Temporal And Spatial Characteristics And Influencing Factors Of Dissolved Nitrogen And Phosphorus Output In Sugarcane Planting Watershed

The extensive use of chemical fertilizers and pesticides in intensive agricultural production,which is the main cause of eutrophication of rivers and lakes and degradation of surface water quality.Guangxi is the largest sugarcane planting area in China,accounting for more than 60 percent of the total sugarcane planting area.According to statistics,the average amount of nitrogen fertilizer applied to sugarcane is 370.5 kg/hm² in Guangxi,which exceed the international limit for safe use of fertilizer.After applying a large amount of chemical fertilizer,it is easy to enter the river with the runoff during the rainfall,resulting in nitrogen and phosphorus loss,harmed the quality and safety of surface water.However,there is still lack the research on the contribution of nitrogen and phosphorus loss from farmland to river in rainfall events at the watershed scale.Therefore,this research selected the sugarcane planting watershed–Nala watershed as the research area,which in the water source area of Kelan reservoir in the Zuojiang river watershed.Monitored the output of dissolved nitrogen and phosphorus(total dissolved nitrogen?TDN?,total dissolved phosphorus?TDP?,ammonium nitrogen?NH₄⁺-N?,and nitrate nitrogen?NO₃^--N?during the rainfall in three sub-basins for one year?from June 1,2018 to May 31,2019?,and two sub-basins in the upstream?S2,S3watershed?and one sub-basin in downstream?S1watershed?.At the same time,combined with unmanned aerial vehicle to investigate the vegetation coverage in Nala watershed.Investigated and summarized the agricultural management activities such as fertilizer application amount,fertilization method and time.Discussed the spatial and temporal characteristics of dissolved nitrogen and phosphorus output and the relationship with fertilization,rainfall and vegetation coverage in sugarcane planting watershed in intensive agricultural region to provides basic scientific basis and reasonable suggestions for controlling agricultural non-point source pollution in this area.The main conclusions are as follows:?1?The concentration of dissolved nitrogen and phosphorus had obvious seasonal changes in Nala watershed.The peak concentration of dissolved nitrogen and phosphorus was from June to September in 2018?TDN 9.92?28.01 mg/L,TDP 0.01?1.13 mg/L,NH₄⁺-N 0.45?3.94 mg/L,NO₃^--N 7.79?20.38 mg/L?,Nala watershed entered the dry season after September 2018 and the concentration of dissolved nitrogen and phosphorus remained at a low level?TDN 3.22?8.48 mg/L,TDP 0.03?0.05 mg/L,NH₄⁺-N 0.09?0.69 mg/L,NO₃^--N 2.88?7.53 mg/L?.The annual average concentrations of TDN,TDP,NH₄⁺-N and NO₃^--N In Nala watershed were 13.56 mg/L,0.12 mg/L,1.12 mg/L and 10.99 mg/L,respectively.NO₃^--N is the main form of TDN,and NO₃^--N concentration was significantly positively correlated with TDN concentration?p<0.01?;NH₄⁺-N concentration were not significant with TDN concentration?p>0.05?.In 70 percent rainfall events,TDN and NO₃^--N concentration in the upstream?S2,S3 watershed?was greater than the downstream?S1watershed?;the spatial distribution of TDP concentration is different with TDN and NO₃^--N concentration:In 60 percent rainfall events,TDP concentration in the downstream?S1 watershed?was greater than the upstream?S2,S3 watershed?;The spatial distribution of NH₄⁺-N concentration had no obvious law in different rainfall events.There was no significant difference in the concentration of TDN,TDP,NH₄⁺-N and NO₃^--N between S1,S2 and S3 watershed?p>0.05?.?2?The annual outputs flux of TDN,TDP,NH₄⁺-N and NO₃^--N in Nala watershed were 41.43 kg/?hm²·a?,0.35 kg/?hm²·a?,3.83 kg/?hm²·a?and,33.83 kg/?hm²·a?,respectively.NO₃^--N is the main output form of TDN,accounting for 81.65%of the annual output flux of TDN;the output of NH₄⁺-N