Nitrogen And Phosphorus Transfer,Utilization And Stoichiometry In Black Soil Paddy Fields Under Different Water And Nitrogen Management Modes

The migration and transformation of carbon,nitrogen and phosphorus in agro-ecosystems plays a crucial role in biogeochemical cycles,multi-element balance and nutrient limitation,and can also reveal the maintenance mechanism of ecosystem function and stability in changing environments.With the northward shift of China’s grain production center,Heilongjiang Province has gradually become China’s main grain production base and strategic grain reserve base.The increasing consumption of agricultural water resources and the excessive application of chemical fertilizers seriously restrict the sustainable development of the local rice field ecosystem,not only reducing water and fertilizer Utilization,the loss of nitrogen and phosphorus will also lead to eutrophication of rivers and lakes,groundwater pollution and other environmental problems.In this study,the characteristics of nitrogen and phosphorus runoff,leakage and leaching in black soil paddy fields under different water and nitrogen management modes,the law of rice yield formation and nitrogen,phosphorus absorption and utilization in black soil paddy fields under different water and nitrogen management modes,the accumulation and stoichiometry of plant and soil carbon,nitrogen and phosphorus in black soil paddy fields under different water and nitrogen management modes,were investigated.Three irrigation models,conventional flooded irrigation（F）,shallow wet irrigation（W）and controlled irrigation（C）,and four nitrogen application levels 0,85,110,135 kg/hm²（N0,N1,N2,N3）,were set up in the experiment,a total of 12 treatments.The main contents of this research included:（1）Dynamic changes of nitrogen and phosphorus in surface water and leakage water,nitrogen and phosphorus output load and loss rate in paddy field during the growth period under different water and nitrogen management modes.（2）The effects of different water and nitrogen coupling models on dry matter,yield,nitrogen and phosphorus uptake and transport,water,nitrogen and phosphorus use efficiency.（3）The effects of different water and nitrogen management patterns on carbon,nitrogen and phosphorus contents,accumulation,allocation ratio,stoichiometric ratio and nitrogen,phosphorus nutrient restriction of rice plants during main growth periods.（4）The effects of different water and nitrogen management modes on soil carbon,nitrogen and phosphorus contents,reserves,stoichiometric ratio,stratification rate and yield in the 0～60 cm soil layer after rice harvest.（5）Based on the fuzzy matter element and entropy weight method was used to comprehensively evaluate the 7 indicators,and screen the optimal water and nitrogen management mode.It provides theoretical basis and technical reference for the establishment of a high-efficiency water and nitrogen management method for regional water-saving and fertilizer-saving,yield increase and emission reduction.The main conclusions were as follows:（1）The variation trends of TN,NH₄⁺-N and NO₃^--N concentrations in the surface water of paddy fields were similar.The concentrations reached the peak on the 1st day after fertilization,and gradually became stable after 7 days.The average concentrations of TN,NH₄⁺-N and NO₃^--N in the paddy field surface water under the controlled irrigation mode were increased by 23.63%,10.19%and 10.50%compared with the shallow wet irrigation,and 32.01%,22.23%and 5.63%compared with the conventional flooded irrigation.Nitrogen application significantly increased the concentrations of TN,NH₄⁺-N and NO₃^--N in the paddy field surface water（P<0.05）.The overall change trend of TP and DP concentrations was similar.The concentration reached the peak on the 1st day after basal fertilizer application,decreased with the days after rice transplanting,and decreased to less than 1/5 of the peak concentration on the 17th day.The average concentration of TP and DP in the paddy field surface water in the controlled irrigation mode was increased by 10.75%and 5.84%compared with the shallow wet irrigation,and 18.50%and 23.77%compared with the conventional flooded irrigation.Nitrogen application had no significant effect on the concentrations of TP and DP in the field water（P>0.05）.NH4+-N and PP are the main forms of nitrogen and phosphorus in paddy field surface water.The average runoff loss of TN and TP in the conventional flooded irrigation mode was increased by 16.51%and 20.53%,respectively,compared with the shallow wet irrigation,and increased by 81.07%and72.90%,respectively,compared with the controlled irrigation.The runoff loss of TN,NH₄⁺-N and NO₃^--N increased with the increase of nitrogen application,while the runoff loss of TP and DP did not show obvious regularity with the increase of nitrogen application.