Nitrogen dynamic transport processes shaped by watershed hydrological functional connectivity

Nitrogen (N) pollution is the major type of non-point pollution in watersheds. The nitrogen transport process is significantly controlled by the watershed hydrological connectivity under different rainfall conditions. Changes in hydrological connectivity were controlled by extreme rainfall events, resulting in a more uncertain response mechanism for dynamic nitrogen transport. In this study, four typical rainfall events were selected based on long-term positional monitoring of a small watershed using high-frequency sampling (long duration rainfall events E1, E2; short duration rainfall events E3, E4). Characterizing spatial and temporal changes in hydrological connectivity under different types of rainfall events. Revealing how hydrological connectivity shape the nitrogen dynamic processes. The results showed that the hydrological functional connectivity was better for long duration rainfall events than for short duration rainfall events as influenced by rainfall events (E1, E2: 4.58, 5.5; E3, E4: 5.25, 5.18). The difference in hydrological connectivity not only made the average and peak nitrogen concentration load under the long duration rainfall events higher than that under the short duration rainfall events. It also resulted in different nitrogen source composition, with soil nitrogen (SN) and manure & sewage (M&S) being the main sources under long duration rainfall events (SN: 28.3%, 25.5%; M&S: 41%, 53%), whereas M&S dominated under short duration rainfall events (37%, 67%). The antecedent precipitation index (API) also constrained the timing of onset of hydrological functional connectivity, in addition to rainfall events influencing watershed hydrological connectivity. The landscape characteristics of the watershed also affected the spatial and temporal characteristics of hydrological connectivity, which shaped the nitrogen source-sink transition relationship in the watershed. Based on the differences in the response of nitrogen transport characteristics to hydrological connectivity under different rainfall events. Considering hydrological connectivity dimensions and intervention can help to achieve precise management of nitrogen in the watershed.