Carbon Stocks and Carbon Stock Changes in German Forest Soils

Abstract

Forest soils play an important role in the active carbon (C) cycle of terrestrial ecosystems as they store one third of the global organic carbon. Therefore, the sequestration of atmospheric carbon in soils as stable organic matter is discussed as a potential contribution to mitigate atmospheric CO2 concentrations. The carbon dynamic in forest ecosystems is expected to be a result of environmental as well as human-induced factors. Germany's forest soils contained in the organic layer and the mineral soil down to 90 cm on average 117.1 {\textpm} 1.7 Mg C ha–1 which has been increased significantly since the NFSI I by 0.75 {\textpm} 0.09 Mg C ha–1 year–1 resulting in a total increase of 11.3 Mg C ha–1. Structural equation modelling was performed to analyze direct and indirect factors affecting organic carbon stocks and organic carbon stock changes. The pathway analyses revealed a variation of carbon stocks in the organic layer that was especially controlled by tree species. Organic layers under broadleaf trees stored less carbon than under coniferous trees, while tree species effects on carbon stocks of mineral soil were comparatively less pronounced. Soil carbon stocks were furthermore affected by site conditions. An effective selection of tree species combined with specific site conditions may therefore enhance carbon sequestration potential of soils. We found specific effects of nitrogen deposition and forest liming on carbon stock changes. The additional nitrogen has the potential to increase sequestering carbon by an increase in productivity and accumulation of soil organic matter through increased litter production, while liming may both stimulate and inhibit soil respiration depending on various environmental conditions. Altogether, the results showed that further research is needed to identify the most important factors affecting turnover of soil organic matter in respect to the impact of anthropogenic effects as forest stand management, liming, or atmospheric nitrogen deposition especially on dynamics of microbial communities as well as on recalcitrance and stabilization of soil organic matter.