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Geese impact on the nitrogen cycle and especially on the fate of litter nitrogen in Artic wetlands

Loonen, M., L. Fivez, P. Meire, I. Janssens and P. Boeckx. 2014.


Due to land use changes and reduced hunting pressure in their wintering grounds, goose numbers increased dramatically over the past 50 years. To understand the consequences of these changes, studies on ecosystem processes of the breeding grounds in the Artic are indispensable. A key process affected by herbivores is decomposition, which in turn influences nutrient cycling and thus plant growth. Here, we investigated the influence of geese on the nitrogen cycle. In Spitsbergen (78° 55' N, 11° 56' E), we used paired long-term exclosures and control plots. Nitrogen incorporation from decomposing litter was studied by tracing the fate of 15N originating from 15N-labelled moss and grass litter. In this study we found indications of geese (grazing) impacting on almost all levels of nitrogen cycling. Geese change the start material for decomposition and nitrogen mineralisation by enhancing the nitrogen concentration and by redistribution of nitrogen among the different ecosystem compartments. Although goose grazing did not significantly alter nitrogen release from moss or grass litter, geese might indirectly have an impact on nitrogen release rates from plant litter by suppressing the production of grass litter, which was found to release nitrogen more readily than moss litter. Moreover, the fate of litter nitrogen varied through at least two mechanisms: i.e. the suppression of grass litter production and the reduction of the moss layer. Indeed, in this study a strong indication was found that nitrogen from grass litter is partly intercepted by the moss layer when it, after decomposition, migrates down to the rooting zone of vascular plants. In absence of geese the moss layer is thicker and more nitrogen from grass litter is intercepted. Already after one winter goose effects on release rates and redistribution from litter nitrogen were found. This means that geese even impact on the nitrogen cycle outside the growing season, when they overwinter further south, and underlines the need for more research over winter times.