Fate of airborne nitrogen in heathland ecosystems: a 15N tracer study

In the present study, we analyze the fate of airborne nitrogen in heathland ecosystems (NW Germany) by means of a ¹⁵N tracer experiment. Our objective was to quantify N sequestration and N allocation patterns in an ecosystem that is naturally limited by N, but that has been exposed to airborne N inputs exceeding critical loads for more than 3 decades. We hypothesized that the system has a tendency towards N saturation, which should be indicated by low N sequestration and high N leaching. We analyzed ¹⁵N partitioning (aboveground biomass and soil horizons) and investigated ¹⁵N leaching over 2 years following a ¹⁵N tracer pulse addition. ¹⁵N tracer recovery was 90% and 76% in the first and second year, respectively. Contrary to our expectations, more than 99% of the tracer recovered was sequestered in the biomass and soil, while leaching losses were <0.05% after 2 years. Mosses were the most important short‐term sink for ¹⁵N (64% recovery in the first year), followed by the organic layer. In the second year, the moss layer developed from a sink to a source (23% losses), and soil compartments were the most important sink (gains of 11.2% in the second year). Low ¹⁵N recovery in the current year's shoots of Calluna vulgaris (<2%) indicated minor availability of ¹⁵N tracer sequestered in the organic layer. N partitioning patterns showed that the investigated heaths still have conservative N cycling, even after several decades of high N loads. This finding is mainly attributable to the high immobilization capacities for N of podzols in soil compartments. In the long term, the podzol A‐ and B‐horizons in particular may immobilize considerable amounts of incoming N. Since N compounds of these horizons are not readily bio‐available, podzols have a high potential to withdraw airborne N from the system's N cycle.