Effects of preferential flow on soil nutrient transport in karst slopes after recultivation

Abstract

In response to the global food shortage, a large amount of abandoned land in karst areas has been reclaimed as cultivated land, causing severe nonpoint source pollution. Preferential flow-driven soil nutrient transport on karst slopes remains poorly studied, though it is a major factor in nonpoint source pollution, as it responds to changes caused by reclamation. We explored the characteristics of soil preferential flow differences in recultivated land, grassland, and shrubland from returning farmland by dye tracer experiments and quantitatively examined the effect of preferential flow on nutrient transport. Under the condition of 40 mm precipitation, the preferential flow paths (PFPs) of the three types of plots were mainly distributed from 0 to 40 cm. The total porosity in the 20–40 cm soil layer was significantly reduced by reclamation, and the number of preferred flow paths in the 20–40 cm soil layer was significantly reduced from 60 to less than 10, which was significantly less than that in grassland and shrubland. But, reclamation results in the transport of more soil nutrients by preferential flow. The contribution rate of preferential flow to other nutrient indexes in the reclaimed land, in contrast to grassland and shrubland, was lower than zero, except for organic matter and total potassium. Moreover, when the PFP was connected to the rock–soil interface, the soil water can leak underground through the rock–soil interface quickly. Therefore, our findings indicated that reclamation reduces the distribution depth of the PFP. Still, the connection of soil preferential flow to the rock–soil interface increases the transport of soil nutrients to deep fissures and even underground rivers, thereby causing recultivated land to become one of the main sources of groundwater pollution in karst areas.