# Carbon and Nitrogen Pools and Fluxes in Adjacent Mature Norway Spruce and European Beech Forests
## 4 Discussion
### Tree Species Effects on C Cycling  
Annual litterfall was higher in the beech stand compared to the spruce stand. Spruce litterfall flux was comparable to other studies on temperate forests of similar age, whereas beech litterfall was higher at our investigated stand [48; 49]. The relatively high measured litter flux in the beech stand was a result of high crown area and dimensions of beech trees in the plot. Litterfall is an important component of forest ecosystem NPP and nutrient cycling [50]; thus, it is highly important for forest management strategies to sequester C [51]. Our results suggest that old growth broadleaf forest could deliver a substantial amount of assimilated C to the forest floor compared with lower inputs in managed forests. Surprisingly similar fluxes of the DOC from the forest floor were measured in the two forest stands. DOC flux represented 11% of annual litterfall flux in the spruce stand and 6% of annual litterfall flux in the beech stand. This can be interpreted as showing that spruce litter is an important source of soil solution DOC [52]. As for the forest floor, similar seepage fluxes of DOC were observed in the mineral soil. A significant reduction of DOC flux occurred from top soil to 40 cm depth, by roughly 80%. Whether it is absorbed in the mineral soil or respired needs further research. However, despite comparable fine soil pools in both forest floor and mineral soil, significantly higher C concentration in deeper mineral soil (20-40 cm) caused a significantly higher soil C pool in the beech stand compared to the spruce stand. As we did not observe significantly higher DOC flux from decaying litter in the beech stand compare to the spruce stand, we suggest that higher soil C pool under the beech forest was a result of long-term accumulation of recalcitrant SOM that originated from root and mycorrhizal decomposition [15; 16]. Simultaneously, higher contents of oxalate-extractable Fe and Al were found in the beech mineral soil [53], suggesting higher sorption capacity in this stand, favouring C sequestration in the beech stand over the spruce stand. Contrary to previous paired studies (e.g., [13]), we did find a higher C stock in the forest floor of the beech stand compared to the spruce stand. In an earlier study, we documented partial loss of forest floor C in the spruce stand as a result of recovery from acidification [54]. In the early 1990s, forest floor C mass was 58 t-ha\({}^{-1}\), comparable to the current beech stand, but it has since declined by 47%. We do not know whether a similar proportional decline occurred at the beech stand due to the lack of available historical data. We can only suggest that relatively high N availability at the beech stand (low soil C/N, some inorganic N leaching) may have stimulated greater stabilization into humus through a combination of chemical reactions and enzyme inhibition [55; 56], thus preventing SOM loss from the forest floor after acidification retreat. Soil temperature explained 78% and 72% of temporal variation in soil respiration in the spruce and beech stands, respectively. Soil moisture did not provide additional explanatory power in multiple regression analysis, suggesting low moisture stress in our mountainous forest ecosystems. Q\({}_{10}\) was slightly higher in the beech stand (4.2) compared to the spruce stand (3.7), but both values were close to the Q\({}_{10}\) values reported for other temperate ecosystems [23; 41; 57]. Based on our calculations, potential respiration at 0 \({}^{\circ}\)C was higher in the spruce (0.89 g:m\({}^{-2}\cdot\)day\({}^{-1}\) of C) than in the beech stand (0.78 g:m\({}^{-2}\cdot\)day\({}^{-1}\) of C) and similar at 10 \({}^{\circ}\)C. Thus, annual CO\({}_{2}\) soil efflux was higher in the spruce stand compared to the beech stand, due to the calculated higher soil respiration in winter and spring periods. One of the assumptions when calculating TBCA is that the total soil C pool is in a steady state.