Seasonal rainfall variation disrupted the balance of carbon and nitrogen coupling loss in subtropical acidified watershed

Carbon and nitrogen coupling loss is a critical ecological process influenced by biogeochemical interactions, while the balance is frequently disrupted by significant seasonal variability in subtropical watersheds. This study investigated the mechanisms driving carbon‒nitrogen coupled loss across the land-river-estuary continuum in a representative subtropical acidic watershed in southern China. Through multi-season sampling, field research, and a distributed hydrological model, we analyzed spatiotemporal patterns of carbon and nitrogen concentrations in surface water, bed sediments, and vertical sediments. This study revealed a strong coupling effect between carbon and nitrogen loss during the dry season (R = 0.702, P < 0.01). In contrast, seasonal factors such as rainfall in the wet season disrupted this coupling balance. The total organic carbon concentration in surface water increased significantly (with an average value of 2.26 mg/L), whereas the proportion of ammonium nitrogen decreased by 50% due to enhanced nitrification and dilution effects. Spatial analysis highlighted the downstream accumulation of pollutants, which were driven primarily by diffuse agricultural sources and sediment deposition. Redundancy analysis identified precipitation pH, soil pH, and the vegetation index (NDVI) as key drivers of these processes. This study underscores the vulnerability of carbon-nitrogen coupling to seasonal hydrological shifts and anthropogenic pressures. Exploring the seasonal characteristics and driving mechanisms of carbon and nitrogen coupling loss can further elucidate the factors influencing watershed diffuse pollution and provide valuable insights for formulating effective watershed management strategies.