is smaller,accounting for 9.25%of the annual output flux of TDN.The output of dissolved nitrogen and phosphorus showed obvious seasonal variation in Nala watershed,with the largest output flux from June to September in 2018,and the output fluxes of TDN,TDP,NH₄⁺-N and NO₃^--N in this period accounted for 89.19%,89.18%,92.03%and 88.71%of the annual output fluxes respectively.From October 2018 to May 2019,the output flux of dissolved nitrogen and phosphorus was relatively low,the output fluxes of TDN,TDP,NH₄⁺-N and NO₃^--N accounted for 10.81%,10.82%,7.97%and 11.29%of the annual output flux respectively.In different rainfall events,the spatial distribution of dissolved nitrogen and phosphorus output flux have not obvious law in S1,S2 and S3 watershed.There was no significant difference in the output fluxes of TDN,TDP,NH₄⁺-N and NO₃^--N in S1,S2 and S3 watershed?p>0.05?.The annual output of dissolved nitrogen and phosphorus in the downstream?S1watershed?of Nala watershed was greater than the annual output of dissolved nitrogen and phosphorus in the upstream?S2 and S3 watershed?at the annual scale,and the output sequence was S1>S2>S3.?3?Fertilization,rainfall and vegetation coverage are the main factors that affect dissolved nitrogen and phosphorus output concentration,and the effect of fertilization was more obvious.In this research,selected two rainfall events with similar rainfall:one rainfall event on July 24,2018?with fertilization?and the other rainfall event on August 16,2018?without fertilization?.The concentrations of TDN,TDP,NH₄⁺-N and NO₃^--N in the rainfall event on July 24,2018 were 2.42,8.02,8.77 and 2.17 times that of the rainfall event on August 16,2018,it had shown that that fertilization significantly increased the concentration of dissolved nitrogen and phosphorus in rainfall runoff.The distribution of rainfall determines the distribution characteristics of dissolved nitrogen and phosphorus loss,but the concentration of TDN,TDP,NH₄⁺-N and NO₃^--N were not significantly correlated with rainfall?p>0.05?.The concentration of TDN,NH₄⁺-N,NO₃^--N was not significantly correlated with vegetation coverage?p>0.05?;TDP concentration was significantly negatively correlated with vegetation coverage?p<0.01?.?4?Fertilization,rainfall and vegetation coverage are the main factors that affect the output flux of nitrogen and phosphorus,and the effect of fertilization was more obvious.In this research,selected two rainfall events with similar rainfall:one rainfall event on July 24,2018?with fertilization?and the other rainfall event on August 16,2018?without fertilization?.The output flux of TDN,TDP,NH₄⁺-N and NO₃^--N in the rainfall event on July 24,2018 were 2.47,3.74,10.26 and 2.13 times that of the rainfall event on August 16,2018,it had shown that fertilization significantly increased dissolved nitrogen and phosphorus loss.Dissolved nitrogen and phosphorus loss was affect by rainfall,the output flux of TDN,TDP and NO₃^--N was significantly positively correlated with rainfall?p<0.01?;the output flux of TDP was significantly correlated with rainfall?p<0.05?,the output flux of NH₄⁺-N was not significantly correlated with rainfall?p>0.05?.The output flux of TDN,TDP and NO₃^--N was significantly negatively correlated with vegetation coverage?p<0.01?,while the output flux of NH₄⁺-N was not significantly correlated with vegetation coverage?p>0.05?.?5?Dissolved nitrogen and phosphorus concentration were significantly different in different periods.In the process of typical rainfall,the concentration of TDN and NO₃^--N was significantly negatively correlated with runoff?p<0.01?,TDP concentration was significantly correlated with runoff?p<0.05?,while NH₄⁺-N concentration was not significantly correlated with runoff?p>0.05?.The concentration of TDN and NO₃^--N in the late period of rainfall was greater than that in the early period of rainfall.