Biological Regulation of Subsoil C-cycling under Field Conditions

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Two soil transfer experiments done in the first phase provided evidence of depth specific carbon incorporation of either cellulose or root derived carbon into different groups of soil microorganisms. Fungal abundance was restricted in subsoils, but fungi incorporated more carbon than bacteria in both surface and subsoils. Consequently, we need to better understand the release of carbon from specific groups of soil microorganisms as well as the transport of microbial residues within the soil profile to adequately model microbially mediated C pathways in soil profiles. Data from the two joint experiments (observatories and DOC injection) will be combined with the exposure of labelled bacteria and fungi at different depths (see Task 1 and 2), making it possible to characterize group specific turnover times as well as transport of microbial residues at two different levels of resolution (temporal scale: either weeks or years, spatial scale: centimeters or meters). Since we have learned that fungal life in subsoils is restricted and that transfer of deeper subsoil into shallower subsoil stimulates fungal abundance, we need to explore further the dynamics and drivers of fungal communities in subsoils. Using next generation sequencing of fungal internal transcribed spacers (ITS) we will define fungal guilds based on their main source of C; saprotrophic fungi are those that gain C from dead organic material, and root-associated fungi are those that gain C directly from their host plant. We will relate fungal key players to the production of specific enzymes and identify them as either saprotrophs (producing hydrolytic and oxidative enzymes) or decomposers (producing oxidative enzymes only). In the context of the research unit, our data will clarify the importance of bacterially and fungally driven stabilisation of organic substances in subsoils and provide a better understanding of long-term carbon storage in forest ecosystems.