Risk and driving factors of nitrogen and phosphorus runoff losses under extreme rainfall conditions: sludge application strategies in forest soils

Sewage sludge (SS), enriched with organic matter (OM), nitrogen (N), and phosphorus (P), holds the potential for rehabilitating degraded forest soils, yet poses non-point pollution risks through rainfall-induced nutrient migration. We systematically quantified N and P losses under five SS application rates using six simulated rainfall events (90 mm·h⁻¹). Results showed that SS amendment increased interflow and sediment erosion, while reducing surface runoff. Cumulative nutrient losses increased proportionally with the application rates. Surface runoff was responsible for 43.5 % of total nitrogen (TN) transport, whereas sediments mediated 96.2 % of P loss. Notably, interflow exhibited TN concentrations that were 6–70 times higher than other pathways, and its contribution to N loss increased from 16.2 % to 40.6 % following SS application. The first flush effect on ammonium nitrogen and nitrate nitrogen (AN and NN) was detected in the surface runoff, while increased exports of organic nitrogen (ON) and AN were observed in interflow. SS application preferentially enriched N and P in 0.25–1 mm sediment particles, raising the percentage of TN carried by this fraction from 12.5 % to 37.9 % and that of total phosphorus (TP) from 11.8 % to 35.8 %. Multivariate analysis indicated SS application rate, rainfall frequency, and particle size as the key drivers that modulate nutrient loss by modifying soil porosity. Ecological risk assessments recommend limiting SS application to ≤6 kg·m⁻² in eucalyptus plantations to reduce nutrient leaching, protect groundwater, and adjacent aquatic environments. Our findings provide important insights into how SS can be sustainably applied as a soil amendment, while minimizing the environmental risk of N and P loss from forest soils through runoff.