Nitrogen effects on soil organic matter turnover at the molecular level
Atmospheric nitrogen (N) deposition has increased three- to fivefold over the last century due to human activity, mainly by fossil fuel burning and fertilizer application. In terrestrial ecosystems, the availability of N feeds back on soil carbon (C) dynamics, by influencing processes that lead to changes in C fluxes and consequently pool sizes. Since soils represent the largest C pool in terrestrial ecosystems, there is a strong need to understand the underlying processes, because major N-induced changes in soil C storage can affect global C dynamics. Within this project, we are testing the hypothesis that decomposition processes of individual organic molecules respond differently to N, depending on their internal N content. According to this hypothesis, decomposition of microbially-derived compounds with high N content, would respond differently to N additions, in contrast to plant-derived compounds with low N content.
We have the opportunity to use soil samples from a four year CO2 enrichment and N deposition experiment with model forest ecosystems growing on two soil types (Figure 1). Ecosystems were fumigated with 13C-labeled CO2 and treated with two levels of 15N-labeled fertilizer (Figure 2), which enables us to apply compound-specific stable-isotope-analysis of individual organic compounds. We will study the influence of N deposition on composition and turnover of plant-derived lignin and microbially-derived amino sugars in soil. Additionally, physical fractionation of soil samples prior to analysis will identify soil compartments where important changes occur. These information may also be used to test general hypotheses on stabilization mechanisms of soil organic matter that are currently discussed.
