Interception And Removal Effects Of The Different Types Of Agriculture Drainage Ditches On Nitrogen And Phosphorus In Dianchi Lake Watershed

Agricultural non-point source pollution in the Dianchi Basin has become the main pollution sources of lake eutrophication. As the sink of the agricultural non-point source pollution in the upstream and the source of the downstream water body, water quality of the ditch significantly affects eutrophication of rivers and lakes. In order to understand the output characteristics of non-point source pollution in farmland and control of nitrogen and phosphorus pollution, the Chai Rive sub-basin within the Dianchi watershed was selected as the study area, and nitrogen and phosphorus concentration variation characteristics of the runoff and sediment of different types of agriculture drainage ditches in different planting areas were analyzed based on the survey and monitor of soil and water samples in natural and simulated runoff. The main results are as follows:(1) The different types of agriculture production and planting led to the different nutrient concentrations of ditch sediment. Overall, the order of total nitrogen, available nitrogen, total phosphorus, and organic matter contents of ditch sediment was greenhouse area> vegetables planting area> maize planting area; and the sequence of the available phosphorus concentration was maize planting area> greenhouse area> vegetables planting area. Due to the speciality of the study area, the higher phosphorus content was displayed, and the highest content of total phosphorus and available phosphorus were9g/kg and1.59g/kg.(2) Under the natural rainfall condition, the close relationships were found between the ecological characteristics of ditches, different planting conditions and the change of nitrogen, phosphorus contents in surface runoff. The contents of total nitrogen and soluble nitrogen in vegetable area drainage runoff were higher than greenhouse area and maize area. As for total phosphorus and dissolved phosphorus, the contents of greenhouse area ditches were higher than others, and the contents in ecological ditches were higher than in soil ditches. The results of two-way analysis of variance showed that the agriculture planting type is the main factors in influence of the changes of nitrogen and phosphorus runoff contents, especially for the changes of dissolved nitrogen and phosphorus, total nitrogen content.(3) In the simulated rainfall conditions, the close relationships were found between the type of ditches, different runoff volume and the change of nitrogen, phosphorus contents in surface runoff. As for nitrogen (total nitrogen, soluble nitrogen, nitrate nitrogen and ammonium nitrogen), the concentration of nitrogen decreased in different degree in ecological ditch,and changes of nitrogen concentration in the cement ditch is not obvious during the experience process.when the experience time was in1to3hour, nitrogen concentration in the ecological ditch was higher than that of cement ditch, which shows that there was some released nitrogen in ecological ditch; when the experience time was after the1to3hour, the nitrogen concentrations of ecological ditch runoff was less than the cement ditch, which suggests the interception and removal effects of ecological ditch may be related to the duration of runoff time. As for phosphorus in runoff,, the runoff concentration in cement ditch is higher than that of the ecological ditch because of the difference of early runoff phosphorus erosion, but phosphorus concentration in the cement ditch showed obviously decreased than ecological ditch with the passage of time. Two-way analysis of variance displayed that the runoff volume is the main factor influencing the changes of nitrogen in ditch, and ditch type is the main factors influencing the change of phosphorus.(4) In the simulated rainfall conditions, the nitrogen and phosphorus reduction capacity of different ditch were associated with the runoff volume and duration. The reduction ratio of total nitrogen and soluble nitrogen in cement ditch was better than that of ecological ditch. Under the same ditch, the reduction ratio of total nitrogen and soluble nitrogen in the mean runoff volume was better than that of large runoff volume. As for nitrate nitrogen and ammonium nitrate, the reduction ratios of different types of ditch in the mean runoff volume were all better than that of large runoff volume because of some released nitrogen. The higher ratios of released phosphorus were also found in all different combinations of ditch type and runoff volume except the ecological ditch in large runoff volume. The time effects of different types of ditch showed that the positive reduction ratio happened after3hours in mean runoff volume.Based on the above results, some recommendations should pay attention to the following aspects in control of agricultural non-point source pollution in Dianchi watershed:(1) the optimization of local agricultural planting structure;(2) if do not change the original planting structure, some measures and managements should be taken in greenhouse area and vegetable cultivation area, such as sediment dredging and cleaning of the agriculture runoff;(3) if sediment dredging is impossible, in order to increase the runoff retention time, the existing trench improvement or enhancement of ecological level should be taken to reduction efficiencies of non-point source pollutants;(3) due to the most of ditches in Dianchi Watershed are shallow and have short flow, the ditch system should be optimized or connected for improving the nitrogen and phosphorus reduction capacity;(4) the phosphorus reduction ability is weak for the ditch system, even strong released phosphorus existed, therefore, it is necessary to strengthen the construction of ecological ditch technology for phosphorus reduction.