Carbon - Nitrogen interactions in forest ecosystems. Final report

Abstract

Databases on carbon (C) and nitrogen (N) fluxes and pools in European forests were compiled for 400 sites and explored thoroughly to create empirical models that predict C accumulation and N retention/nitrate leaching from N input, climate, and ecosystem characteristics. For nitrate leaching, analyses show that there is a threshold N deposition of 8-10 kg N/ha/yr below which almost no leaching occurs. The important parameters that determine N leaching (and thus N retention) are: N deposition, the organic layer carbon to nitrogen ratio (C/N ratio) and annual temperature. At low C/N ratios (below 23) N input determines N leaching. At higher C/N ratios both N input and temperature are important. Adding more sites throughout the project did not change these relationships and they were robust in validation tests. Based on a ‘N balance’ approach, estimates of soil C sequestration rates were calculated by multiplying soil N retention with soil C/N ratio. The mean for European forest with data was 190 kg C/ha/yr, but these have a geographical bias towards central Europe where the estimated C sequestration rates are highest. An unbiased but more uncertain extrapolation to Europe had an overall mean of 70 kg C/ha/yr. Estimates of C and N sequestration rates in the organic layer of forest soils have been calculated for specific sites based on the ‘limit value’ concept that uses data from studies of the decomposition of organic matter. The method was further validated in CNTER. This method can be upscaled to Europe, and for 150 sites for which data are sufficient, a mean of 400 kg C/ha/yr was obtained. The method has also been applied for the whole of Sweden, where the range in estimated soil C sequestration was 40-400 kg C/ha/yr. Estimates obtained by the N-balance approach throughout Sweden were below those for the limit value but followed the same spatial gradients. Estimates of C sequestration in the organic layer (using the limit value method) are usually higher than those using the N balance approach for two reasons: i) unlike the N balance method, the limit value approach cannot account for negative C sequestration (i.e. a C loss), and ii) the limit value approach estimates C accumulation in the forest floor (which has the highest C accumulation rate of the soil) whereas the N balance approach accounts for the whole soil profile. This is most noticeable after land use change (i.e. afforestation) where an organic layer accumulates, but where C may be lost from the mineral soil. Efforts on modelling C sequestration have shown that traditional concepts for decomposition assuming a steady state at some point yield too low C accumulation. We have further made regional and European estimates of the present C sequestration rates in forest soils using several methods. The estimates have consistently shown that C sequestration rates in soil are low: 0-400 kg C/ha/yr. A probable mean is c. 100 kg C/ha/yr. Converted to a European scale this is 13 Mt C/yr, where trees additionally have a net accumulation in the biomass of c. 70 Mt C/yr. Our estimates are much lower than estimates published earlier based on other approaches. For the CNTER estimates it is assumed that C accumulates with N i.e. that soil C/N ratios do not increase. With the elevated N deposition in Europe, the soil C/N ratios are more likely decreasing, thus C-sequestration rates are probably even lower than we estimated. We find it very important to arrive at well-established soil C sequestration numbers, since the size of the possible biological C sink in Europe is uncertain and under debate. We will seek to continue to improve on our estimates of soil C sequestration rates. To gain insight into C and N interactions, soil and vegetation from long-term field experiments using additions of stable N isotopes have been re-sampled. After ten years, most of the N applied over one year is still present in the soil in amounts not very different from those measured after 1-3 yrs. These experiments allow for a thorough testing and validation of a process model that predicts the fate of N in the ecosystem. When applied to multiple sites we gain an insight in the fate of N that will allow calculation of C accumulation and fluxes in the systems, which cannot be obtained in other ways. Tree species trials, plantation mosaics and felling experiments were re-sampled to gain insight into forest management options (tree species, age and felling regime) for increasing C sequestration and protecting downstream water against eutrophication. The indicators sampled were concentrations of nitrate, DOC and pH in seepage water below rooting zone, forest floor and upper mineral soil C and N pools and CN ratio. The impact of felling was increased with decreasing depth of the organic layer at humid temperate climate. There was no consistent effect of tree species on N leaching between regions in these trials. On a cross-European basis, conifer forests receiving inorganic N in throughfall from 10-25 kg N ha-1 y-1 appear to have enhanced N leaching over hardwood forests receiving the same amount of N deposition