Flow of deposited inorganic N in two Gleysol-dominated mountain catchments traced with 15NO3- and 15NH4+

In two mountain ecosystems at the Alptal research site in central Switzerland, pulses of (NO3)-N-15 and (NH4)-N-15 were separately applied to trace deposited inorganic N. One forested and one litter meadow catchment, each approximately 1600 m(2), were delimited by trenches in the Gleysols. (KNO3)-N-15 was applied weekly or fortnightly over one year with a backpack sprayer, thus labelling the atmospheric nitrate deposition. After the sampling and a one-year break, (NH4Cl)-N-15 was applied as a second one-year pulse, followed by a second sampling campaign. Trees ( needles, branches and bole wood), ground vegetation, litter layer and soil (LF, A and B horizon) were sampled at the end of each labelling period. Extractable inorganic N, microbial N, and immobilised soil N were analysed in the LF and A horizons. During the whole labelling period, the runoff water was sampled as well. Most of the added tracer remained in both ecosystems. More NO3- than NH4+ tracer was retained, especially in the forest. The highest recovery was in the soil, mainly in the organic horizon, and in the ground vegetation, especially in the mosses. Event-based runoff analyses showed an immediate response of (NO3-)-N-15 in runoff, with sharp N-15 peaks corresponding to discharge peaks. NO3- leaching showed a clear seasonal pattern, being highest in spring during snowmelt. The high capacity of N retention in these ecosystems leads to the assumption that deposited N accumulates in the soil organic matter, causing a progressive decline of its C: N ratio.