Regulatory role of permanent gully in runoff dissolved nitrogen and phosphorus transport across rainfall types

Abstract. Tracking the transport of runoff-dissolved nitrogen (N) and phosphorus (P) from upslope farmland to the catchment outlet is vital for controlling non-point source pollution in agroecosystems. However, the hydrological and regulatory roles of permanent gully within catchment in modulating dissolved N and P losses dynamics under natural rainfall conditions remain poorly understood. In this study, runoff and associated losses of dissolved NH₄⁺, NO₃^-, and P were measured at both the gully head and the outlet from 2022 to 2023. The results are as follows: (1) Gully significantly enhanced runoff generation, contributing 36.1 % of total runoff despite occupying only 12.4 % of the area. This contribution varied across rainfall types (Type A, frequent, low-depth, low erosivity; Type B, short duration, high intensity; Type C, long duration, high erosivity) and was highest under Type A (43.2 %) and lowest under Type C (33.8 %). (2) Gully exerted a pronounced dilution effect on the concentrations of dissolved NH₄⁺, NO₃^-, and P, particularly for dissolved NO₃^- (dilution ratio: 0.65). Consequently, gully contributed less to dissolved nitrogen and phosphorus fluxes relative to its contribution to runoff volume, accounting for 31.4 %, 22.4 %, and 31.1 % of dissolved NH₄⁺, NO₃^-, and P fluxes, respectively. (3) Type C rainfall dominated the loss of dissolved N and P. Only 10.2 % of events contributed over 68 % of dissolved N and P fluxes at the catchment scale and markedly increased their loss sensitivity to rainfall compared to Type A and Type B. These sensitivities were also intensified by gully. The study provides new insights into runoff dissolved nutrient interactions within gully systems and offers a foundation for improving nutrient management in gully-dominated agricultural landscapes.