The influences of N₂-fixing Alnus and atmospheric nitrogen deposition on the nitrogen balance in close to natural ecosystems

The aim of this thesis was to arrive at an understanding of how increased availability of reactive nitrogen (Nr) caused by atmospheric N deposition or by the dominance of N2-fixing Alnus species influences the N cycle and associated ecosystem properties in close to natural ecosystems.
In Switzerland, atmospheric N deposition represents large N inputs across many close to natural ecosystems and may induce indirect NO and N2O emis- sions from soils in these ecosystems. In chapter 2 of this thesis, we present a model, based on literature data, for the estimation of the indirect emissions in forests, (semi-)natural grasslands and wetlands. In the second part of this thesis (chapter 3–5), effects of N2-fixing Alnus on ecosystems, and by N2O emission on the atmosphere, are addressed. In chapter 3, we review the causes of the rapid encroachment of Alnus viridis into grassland across the entire Alps, and its effects on biodiversity, ecosystem properties, and economy, and we present management options to reduce the expansion of this shrub. Building on the results from chapter 3, we present a multiyear field study (2009–2012) on the effects of the presence of Alnus viridis on N and C pools, and N fluxes, and associated changes in ecosystem properties in comparison to adjacent pastures. This fieldwork was conducted along a West–East gradient in three valleys in central Switzerland (chapter 4). Increased temperature and Nr availability is thought to increase N2O emissions from soils, at least under laboratory conditions. Whether these assumptions hold for field studies was explored in chapter 5.