Fate characteristics of nitrogen in runoff from a small agricultural watershed on the south of Huaihe River in China

Nitrogen loss laws on the field scale or the watershed scale are often explored independently. Few studies have addressed the fate characteristics of nitrogen in runoff from a “source” farmland to a small watershed outlet during different natural rainfall events. The dynamic processes of surface runoff and the concentrations of three nitrogen forms at outlets of two representative “source” farmlands (paddy field and upland), as well as the total outlets of a small agricultural watershed on the south of Huaihe river in China, were simultaneously monitored for three typical natural rainfall events during one crop season. The results showed that the fate of nitrogen in runoff from the small agricultural watershed varied remarkably with time and space. On the scale of “source” field, the peak flows, surface runoff fluxes, concentration peaks and transfer fluxes of TN, NO₃ ⁻-N and NH₄ ⁺-N from the farmlands for the three typical rainfall events were significantly different, and were all in the magnitude order of heavy rainstorm event > rainstorm event > moderate rain event. For the same rainfall event, due to the influence of the antecedent soil moisture condition, the runoff flux from the paddy field was less than from the upland during the rainstorm, while this relation was reversed during the heavy rainstorm and moderate rain events. Based on the observed rainfall events, the variation extent of the concentrations of three nitrogen forms with time in the runoff water from the upland was all greater than from the paddy field. The nitrogen in runoff water from the upland was transferred dominantly in particulate form in the heavy rainstorm and rainstorm events and in dissolved form in the moderate rainfall event. In contrast, rainfall characteristics had a minimal effect on the main form of nitrogen loss from the paddy field, while dissolved nitrogen was dominant in every rainfall event. In addition, a significant nonlinear relationship between the loss loads of the three nitrogen forms and runoff volumes for each rainfall event was identified (P < 0.01). On the scale of the small watershed, the runoff volumes, average concentrations and loss loads of each nitrogen form also varied consistently with rainfall volume. Dissolved nitrogen was dominant in runoff water at the small watershed outlet in every rainfall event. The surface runoff volumes from the whole small watershed were larger than those from the farmlands accounting for about 80% of the small watershed area; however, the average concentrations and loss loads of each nitrogen form in the runoff of the former were all lower than those of the latter. The main form of nitrogen loss in the runoff of the former was more stable, which indicated that a large number of small ponds and drainage ditches in the small watershed had a strong reduction and retention effect on the “source” farmland nitrogen. Therefore, the retention or establishment of regional landscape structure of “field–ditch–pond” had practical significance on the control of agricultural non-point source nitrogen pollution.