Effect And Mechanism Of Nitrogen And Phosphorus Removal From Agricultural Runoff By The Buffer Zone Of Erhai Lake

Due to the severity of agricultural non-point source pollution and the lack of ecological structure integrity and barrier function of the buffer zone of Erhai Lake, this study used the methods of combining investigation and simulation experiments to optimize aquatic macrophytes, grass varieties, width, slope and other parameters, the mechanism of nitrogen and phosphorus removal from agricultural runoff by the buffer zone was also analyzed.First of all, we investigated the water quality, sediment pollution situation and spatial variability of soil properties of 18 inflowing rivers in the buffer zone of Erhai Lake. The results showed that the average values of p(TN) and p(TP) were 2.12mg/L and 0.24mg/L, respectively. DTN was the main form of TN. p(SRP) accounted for 54.2% of p(TP). Nitrogen and phosphorus were both the main pollution factors influencing the water quality in the buffer zone. The average values of w(TN) and w(TP) in the sediments were 141.66mg/kg and 611.43mg/kg, respectively. ON was the main form of TN. High C/N indicated that sediments were mainly influenced by exogenous sources, such as runoff, human activities and agricultural fertilizer. w(NH3-N) of soils were higher in the outer circle of the buffer zone than that in the inner, while w(NO3’-N) was quite the contrary. The erosion of NO3--N in the soils of the buffer zone was the emphasis of pollution controlling.Based on the absorption of nitrogen and phosphorus, removal efficiency and growth condition of different aquatic macrophyte-river sand systems, which run 35d continuously with farmland agricultural runoff under the hydraulic loading rate of 1 cm/d, the optimal macrophyte species for building the buffer zone were selected. The results showed that Phragmites australis was the best choice among emergent plants, its highest removal efficiency of TN and TP was 74.0%and 96.6%, respectively. There was no significant difference between the removal rate of submerged macrophytes. While, Ottelia acuminata could enhance the economic value, its highest removal efficiency of TN and TP was 62.2% and 90.2%, respectively. Nymphoides peltatum was the best choice among floating plants, its highest removal efficiency of TN and TP was 78.4% and 92.3%, respectively.Sward buffer zone, which consists of sward and soil, had a good purification effect of the village agricultural runoff. Overall consideration, Trifolium repens buffer zone (TRBZ) was better than Festuca arundinacea buffer zone (FABZ) and Cynodon dactylon buffer zone (CDBZ). When the ratio of inflow was 1.0L/s, the average removal efficiency of TN and TP in the surface flow was significantly higher than that in the interstitial flow in the first 3m. The suitable slope of the sward buffer zone was i=0%. Active microbial biomass and community structure in the soils was determined by the method of phospholipid fatty acid (PLFA), the results showed that PLFA contents increased significantly in the TRBZ and the PLFA contents of bacterial biomass, fungal biomass, gram-positive bacteria biomass and gram-negative bacteria biomass showed a significantly positive correlation with the reduction of TN in the agricultural runoff.At last, a whole serials buffer zone which included macrophanerophytes, shrubs, sward and aquatic macrophytes was first built in Erhai Lake basin according to the optimal choice of plant species and structure parameters of pre-tests. It had a good purification effect of the agricultural runoff.