Allocation patterns of airborne nitrogen in mountainous heathlands - A 15N tracer study in the Cantabrian Mountains (NW Spain)

The fate of atmospheric N depositions in heathlands dominated by Calluna vulgaris (L.) Hull in the Cantabrian Mountains (NW Spain) was analyzed in this study. The aim was to identify and quantify allocation patterns of airborne nitrogen in mountain heathland ecosystems by 15N tracer experiment. Four replicated plots were established to analyze 15N partitioning among different compartments selected (Calluna biomass, soil horizons and soil microbial biomass), besides N losses by leaching, using 15N tracer pulse addition. The study was conducted over two years. The recovery of 15N tracer was significantly higher (72%) in the first year compared to the second year (5%). Most 15N was recovered in the soil compartment in both years, mainly in the O-horizon. 15N losses by leaching were negligible over two years, suggesting that the ecosystem was not N saturated. Low 15N tracer recovery was found both in the new shoots of Calluna (0.5%) and the old ones (1.3%) in the short-term. The soil microbial biomass was not an important N sink in these heathlands. This study demonstrates that Cantabrian heathlands have a capacity to immobilize nitrogen in a short time, but that N is transferred out of the ecosystem during the second year.