Characteristics of N and P Losses From Ditch Runoff in Farmlands Different in Planting Patterns Offshore of Lake Erhai

In order to explore effect of planting pattern on nitrogen and phosphorus losses with ditch runoff, one typical irrigation and drainage unit each was selected in a sapling nursery, vegetable garden and a tract of rice paddy field on the west bank of Lake Erhai as objects in the study, nitrogen and phosphorus concentrations in the irrigation water flowing into and ditch runoff flowing out of the irrigation and drainage units were monitored for analysis of effects of planting pattern on nitrogen and phosphorus concentrations in ditch runoff and their relative contribution. Results show that in terms of N and P concentrations of N and P in ditch runoff, the three types of farmlands exhibited an order of vegetable garden > rice paddy field > sapling nursery. The ditch run-offs at the outlets of the ditches from the test units in the paddy field and the sapling nursery were higher in TN but lower in TP than the criteria for Class IV surface water set by GB 3838-2002, while the ditch run-off from the vegetable garden was far above the criteria for Class V surface water in both TN and TP and higher than the irrigation water in concentration of various forms of nitrogen and phosphorus. The situation in the sapling nursery was just the contrary. In the paddy field the ditch runoff was higher than the irrigation water in concentration of various forms of N and P in May and June, while it went opposite in the other months. Inorganic nitrogen and dissolved total phosphorus (DTP) were the main N and P forms in the ditch runoff under all the three planting patterns, accounting for 70.32%-81.49% of TN and 70.33%-79.33% of TP, respectively, and NO₃^--N was the major fraction of inorganic nitrogen, accounting for 75.13%-84.75%. In terms of contribution to TN, NH₄⁺-N, NO₃^--N, TP and DTP in the ditch run-offs, the three types of farmlands followed an order of vegetable garden (56.41%, 85.81%, 44.61%, 66.17% and 64.80%, respectively) > rice paddy field (-4.50%, -15.14%, -10.01%, -0.85% and -0.29%, respectively) > sapling nursery (-89.88%, -64.81%, -96.49%, -72.11% and -69.69%, respectively). All the findings in this study demonstrate a strong need for optimizing spatial distribution of planting in the areas offshore of Lake Erhai to reduce agricultural N and P losses and protect Erhai Lake from eutrophication.