Nitrogen deposition effects on forest ecosystems: Linking N leaching patterns to long-term dynamics of soil C/N ratios in Swiss ICP Forests Level II Plots

Forests play a major role in wood production and other ecosystem services, such as carbon (C) sequestration and filling reservoirs in drinking water quality. However, it is still under discussion to what extent environmental changes, such as elevated nitrogen (N) deposition and related eutrophication, may affect such services.

Our study aimed to assess long-term changes in N and C storage in Swiss forest soils along a gradient in N deposition (about 10 to 30 kg N/ha/y). At five long-term forest ecosystem research plots in Switzerland, which are part of the ICP Forests Level II network, nutrient fluxes (atmospheric deposition, litterfall, soil solution) have continuously been measured since the 1990s. Soil samples were taken from fixed depth layers in the course of soil inventories in the 1990s and 2022.

The observed flux patterns indicated that the forests had reached nitrogen saturation on some sites, resulting in nitrogen leaching. At sites with a higher carbon-to-nitrogen ratio (C/N), we found comparatively lower levels of N leaching. The comparison of the two soil inventories showed that the N concentration in soils has actually increased (and the C/N ratio decreased) on the sites with high C/N ratio and high N deposition. We will discuss the observed accumulation and transformations of organic C and N in these soils and the potential impacts on selected ecosystem services.