# Carbon and Nitrogen Pools and Fluxes in Adjacent Mature Norway Spruce and European Beech Forests
## 4 Discussion  
Both forest stands have been altered by different forest management strategies since the 1850s and by impacts of acid deposition during the 20 th century. Former mixed forests were converted into pure spruce monocultures, with current second rotation of Norway spruce in the spruce stand. The beech stand was transformed by acid deposition to broadleaf monoculture during the 1970s and 1980s by induced selective conifer dieback. It is very likely that in the 1850s, similar site conditions prevailed at both stands. Since then, significantly higher soil C and N stocks have been measured in the beech forest compared to the spruce forest. While standing biomass was higher in the beech stand, due to the relatively high tree density combined with the high wood biomass of the 150 year old beech forest, the biomass N stocks were comparable among stands, resulting in a lower C/N ratio in the spruce stand compared to the beech stand.  
Although estimated historical N inputs were higher in the spruce stand due to higher pollutant scavenging by spruce needles [47], current deposition fluxes of N are similar in both forests. On the contrary, DOC throughfall fluxes were higher in the spruce stand. Few differences were measured in the DOC soil solution fluxes in the soil profiles. Nitrogen fluxes were dominated by DON at both stands, with higher NO\({}_{3}\)\({}^{-}\) losses in the beech stand. Soil respiration dynamics were similar for both forest sites, with slightly higher Q\({}_{10}\) in the beech stand compared to the spruce stand. As a result, annual soil respiration was similar at both sites. Higher litterfall C flux in the beech forest caused lower total belowground carbon allocation (TBCA) compared to the spruce stand. The estimated fraction of TBCA that accounted for belowground net primary productivity (BNPP) was similar in both forest ecosystems. Due to the lower TBCA, both gross primary productivity (GPP) and net primary productivity (NPP) were lower in the beech stand compared to the spruce stand. Due to the similar evapotranspiration (ET) in both stands, water use efficiency (WUE) was higher in the spruce stand compared to the beech stand. Higher N fluxes were measured in the beech stand compared to the spruce stand, together with higher susceptibility of inorganic N losses in the beech stand compared to the spruce stand.  
\begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline  & \(\mathbf{N_{req}}\) & \(\mathbf{N_{req}}\) & \(\mathbf{N_{req}}\) & \(\mathbf{N_{req}}\) & \(\mathbf{Coarse}\) & \(\mathbf{N_{req}}\) & \(\mathbf{N_{req}}\) & \(\mathbf{N_{trans}}\) & \(\mathbf{N_{uptake}}\) & \(\mathbf{N_{insoil}}\) \\  & **Wood** & **Foliage** & **abovegr** & **Roots** & **Roots** & **Belowgr** & **Ntrans** & \(\mathbf{N_{uptake}}\) & \(\mathbf{N_{insoil}}\) \\ \hline  & & & & & & \(\mathrm{kg\,ha^{-1}\,year^{-1}}\) & & & & \\ Spruce stand & 15 & 45 & 60 & 17 & 28 & 45 & 5.7 & 71 & 57 \\ Beech stand & 12 & 116 & 127 & 3.5 & 23 & 26 & 60 & 71 & 74 \\ \hline \hline \end{tabular}
\end{table}
Table 9: Nitrogen allocation in spruce and beech stands (kg N\(\cdot\)ha\({}^{-1}\cdot\)year\({}^{-1}\)). Annual nitrogen requirements (N\({}_{\text{req}}\)) of the wood and coarse roots were derived according to biomass equations. N\({}_{\text{req}}\) of the fine roots was estimated as a 0.14 fraction of NPP following [44]. Annual N translocation (N\({}_{\text{trans}}\)) is the difference between foliage N\({}_{\text{req}}\) and annual N litter production. Net N uptake (N\({}_{\text{uptake}}\)) was calculated as the difference between the sum of N\({}_{\text{req}}\) of aboveground and coarse root production and N\({}_{\text{trans}}\). Total flux of N to the forest floor (N\({}_{\text{insoil}}\)) consisted of litterfall N and throughfall N fluxes.