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 orga nic nitrogen (N) in throughfall, in sail solutions of Gleysols, and in outf lows from experimental sub-catchments as well as from a headwater catchment in Switzerland. Additionally, we studied the effect of increased N deposit ion on dissolved N by applying 3 g NH4NO3-N m(-2) year(-1) to soil plots an d 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 proport ion 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. Co ncentrations of DON in the subsoil were higher under reducing than under ox idising conditions. In the soil solution and in the runoff from all catchme nts, nitrate and DON displayed inverse seasonal patterns with concentration s of NO3- being highest in late winter and those of DON being maximal in su mmer and fall. This difference shows that during periods of increased biolo gical activity, NO3- was retained in the forest ecosystem while the product ion of DON was stimulated. Concentrations of both NO3- and DON were negativ ely correlated with the amount of throughfall. In the outflow of the headwa ter catchment, however, the concentration-discharge relationship was negati ve for NO3-, but positive for DON. The reason for this difference appeared to be the larger contribution of topsoil water at high now, which was poor in NO3- and rich in DON. Experimentally increasing the N deposition increas ed NO3- leaching significantly, but had no effects on DON leaching and on D ON export from the catchments. In conclusion, our results show that the dyn amics of dissolved organic and inorganic N are controlled by different fact ors. (C) 2001 Elsevier Science B.V, All rights reserved.