（2）The variation trends of TN,NH₄⁺-N and NO₃^--N concentrations in the leakage water of paddy field were similar.The concentration reached the peak on the 3rd day after fertilization,and gradually became stable after 7 days of fertilization.The average concentration of TN in leakage water in the controlled irrigation mode was 16.89%higher than that of shallow wet irrigation,and 104.46%higher than that of conventional flooded irrigation.The average concentration of NH4+-N in leakage water of conventional flooded irrigation mode was 87.09%and 54.62%higher than that of shallow wet irrigation and controlled irrigation,respectively.Nitrogen application significantly increased the concentrations of TN,NH₄⁺-N and NO₃^--N in leakage water（P<0.05）.The overall variation trends of TP and DP concentrations in rice leakage water was similar.The concentration reached the peak on the 5th day after basal fertilizer application,decreased with the days after rice transplanting,and decreased to a lower level after 7 days.The average concentration of TP in leakage water in conventional flooded irrigation mode was 34.95%higher than that of shallow wet irrigation,and 26.41%higher than that of control irrigation.The average concentration of TP in leakage water in N1,N2 and N3 treatments was8.11%-13.37%lower than that in N0 treatment.NO₃^--N,DP are the main forms of nitrogen and phosphorus in leakage water.The average leaching loss of TN and TP in the conventional flooded irrigation mode was increased by 7.19%and 57.30%respectively compared with the shallow wet irrigation,and increased by 11.03%and 57.47%respectively compared with the controlled irrigation.The amount of nitrogen leakage and leaching increased with the increase of nitrogen application,and the leakage and leaching of TP and DP in nitrogen application treatment were smaller than those in no nitrogen application.（3）The results showed that under conventional flooded irrigation and shallow wet irrigation,dry matter accumulation increased with the increase of nitrogen application rate,while under controlled irrigation,dry matter accumulation first increased and then decreased with the increase of nitrogen application rate.The nitrogen and phosphorus accumulation in different organs of rice shoot increased with the increase of nitrogen application rate.With the same nitrogen application level,the nitrogen accumulation in leaves,stem sheaths,and panicle of the controlled irrigation mode increased by27.80%～43.42%,18.32%～24.97%,13.85%～24.25%compared with conventional flooded irrigation,increased by 0.96%～13.18%,10.73%～12.86%,10.53%～12.61%compared with shallow wet irrigation.Compared with the controlled irrigation,the phosphorus accumulation of leaves,stem sheaths and panicles under the same nitrogen application rate in the conventional flooded irrigation treatment increased by 4.57%～15.30%,12.55%～24.90%,8.56%～13.44%,and phosphorus accumulation in leaves,stem sheaths and panicles under shallow wet irrigation increased by 1.46%-12.64%,1.55%-10.13%,and0.13%-5.54%.Under the three irrigation modes,the dry matter,nitrogen and phosphorus accumulation rates of rice shoot increased with the increase of nitrogen application rate.The dry matter and nitrogen accumulation rates in the controlled irrigation mode were higher than those in the shallow wet irrigation and the conventional flooded mode,and the phosphorus accumulation rate in the conventional flooded mode was higher than that in the controlled irrigation and the shallow wet irrigation.The initial peak of dry matter,nitrogen and phosphorus accumulation was advanced with the increase of nitrogen application rate.The peak time of the average nitrogen accumulation rate was 11.39 and 15.75 days earlier than that of the average dry matter and phosphorus accumulation rate.Compared with conventional flooded irrigation and shallow wet irrigation,controlled irrigation was more beneficial to increase rice yield,and CN2 treatment had the highest yield.The controlled irrigation model significantly improved the agronomic N use efficiency,partial factor productivity of N and partial factor productivity of P.Under the same irrigation mode,the nitrogen transport rate in leaves and stem sheaths and the contribution rate of nitrogen transport in panicle decreased with the increase of nitrogen application rate.Under the same irrigation mode,the phosphorus transfer amount and phosphorus increase in panicles increased with the increase of nitrogen application rate,the leaves phosphorus transport rate was lower than the stem sheaths transfer rate.The transfer amount of stem sheaths and phosphorus increase in panicles in the conventional flooded irrigation mode were higher than those of the shallow wet irrigation and the controlled irrigation.