Controls on Soil C:N Stoichiometry and Implications for Carbon Accrual.

In a continuously changing environment, where the need to sequester carbon (C) and stabilize reactive nitrogen (N) in soil is an ever more pressing issue, understanding the factors controlling soil C:N stoichiometry and its implications for soil C storage at the larger scale and under different land covers is a fundamental step to inform earth-system modeling and policy maker communities. Since N availability is expected to limit terrestrial C storage in the future, terrestrial ecosystems with a higher soil C:N ratio, such as forests with ectomycorrhizal associations could sequester more soil organic C (SOC). We used European-wide soil (n= 8643), plant cover and traits databases to analyze the biophysical factors controlling the mineral soil C:N ratio in forests and grasslands, and investigate the relationship between soil C:N and SOC storage. Top mineral soil C:N appeared to be a feature of the system: coniferous and mixed forests had consistently higher C:N ratios than broadleaved and grassland soils, and their C:N ratio decreased with increasing pH and temperature. Broadleaved forests and grasslands had lower C:N ratios, largely independent of physical controls. While ectomycorrhizal systems had a significant higher soil C:N ratio than arbuscular mycorrhizal systems, this seemed to be determined by the plant rather than the fungal component of the association. Furthermore, the inclusion of mycorrhizal type did not improve soil C:N model prediction. Across ecosystem types, higher soil C:N tended to be associated with higher soil C stocks. We are conducting soil physical fractionation to test the hypothesis that high C:N soils have a lower relative amount of persistent mineral-protected SOC, since this is known to have a low C:N ratio. We will present these results and expect that promoting soil C sequestration in ectomycorrhizal forest soils leads to the accumulation of particulate organic matter, a relatively short term gain.