3D spatial redistribution and potential release risk of soil nitrogen and phosphorus in a newly built mountainous reservoir watershed

Introduction The spatial distribution of soil total nitrogen (STN) and soil total phosphorus (STP) in newly built reservoir watersheds is a critical issue because the initial inundation of nutrient-rich soils can trigger a trophic upsurge. This research assessed the 3D spatial redistribution characteristics of STN and STP, the impacts of environmental factors, and associated legacy P risks in the Gaopingqiao Reservoir watershed. Methods Soil samples were collected from surface (0-20 cm) and subsurface (20-40 cm) layers across 15 transects covering upper, middle, and footslope positions. Results Results showed that STN and STP concentrations averaged 0.43 and 0.08 g kg - ¹ with coefficients of variation (CV) of 32.25%-37.46% and 57.14%-65.08% respectively. Distinct 3D distribution architectures were revealed where STN exhibited consistent surface enrichment across all slope positions, whereas STP displayed a significant lateral source-sink transition. This transition was characterized by pronounced enrichment at footslopes (0.10 g kg -1 ) compared to upper slopes (0.05 g kg - ¹) and vertical homogeneity in the depositional zone ( P > 0.05). Longitudinal analysis demonstrated a nutrient accumulation trend from upstream to downstream in the left tributary and reservoir area, while the right tributary exhibited fluctuating patterns. Redundancy analysis (RDA) identified slope as the primary environmental driver, with the first axis accounting for 88.31% of the explained variation in nutrient distribution. Discussion These findings demonstrate a significant spatial decoupling between N and P, where footslope positions constitute critical source areas (CSAs) of legacy P risk. The study suggests that watershed management should transition from uniform conservation toward spatially differentiated strategies to ensure water security during the formative years of newly built reservoirs.