（4）The results show that under different water and nitrogen management modes,the contents of carbon,nitrogen and phosphorus in stem sheaths were 35.87%～39.43%,0.44%～2.19%and0.14%～0.32%,respectively,and the contents of carbon,nitrogen and phosphorus in leaves were36.34%～40.83%,0.76%～3.70%and 0.14%～0.36%,respectively and the contents of carbon,nitrogen and phosphorus in panicle were 37.05%～41.72%,0.82%～1.63%and 0.24%～0.39%,respectively.Controlled irrigation could increase the carbon and nitrogen accumulation from jointing booting stage to mature stage,and the phosphorus accumulation during the growth period of conventional flooded irrigation was always higher than that of shallow wet irrigation and controlled irrigation.Under 3irrigation modes,compared with N0,N1,N2 and N3 treatments increased carbon accumulation by31.46%,52.55%and 57.37%,nitrogen accumulation by 52.98%,117.63%and 144.88%,and phosphorus accumulation by 50.28%,79.85%and 93.89%,respectively,at maturity stage.The proportion of carbon,nitrogen and phosphorus in stem sheaths increased first and then decreased,the proportion of carbon,nitrogen and phosphorus in leaves decreased continuously,and the proportion of carbon,nitrogen and phosphorus in panicle increased continuously.Compared with conventional flooded irrigation and shallow wet irrigation,the controlled irrigation mode has less effect on the carbon content of rice plants,but it can increase the nitrogen content in the middle and late stages of rice plant growth,and reduce the plant phosphorus content,thereby reducing rice plant C/N and increasing rice plant C/P and N/P.Nitrogen application significantly increased the nitrogen content of rice plants,slightly increased the phosphorus content of rice plants,and had relatively little effect on the carbon content of rice plants,and then decreased the C/N and C/P of rice plants,and increased the N/P of rice plants.Under conventional flooded irrigation and shallow wet irrigation,shoot plants of rice transition from phosphorus restriction to nitrogen and phosphorus co-restriction and then to nitrogen restriction,while under controlled irrigation,shoot plants of rice only transition phosphorus restriction to nitrogen and phosphorus co-restriction.（5）The results showed that under the same irrigation mode,the contents and reserves of soil organic carbon（SOC）,soil total nitrogen（STN）and soil total phosphorus（STP）decreased with the increase of soil depth after rice harvest.The contents and reserves of SOC and STN increased with the increase of nitrogen application rate,while the contents and reserves of STP decreased with the increase of nitrogen application rate.The contents and reserves of SOC and STN in different soil layers under conventional flooded irrigation were higher than those under shallow wet irrigation and controlled irrigation,and the contents and reserves of STP in different soil layers under controlled irrigation were higher than those under shallow wet irrigation and conventional flooded irrigation.The interaction of water and nitrogen had no significant effect on the contents and reserves of SOC,STN and STP（P<0.05）.The average values of soil C/N,C/P,and N/P in the 0～60 cm soil layer in the study area were16.18,28.71,and 1.84,respectively,among which C/P and N/P were lower than the national average values of farmland soil.Soil C/N increased with the increase of soil depth,and decreased with the increase of nitrogen application rate.Soil C/P and N/P decreased with the increase of soil depth,and increased with the increase of nitrogen application rate.The C/N,C/P and N/P of all soil layers under conventional flooded irrigation were higher than those under shallow wet irrigation and controlled irrigation.Nitrogen application increased SOC,STP contents and reserves,C/N stratification rate in different soil layers.Choosing an appropriate water and nitrogen management model in black soil paddy fields can increase rice yield,improve water,nitrogen and phosphorus use efficiency,and reduce the risk of nitrogen and phosphorus loss while maintaining soil fertility.Through the fuzzy matter element and entropy weight model screening,it was found that the CN2 treatment is optimal.The controlled irrigation mode with 110 kg/hm² nitrogen fertilizer can achieve the purpose of saving water and fertilizer,increasing yield,and reducing emissions,and can be promoted as an economical and applicable rice water and nitrogen management mode in black soil areas.