Response characteristics of dissolved nitrogen and phosphorus migration to outcrop bedrock pattern in karst slopes under individual rainfall

Abstract

The severe non-point pollution threatens to karst aquifers, which supply freshwater resources for approximately 25 % of the global population. The unique natural landscape of diverse rock outcroppings on karst slopes significantly influences hydrological processes and alters the solute transport characteristics. However, there is still insufficient understanding of the impact of bedrock outcroppings on solute transport. This study aimed to investigate the spatial patterns of dissolved nitrogen and phosphorus transport in karst slopes, considering the effects of bedrock morphology, bedrock patterns, and rainfall type. The findings indicated that sub-surface runoff production was associated with higher concentrations of dissolved total nitrogen (TN) (7.72 mg·L^-1–30.88 mg·L^-1) and that the soil–bedrock interface became the primary pathway for TN migration, particularly during moderate rainfall (10–25 mm·d^-1), achieving efficiencies of 51.54 % to 91.92 % depending on bedrock distribution patterns. Conversely, surface runoff had 1.05–2.61 times more dissolved phosphorus (TP) concentration than sub-surface runoff, soil-rock runoff, and underground pore fissure runoff, with surface pathway losses being the main TP loss channel. The TN and TP positively correlation with rainfall, with correlation coefficients exceeding 0.85 across different bedrock pattern treatments. Power function analysis revealed exponents of 3.58 and 3.34 for TN and TP loss fluxes, indicating higher vulnerability of dissolved TN to runoff losses. Moreover, surface dissolved TN and TP losses were greater in the bedrock morphologies 1:2 aspect ratio than 1:1 aspect ratio and were aggregated distribution > uniform distribution > centered distribution in various bedrock distribution patterns. The findings suggest that minimizing the scattered distribution of bedrock on karst slopes has potential for decreasing influxes of dissolved substances into underground aquatic ecosystems.