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To evaluate the pathways and dynamics of inorganic nitrogen (N) deposition in previously N-limited ecosystems, field additions of 15 N tracers were conducted in two mountain ecosystems, a forest dominated by Norway spruce ( Picea abies ) and a nearby meadow, at the Alptal research site in central Switzerland. This site is moderately impacted by N from agricultural and combustion sources, with a bulk atmospheric deposition of 12 kg N ha −1 y −1 equally divided between NH 4 + and NO 3 − . Pulses of 15 NH 4 + and 15 NO 3 − were applied separately as tracers on plots of 2.25 m 2 . Several ecosystem pools were sampled at short to longer-term intervals (from a few hours to 1 year), above and belowground biomass (excluding trees), litter layer, soil LF horizon (approx. 5–0 cm), A horizon (approx. 0–5 cm) and gleyic B horizon (5–20 cm). Furthermore, extractable inorganic N, and microbial N pools were analysed in the LF and A horizons. Tracer recovery patterns were quite similar in both ecosystems, with most of the tracer retained in the soil pool. At the short-term (up to 1 week), up to 16% of both tracers remained extractable or entered the microbial biomass. However, up to 30% of the added 15 NO 3 − was immobilised just after 1 h, and probably chemically bound to soil organic matter. 16% of the NH 4 + tracer was also immobilised within hours, but it is not clear how much was bound to soil organic matter or fixed between layers of illite-type clay. While the extractable and microbial pools lost 15 N over time, a long-term increase in 15 N was measured in the roots. Otherwise, differences in recovery a few hours after labelling and 1 year later were surprisingly small. Overall, more NO 3 − tracer than NH 4 + tracer was recovered in the soil. This was due to a strong aboveground uptake of the deposited NH 4 + by the ground vegetation, especially by mosses. |
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