Minerals and their interactions with dissolved organic matter as regulators of microbial necromass stabilization in soil

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Microbial necromass is presumed to significantly contribute to stable soil organic matter. Yet, the so-called ‘entombing effect’, i.e. the built-up and subsequent stabilization of necromass-derived matter, is still not well understood on a mechanistic level. The sorption of dissolved organic matter to reactive mineral surfaces is widely known as a major process of soil carbon stabilization. Herein, we thus plan (i) to study production and reactivity (biodegradability and sorptivity) of dissolved organic matter along the decomposition continuum from plant litter to microbial necromass, and (ii) to concurrently examine how sorptive stabilization of organic compounds shapes the structure of the microbial community, and its carbon and energy use. The project will, for the first time, describe interactions between necromass formation and dissolved organic matter sorption on basis of a quantitative assessment of matter and energy fluxes. It will, therefore, contribute to Central Hypotheses C (Boundary conditions) and also B (Biological complexity) of the SPP 2322. The proposed research links direct in situ decomposition of plant litter and microbial necromass with in-depth analyses of underlying processes to identify all steps of necromass stabilization. This includes: (i) incubation experiments on the production of dissolved organic matter from decomposing plant litter and microbial necromass (bacteria and fungi), (ii) sorption experiments using different pristine minerals, (iii) biodegradability tests for dissolved organic matter before/after sorption, (iv) tests of the stability of mineral-bound organic matter, and (v) incubations of carbon-labelled plant litter and microbial necromass in natural soil. This step-wise experimental approach enables to quantify and explain differences in the transfer efficiency of carbon and energy from variable organic sources to mineral surfaces. The determination of the molecular composition and energy content of dissolved and sorbed organic matter as well as the analysis of the microbial community using state-of-the-art methods, such as FT-ICR-MS and XPS among others, allow for gaining novel insight into the mechanisms of necromass stabilization in soil. The project will add to our understanding how minerals regulate microbial energy use, and thus, contribute to the development of new-generation soil carbon models with explicit representation of microbial turnover.