Effect of simulated rainfall intensities and underground pore fissure degrees on soil nutrient loss from slope farmlands in Karst Region

Abstract: Karst, which is characterized by a double-layer structure of surface and underground spaces, is a landscape formed through dissolution of soluble rocks, including limestone, dolomite and gypsum. Due to interaction of natural factors and human activities, the surface presents rocky desertification landscape with shallow soil layer, discontinuous regolith and even large area bare rocks, meanwhile, the underground space is characterized by sinkholes, closed depressions, subterranean drainage, caves and fissures. Undoubtedly, rainfall intensity acts as a driving force for surface and underground runoff and sediment yield. Underground pore fissure is an important way of soil and water loss on slope in Karst Region. Due to the difficulty in measuring underground leakage of soil and water in the underground space, there is very little study on the underground pore fissure leakage of soil nutrients in Karst Region. Therefore, people's understanding on rainfall intensity and underground pore fissure influencing soil nutrients loss is not enough in Karst Region. This paper aimed to study the effect of rainfall intensities and underground pore fissure degrees on nitrogen, phosphorus and potassium loss in runoff and sediment from slope farmland in Karst Region using the method of artificial simulation rainfall experiments. A steel tank (length of 4.0 m× width of 1.5 m× depth of 0.35 m) was built to simulate the surface micro-topography and underground pore fissure characteristics of karst slope in laboratory. Results indicated that: 1) Rainfall intensity had a significant (P<0.05) impact on surface runoff and sediment yield, and the runoff depth and sediment yield increased with the increase of rainfall intensity. The critical rainfall intensity for yielding surface runoff and sediment changed between 30 mm/h and 50 mm/h. Underground pore fissure degree had no obvious changes with surface and underground runoff and sediment yield. Specifically, soil erosion type on karst slope farmland was a process of transition from underground leakage to surface erosion. Total runoff yield was dominated by underground runoff, but total sediment production was dominated by surface sediment on slope farmland in Karst Region. 2) Except for surface runoff yield events, rainfall intensity showed certain effects on nutrients concentration of surface runoff and sediment, in which total nitrogen (TN) and total potassium (TK) concentration of surface runoff decreased with rainfall intensity increasing while TN concentration of surface sediment increased with increasing rainfall intensity. Under same conditions, a few differences in TN and total phosphorus (TP) concentrations were measured between surface runoff and underground runoff. However, the TK concentration in surface runoff was higher than underground runoff, which indicted that potassium was prone to loss with surface runoff. 3) Rainfall intensity had significant (P<0.05) effects on load of nutrients loss in surface runoff, load of TN loss in underground runoff and total load of TN loss in runoff. Meanwhile, underground pore fissure degree also had a significant (P<0.05) effect on load of TN loss in underground runoff while had no obvious effect on load of TP and TK in underground runoff, load of nutrients in surface runoff and total load in runoff. Total load of nutrients loss for TN and TP were dominated by losing in runoff, but total load of TK loss was dominated by losing in sediment on karst slope. In a word, rainfall intensity was an important influence factor of soil nutrient loss on karst slope. Underground pore fissure degree had little effect on soil nutrient loss but it was the main way of soil nutrient loss on karst slope farmland. These results of this study could provide some basic parameters and scientific basis for revealing the mechanism and controlling beginning of soil nutrients loss on karst slope farmland.