Contrasting dynamics of dissolved inorganic and organic nitrogen in soil and surface waters of forested catchments with Gleysols

In this study, we investigated the dynamics of dissolved inorganic and organic nitrogen (N) in throughfall, in soil solutions of Gleysols, and in outflows from experimental sub-catchments as well as from a headwater catchment in Switzerland. Additionally, we studied the effect of increased N deposition on dissolved N by applying 3 g NH 4NO 3–N m −2 year −1 to soil plots and to one of the sub-catchments. Dissolved organic N (DON) was the dominant form of total dissolved N (TDN) in the soil and surface waters. The proportion of DON in TDN increased from the throughfall down to the subsoil, which indicates that the retention of DON was lower than that of inorganic N. Concentrations of DON in the subsoil were higher under reducing than under oxidising conditions. In the soil solution and in the runoff from all catchments, nitrate and DON displayed inverse seasonal patterns with concentrations of NO 3 − being highest in late winter and those of DON being maximal in summer and fall. This difference shows that during periods of increased biological activity, NO 3 − was retained in the forest ecosystem while the production of DON was stimulated. Concentrations of both NO 3 − and DON were negatively correlated with the amount of throughfall. In the outflow of the headwater catchment, however, the concentration–discharge relationship was negative for NO 3 −, but positive for DON. The reason for this difference appeared to be the larger contribution of topsoil water at high flow, which was poor in NO 3 − and rich in DON. Experimentally increasing the N deposition increased NO 3 − leaching significantly, but had no effects on DON leaching and on DON export from the catchments. In conclusion, our results show that the dynamics of dissolved organic and inorganic N are controlled by different factors.