Regulatory role of permanent gullies in dissolved nitrogen and phosphorus transport under different rainfall types

Abstract. Understanding how permanent gullies regulate the transport of dissolved ammonium (NH4+), nitrate (NO3-), and phosphorus (P) in runoff delivered from agricultural hillslopes under different rainfall types is essential for controlling non-point source pollution in agroecosystems. In this study, we selected two agricultural catchments, each containing a single permanent gully, and monitored runoff at the gully head and the gully outlet during the rainy seasons of 2022 and 2023. Runoff samples were filtered through 0.45 µm membrane filters and analyzed for dissolved NH4+, NO3-, and P concentrations, and the corresponding nutrient transport fluxes were then calculated. Based on event-scale rainfall characteristics, including rainfall depth, duration, average intensity, maximum 30 min intensity, and erosivity, rainfall events were classified using the k-means method to examine how different rainfall types influenced the role of gullies in the transport of dissolved NH4+, NO3-, and P. The results showed that: (1) Gullies significantly enhanced runoff generation, contributing 36.1 % of total runoff despite occupying only 12.4 % of the catchment 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) Gullies exerted a pronounced dilution effect on dissolved NH4+, NO3-, and P concentrations, particularly on dissolved NO3- (dilution ratio: 0.65). Consequently, the contribution of gullies to dissolved NH4+, NO3-, and P transport fluxes was lower than that to runoff volume, accounting for 31.4 %, 22.4 %, and 31.1 % of dissolved NH4+, NO3-, and P transport fluxes at the outlet, respectively. (3) Type C rainfall dominated the transport of dissolved NH4+, NO3-, and P. Only 10.2 % of events contributed over 68 % of dissolved NH4+, NO3-, and P transport fluxes at the catchment scale and markedly increased their transport sensitivity to rainfall compared to Type A and Type B. These sensitivities were also intensified by gullies. These findings highlight the importance of prioritizing permanent gullies and high-erosivity rainfall events in strategies to reduce dissolved nutrient losses from agricultural catchments.