模拟降雨下喀斯特坡耕地土壤养分输出机制

喀斯特区坡耕地水土及养分流失不仅是该区土地质量退化、土地生产力衰退主要原因,同时也是该区地下水质污染的重要因素。为揭示喀斯特坡耕地地表和地下二元空间结构下的土壤养分流失机制,以喀斯特坡耕地为研究对象,通过模拟其地表微地貌及地下孔(裂)隙构造特征,采用人工模拟降雨试验研究不同雨强下喀斯特坡耕地地表及地下水土及其氮、磷、钾流失特征。结果表明:(1)小雨强(50mm/h)和中雨强下(70mm/h),喀斯特坡耕地坡面产流主要以地下产流为主;大雨强下(90mm/h),地表径流高于地下径流;产沙方式则表现为由小雨的地表和地下产沙并重到中大雨强的地表产沙为主的一个转变过程。(2)在降雨侵蚀过程中,径流各养分输出浓度均表现出一定的初期冲刷效应,受土壤吸附作用影响,雨强对全钾(TK)和全氮(TN)的影响较全磷(TP)明显。(3)地表径流、地表泥沙和总泥沙各养分输出负荷均随雨强增大而增加,坡面径流泥沙总的TK输出负荷以泥沙为主,而TN和TP输出负荷则以径流为主;TP和TN在径流的输出负荷上以地下径流输出为主(其中TP地表负荷比在11.6%—46.2%,TN在7.0%—48.5%之间),而TK则以二者并重(地表负荷比在43.5%—57.0%之间);各养分在泥沙的输出负荷上则均以地表泥沙流失为主,其负荷比均在54.5%以上。研究结果可为喀斯特区坡耕地水土流失及养分流失的源头控制提供基本参数和科学依据。

Output mechanism of soil nutrients from karst slope farmland under simulated rainfall

Soil water and nutrient loss are not only the main reasons for degradation of land quality and declines in productivity, but also important factors contributing to the pollution of underground water in karst regions. Compared with non-karst areas, the special structure of karst slopes requires a different theory regarding soil nutrient loss. Previous studies have determined that rainfall is the primary external factor resulting in soil water and nutrient loss on slopes. Hence, determination of the mechanism of soil erosion and nutrient loss will allow for the elucidation of output characteristics of soil nutrients from karst slope farmlands under different rainfall intensities. In addition, understanding the direction and vehicle of nutrient loss could clarify the method of soil nutrient transfer on karst slopes. However, it is difficult to directly observe underground runoff and sediment on karst slopes (i.e., there is no feasible research method). Using karst slope farmlands as an example, the surface micro-topography and structural characteristics of underground pores and fissures were simulated in a laboratory based on field investigations on slope farmlands in the Guizhou Province karst areas. The goal of this study was to determine the characteristics of nitrogen, phosphorus, and potassium loss in runoff, as well as the sediment yield from surface and underground in karst slope farmlands under different rainfall intensities using simulated rainfall experiments. The results indicated that:1) For light (50mm/h) and moderate rainfall intensity (70mm/h), runoff on karst slopes was primarily dominated by underground runoff; for heavy rainfall intensity (90mm/h), both surface and underground runoff were nearly equal, with the surface water runoff being slightly higher than that of underground runoff. Sediment yield changed from equivalent surface and underground sediment under the light rainfall intensity to surface sediment production being dominant at greater rainfall intensities. 2) The concentration of each nutrient in runoff exhibited an initial scouring effect during the process of rainfall erosion on karst slope farmlands. Influenced by the effect of soil absorption, the effect of rainfall intensity on total potassium (TK) and total nitrogen (TN) concentration in runoff was substantially greater than that of total phosphorus (TP). 3) The output load for each nutrient in surface runoff, surface sediment, and total sediment increased with increasing rainfall intensity. The output load of TK was dominated by sediment, but those of TN and TP were dominated by runoff on karst slope farmlands. With respect to runoff output, the output load of both TN and TP were dominated by underground runoff. The percentage of the TP load in the surface runoff relative to total runoff ranged from 11.6% to 46.2% and that of TN ranged from 7.0% to 48.5%; however, the output load of TK was equal between the surface and underground water, with the percentage of surface runoff relative to total runoff ranging from 43.5% to 57.0%. The sediment load for all nutrients was dominated by surface sediment, and the percentage of each nutrient in the surface sediment relative to total sediment was more than 54.5%. These results could provide basic parameters and a scientific basis to initiate control for soil water and nutrients loss on karst slope farmlands.