ABSTRACT The soil–epikarst zone represents the transition zone between the surface soil and the underlying karst topography. High permeability and complex hydrogeology govern water and solute fluxes. The mechanisms governing subsurface leakage and nitrogen transport in response to rainfall are unclear. Experiments simulating laboratory conditions were performed to assess the impact of rainfall and the soil–epikarst structure on subsurface leakage and nitrogen loss. Three rainfall amounts (15, 30, and 60 mm) and three rock proportions (45%, 55%, and 65%) were used to construct the soil–epikarst structure. Under the conditions of this study, subsurface leakage in the soil–epikarst zone occurred at 15 mm of rainfall (subsurface leakage: 57.8%–73.7%), and rainfall ranging from 30 to 60 mm significantly increased the extent of such leakage (subsurface leakage: 75.5%–77.6%). Additionally, the typical water holding capacity of the soil–epikarst structure ranges from 40 to 60 cm. The extent of nitrate nitrogen (NO 3 − –N) and ammonium nitrogen (NH 4 + –N) loss differed across rainfall amounts. As rainfall increased, NO 3 − –N loss also increased, whereas NH 4 + –N loss initially decreased but subsequently increased. Notably, nitrogen loss was exacerbated at a 55% rock proportion, corresponding to the highest subsurface leakage and leakage coefficient, whereas both subsurface leakage and nitrogen loss were attenuated at 45% and 65% rock proportions. In the soil–epikarst zone, inorganic nitrogen loss occurred predominantly in the form of NO 3 − –N. The amount of NH 4 + –N loss ranged from 28.35 to 35.50 mg, and the amount of NO 3 − –N loss ranged from 94.89 to 227.23 mg. The percentage of N loss ranged from 0.35% to 0.97%. Structural equation modeling revealed that the rock proportion, a quantitative indicator of the soil–epikarst structure, has a significant direct negative effect on total N loss, and this effect is associated with its influence on NO 3 − –N and NH 4 + –N losses, both of which positively contribute to total N loss. Rainfall regulates subsurface leakage and the transport of inorganic nitrogen in subsurface flow, thereby influencing total N loss. In conclusion, when rainfall ranges from 30 to 60 mm, NO 3 − –N accounts for more than 70% of the total N loss, and a 55% rock proportion exacerbates nitrogen loss, with the total N loss rate reaching 75.14%. This research enhances the understanding of subsurface leakage and nitrogen loss in the soil–epikarst zone of karst areas with varying rainfall and may guide the development of efficient water and nitrogen management approaches to minimize nitrogen pollution of water sources.
Yang et al. (Mon,) studied this question.