Soil thickness reshapes the spatial patterns of soil total nitrogen concentration and stocks in a typical karst catchment

Karst regions are both ecologically fragile areas and important agricultural production zones in China, where soil nitrogen pools play a critical role in sustaining ecosystem productivity and regulating carbon-nitrogen cycling. However, highly fragmented soil cover and generally shallow soil layers hinder accurate characterization of regional soil nitrogen pools and their underlying controls. Using 2, 755 soil profiles from the Houzhaihe karst catchment in Guizhou Province, China, we integrated spatial interpolation, ANOVA, regression analysis, and structural equation modeling to examine the spatial patterns and potential drivers of soil total nitrogen (STN) concentration and soil total nitrogen (STN) stocks. The results revealed a pronounced concentration-stock divergence across both landform units and land-use types. STN concentration was approximately twice as high in karst peak-cluster areas as in depressions (3.19 vs. 1.60 g kg^-¹), whereas STN stocks were 21.15% lower in peak-cluster areas (9.08 vs. 11.00 t ha^-¹). Similarly, forestland exhibited the highest STN concentration (3.53 g kg^-¹), whereas STN stocks were highest in paddy fields and upland cropland (13.40 and 10.60 t ha^-¹), indicating greater current nitrogen storage in agricultural soils. Regression analysis further showed that STN concentration and STN stocks were not linearly correlated (p = 0.434) but exhibited a significant hump-shaped relationship ( $R_{adj}^2=0.03$ , p < 0.001), indicating a nonlinear concentration-stock decoupling. Structural equation modeling identified soil thickness as a key structural factor potentially driving this decoupling, exerting opposite effects on STN concentration (β = -0.20) and STN stocks (β = 0.60) by increasing the effective soil mass while diluting soil nitrogen concentration. These findings support a dual concentration-stock framework for evaluating karst soil nitrogen pools and inform agricultural nitrogen management and ecological restoration.