Carbon and Nitrogen Pools and Fluxes in Adjacent Mature Norway Spruce and European Beech Forests

We compared two adjacent mature forest ecosystem types (spruce vs. beech) to unravel the fate of assimilated carbon (C) and the cycling of organic and inorganic nitrogen (N) without the risk of the confounding influences of climatic and site differences when comparing different sites. The stock of C in biomass was higher (258 t·ha−1) in the older (150 years) beech stand compared to the younger (80 years) planted spruce stand (192 t·ha−1), whereas N biomass pools were comparable (1450 kg·ha−1). Significantly higher C and N soil pools were measured in the beech stand, both in forest floor and mineral soil. Cumulative annual CO2 soil efflux was similar among stands, i.e., 9.87 t·ha−1·year−1 of C in the spruce stand and 9.01 t·ha−1·year−1 in the beech stand. Soil temperature explained 78% (Q10 = 3.7) and 72% (Q10 = 4.2) of variability in CO2 soil efflux in the spruce and beech stand, respectively. However, the rather tight N cycle in the spruce stand prevented inorganic N losses, whereas losses were higher in the beech stand and were dominated by nitrate in the mineral soil. Our results highlighted the long-term consequences of forest management on C and N cycling.