农田排水口高度对地表径流氮磷流失的影响

洱海流域农田径流氮磷污染严重，大量氮磷污染物随雨水进入洱海，导致洱海水质雨季下降。为从源头控制氮磷污染物的输出，该研究采用人工模拟降雨的方法，探究5、10、15、20、25 cm 5种不同高度的排水口对农田径流氮磷流失的控制作用。结果表明，农田排水口较低会造成产流初期硝态氮和颗粒态氮浓度升高，将排水口高度提高到15 cm以上可有效降低径流中各形态氮磷浓度，并稳定在较低水平；排水口高度从5 cm提高至15～25 cm产流中总氮、颗粒态氮、铵态氮、硝态氮流失量分别降低了85.60%～93.13%、88.39%～95.77%、84.59%～91.72%、63.05%～65.15%，总磷、颗粒态磷流失量分别降低了86.75%～92.66%，61.64%～94.61%，且排水口设置在15 cm高度处氮、磷流失量削减效果突出，在15 cm基础上继续提高排水口不会对氮磷流失量产生明显影响。综上所述，将排水口提高到15～25 cm对农田径流污染控制效果优越。结合洱海流域多年降雨资料及建设成本，推荐将农田排水口设置于距土壤表面15 cm高度处，对控制农田养分流失，减少面源污染起到显著效果。

Effects of the heights of farmland drainage outlets on nitrogen and phosphorus loss from surface runoff

The Erhai Lake is the second second-largest freshwater resource in the Yunnan Plateau (southwest China). The lake is also a national protected area and drinking water source for Dali residents. The water quality of the Erhai Lake is has gradually aggravated eutrophication, even above an acceptable level in the rainy season, due mainly to global natural conditions and intensified human activities in recent years. Specifically, nitrogen (N) and phosphorus (P) are the main factors causing eutrophication. Furthermore, the rice planting area accounts for about 10% of the total area of Erhai Lake Basin, a typical agricultural basin. The rice planting season is in the peak month of rainfall in the basin, where rainstorm events occur frequently. The loss of nutrients in farmland subjected to rainstorms has been a key environmental factor to determine the water quality of Erhai Lake in the rainy season. Much effort has been dedicated to the loss of nitrogen and phosphorus in the farmland of the Erhai Lake Basin, particularly on fertilization management and rotation mode. Farmland drainage outlets can serve as the channels for terrestrial pollutants to enter rivers, lakes, and other water bodies. However, only a few studies focused on the farmland drainage outlets for pollution prevention and flood control. Taking the height of the drainage outlet in the Erhai Lake Basin as a research object, this study aims to find an effective way to reduce the nitrogen and phosphorus loss from surface runoff in farmland. An artificial rainfall simulation was also adopted. Five drainage outlets were set with different heights, according to the height of the farmland drainage outlet (5-10 cm from the soil surface), and the height of the rice plant in the peak period of rainfall in the study area. The bottom distances of the drainage outlet were 5, 10, 15, 20, and 25 cm from the soil surface. An investigation was finally made to evaluate the control of drainage outlets at different heights on nitrogen and phosphorus losses in farmland runoff. The results showed that: 1) The low drainage outlet of farmland resulted in the increase of nitrate nitrogen and particulate nitrogen concentrations at the early stage of runoff generation. The drainage outlet height of more than 15 cm effectively reduced the concentrations of nitrogen and phosphorus in various forms, all of which be stabilized at a low level. 2) The losses of nitrogen and phosphorus were significantly reduced, with the increase in the height of farmland drainage outlets. A bBetter control was achieved at the height of the farmland drainage outlet increasing from 5 to 15-25 cm. Specifically, the losses of total nitrogen, particulate nitrogen, ammonium nitrogen, and nitrate nitrogen reduced by 85.60%-93.13%, 88.39%-95.77%, 84.59%-91.72%, 63.05%-65.15%, respectively. The losses of total phosphorus and particulate phosphorus decreased by 86.75%-92.66%, 61.64%-94.61%, respectively. Moreover, there was an extremely high reduction of nitrogen and phosphorus losses, when the farmland drainage outlet was set at a height of 15 cm. But there was no significant change over 15 cm. 3) The main form of nutrient loss was the dissolved state in the paddy field. The dissolved nitrogen and phosphorus losses accounted for 52.37%-83.64% and 67.83%-92.29% of the total nitrogen and phosphorus losses, respectively. The inorganic nitrogen loss in runoff was mostly ammonium nitrogen, accounting for 47.85%-80.80% of the total nitrogen loss. Consequently, the drainage outlet at the height of 15-25 cm can be expected to achieve a superior performance for the runoff pollution control in farmland. Anyway, it is strongly recommended to be 15 cm high for the farmland drainage outlet in the Erhai Lake Basin. This finding can provide a significant support to control nutrient loss in farmland, thereby to improve improving the ecological environment.