation paddy fields.The main findings of the present study are as follows:（1）The p H in the surface water was 6.47–8.56,and the concentrations of total nitrogen（TN）and total phosphorus（TP）were 0.06–31.47 mg L^-1 and 0.01–1.53 mg L^-1,respectively.The concentration of N and P in the surface water was the most influenced by basal fertilizer,followed by the tillering fertilizer.The concentrations of TN and TP in runoff were 0.01–1.18mg L^-1and 0.03–0.19 mg L^-1,respectively.N and P loss through runoff within one week of basal or tillering fertilizer application caused the greatest threat to the surrounding water environment pollution.The concentrations of TN and TP in the leachate were 0.09–11.04 mg L^-1and 0.01–1.37 mg L^-1,respectively.The highest concentration of N and P in the leachate were observed after basal fertilizer application.The risk of N and P loss through leachate was highest at this time.The loss rates of N and P through runoff and leachate ranged from 3.6–10.5%and 0.9–2.9%,respectively.Ammonium nitrogen（NH₄⁺-N）and dissolved phosphorus（DP）were main forms of N and P loss,respectively.（2）Alternate wetting and drying irrigation（AWD）had significant effects on p H,N and P loss,water and nitrogen use efficiency,and rice growth.Averaged across three years,compared with CF,AWD reduced the volume of irrigation,runoff,and leachate by 22.1%,14.6%,and 20.1%,respectively;AWD decreased p H by 0.11,and average concentration of TN and TP by 3.3%and 5.0%,respectively;AWD also lowered the loss of TN and TP by 32.3%and 20.0%in runoff,and 30.3%and 27.5%in the leachate,respectively.Moreover,compared to CF,AWD increased irrigation water use efficiency（IWUE）,total water use efficiency（TWUE）,agronomic nitrogen use efficiency（AEN）,and fertilizer nitrogen recovery efficiency（REN）by 26.4%,8.2%,12.1%,and 11.1%,respectively;AWD also increased root average diameter（RAD）,grain filling percentage（GF）,and 1000-grain weight（TW）.（3）Zeolite application had significant effects on p H,N and P loss,water and nitrogen use efficiency,and rice growth.Averaged across three years,compared with NU,NZ reduced p H by 0.11,and average concentration of TN and TP by 17.8%and 12.5%,respectively;NZ also lowered the loss of TN and TP by 10.7%and 15.0%in runoff,and 16.3%and 2.1%in leaching,and decreased N and P loss rate by 1.2%and 0.1%,respectively.Additionally,compared to NU,NZ increased plant height,number of tillers,grain yield（GY）,IWUE,TWUE,AEN,partial factor productivity of nitrogen（PFPN）by 0.8%,5.0%,3.2%,3.3%,2.9%,7.2%,16.9%,and 3.2%,respectively.（4）Controlled-release N fertilizer application had significant effects on p H,N and P loss,water and nitrogen use efficiency,and rice growth.Averaged across three years,compared with NU,NP reduced p H by 2.9%,and average concentration of TN and TP by 51.1%and 16.7%,respectively;NP also lowered the loss of TN and TP by 8.7%and 10.0%in runoff,26.1%and10.6%in leaching,and decreased N and P loss rate by 1.9%and 0.2%,respectively.Additionally,compared to NU,NP increased plant height,number of tillers,grain yield（GY）,IWUE,TWUE,AEN,REN,PFPN by 3.9%,15.2%,6.6%,6.9%,7.1%,14.8%,28.8%,and 6.7%,respectively.（5）The interaction of water-nitrogen managements had no significant effects on rice yield,use efficiency of water and nitrogen,P loss,N and P uptake,and aboveground dry matter accumulation during rice yellow maturity,and had significant effects on TN concentration in the surface water,TN and NH₄⁺-N loss in runoff,and TN loss in the leachate.Compared with NU,NP reduced more TN concentration in the surface water,TN and NH₄⁺-N loss in runoff under CF,and increased the risk of NO₃^--N loss under AWD.NZ reduced more NH₄⁺-N loss in runoff and TN loss in leachate under CF.This indicated that zeolite with molecular sieve function were more effective in reducing N（especially NO₃^--N）loss through leachate.Altogether,the three-year results showed that the water-nitrogen managements of AWD combined with controlled-release N fertilizer performed best in reducing N and P loss in the leachate,NH₄⁺-N loss in the water environment,and promoting aboveground plant growth and grain yield.The water-nitrogen managements of AWD combined with zeolite performed best in reducing N and P loss in runoff,and NO₃^--N loss in the water environment.AWD combined with zeolite or controlled-release N fertilizer effectively and stably reduced the pollution threat of N and P loss to surrounding water environment,promoted N and P supply and absorption by rice,and improved use efficiencies of water and nitrogen.The present results enriched water and nitrogen management strategies and provided a comprehensive and systematic theoretical support for controlling agricultural non-point source pollution.