Nutrient fluxes and speciation shaped by source-hydrology coupling in the Pearl River Basin

Nutrient fluxes exhibit significant spatiotemporal heterogeneity, driven by the dynamic coupling of hydrological processes and biogeochemical cycles. To elucidate the mechanisms controlling nutrient species, we conducted a process-based investigation of the Pearl River Basin using simulations from the Soil and Water Assessment Tool. Our findings reveal that surface nitrate and soluble phosphorus are primarily transported by surface flow, whereas particulate inorganic phosphorus is predominantly regulated by sediment transport. These transport mediators, coupled with soil nutrient pools and fertilizer use, govern the fluxes of nutrient species. Fertilizer use, despite being less abundant than soil pools, exerts a stronger influence due to its closer coupling with transport mediators. In contrast, lateral nitrate flux is largely controlled by soil nitrate pools, with smaller contributions from the coupling between lateral flow and fertilizer use. Organic nutrient species, on the other hand, are primarily regulated by sediment transport in conjunction with plant residues and show minimal dependence on fertilizer inputs. Furthermore, the dominant influence of transport mediators, such as flow and sediment, results in a pronounced wet-season dominance in annual nutrient loads across all nutrient species. Land surface processes further shape the spatial patterns of nutrient fluxes. Surface and lateral flows are most active in regions with high precipitation, with surface flow dominating in urban and agricultural areas, while lateral flow is more prominent in clay-rich forests. Sediment yield is highest in clay-rich urban and agricultural landscapes. Soil nitrate pools and plant residues are abundant in forested regions, whereas inorganic phosphorus pools are elevated in pasturelands. Overall, this study provides critical insights into nutrient dynamics within heterogeneous watersheds, highlighting the interplay between transport processes and nutrient sources across diverse terrestrial landscapes.