Effects of Land Use Types on Nitrogen and Phosphorus in Rivers of the Huixian Karst Wetland in the Lijiang River Basin

When the factors affecting the spatial and temporal variation of nitrogen and phosphorus in rivers of karst watershed are considered,the unique karst geomorphic and its accompanying land use features of underlying surface cannot be ignored.To reveal the effects of karst feature and land use types on nitrogen and phosphorus of the Huixian karst wetland rivers,water samples from Mudong River,Huixian River and Xiangsi River were collected monthly for nitrogen and phosphorus analysis from October 2017 to September 2021.Based on the statistical analysis of river N and P concentration in catchments ranging from 21.79 to 376.42 km~2,in combination with Pearson and Grey correlation analysis,the relationship between N,P concentrationof rivers and the area proportion of landuse type,karst landform,as well as the hydrometeorology factors was studied.The results showed that （1） During the monitoring period,total nitrogen（TN）pollution was more serious than total phosphorus（TP）pollution.The concentrations of TN and TP varied from 0.30 to 27.55 mg/L and 0.03 to 2.49 mg/L,respectively,with 70.3%of 518 samples had TN＞1.0 mg/L and 35.1%had TP＞0.2 mg/L.Under the influence of hydrometeorology,the peak concentrations of TN and TP appeared in the dry season,and the mean concentrations decreased in the wet season.（2） The nitrogen and phosphorus in rivers mainly came from agricultural land（include paddy field,dry land,and orchard）which was significantly affected by human activities.Wetland and ponds contribute to nitrogen and phosphorus reduction.The covered karst area has a positive effect on nitrogen and phosphorus emission levels.（3） The correlation between the concentrations of phosphorus（TP and TDP）and precipitation is stronger than that of nitrogen（NH₄⁺-N,NO₃^-N and TN）,especially in the wet season.This research helps to improve the knowledge about the status and main influence factors of N and P pollution in Huixian karst wetland rivers.It is important to pay attention to nitrogen and phosphorus in karst wetland system for water quality safety and management.