Interactions Between the Carbon and Nitrogen Cycles and the Role of Biodiversity: A Synopsis of a Study Along a North-South Transect Through Europe

Abstract

The C and N cycles of European forests, except in the boreal region, are not balanced, i.e. resource supply exceeds resource use, esp. at the decompn. level. Biodiversity is not limiting these processes, and the soil communities adapt to the soil environment readily, with a change in species compn. at similar levels of diversity. The biodiversity of soil organisms did not regulate the C and N flux, and other soil chem. factors, perhaps base cation supply, Al or P and pH regulate and limit the biol. activity, such that resource use and supply do not match. Anthropogenic N deposition does not alter the short-term C and N fluxes significantly, because the internal fluxes within the ecosystem are much larger than the extra supply and plant prodn. can cope with the supply under most conditions. Chronic atm. deposition of N from air pollutions can trigger processes in the soil, which will lead in the long run to changes in the soil chem. environment, altering the resource use beyond a species-specific threshold. This threshold appears to be at about 100 kg N ha-1 a-1, with regard to inorg. N. It does not matter if this supply originates from ecosystem internal or external processes. If N mineralization rates are low, e.g. due to past land use, external supply may not be harmful, while if N mineralization rates are high, the extra effect of N deposition will lower the site productivity. Canopy uptake of N will contribute to a high N mineralization rate and thus eventually lead to, or enhance the unbalance between supply and demand. The study of the ecosystem internal transfers of C provide ests. of the ecosystem net C balance which is important in view of the Kyoto protocol. C sequestration can be managed and increased by maintaining diversity in terms of species and tree ages on the same area, and by reducing N availability, since NBP was neg. related to the C content in the org. layer. There is no upper limit to which C can be stored in the forest soil, unless other environmental conditions change and support C immobilization, such as disturbances. The av. rate of net ecosystem productivity (NEP) (including woody biomass increments) was about 5 t C ha-1 a-1 and the rate of NBP (excluding woody biomass increments and neglect- ing decompn. after logging) was 1.4 t C ha-1 a-1. These rates are higher than ests. based on the increase in stable humus and black carbon, mainly because of unknown mineralization after logging. Since measurements were carried out across a broad range of climatic and environmental conditions, these rates are extrapolated across Europe. There is a sequestration of 0.7 Gt C a-1 (measured as NEP) including stem-wood increments. When excluding stem-wood growth because it will be harvested, NBP of European forest, as measured by the increase of soil C (including possible effects of logging) is in the order of 0.2 Gt a-1, which is almost 20% of the fossil fuel consumption of the EEC. It exceeds the 8% redn. commitment by the Kyoto proto- col, but carbon gains in managed forests cannot be accounted for