Driver of microbial nutrient turnover in mineral soil, rhizosphere and forest floors

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The forest floor (FF) is the boundary between mineral soil and the atmosphere and is thus important for nutrient transport from the surface to the mineral soil. FF properties influence nutrient concentrations and quality as well as oxygen and water availability. The turnover of nutrients by microorganisms is regulated on the level of nutrient stoichiometry of ecosystems and genetic operon structure of single microorganisms who pursue different strategies to regulate efficient nutrient use. Thus, nutrient availability and community composition are closely interlinked. Consequently, changes in nutrient availability and habitat structure because of increasing temperature feedback on microbial communities in terms of their taxonomic and functional composition as well as trophic interactions. The aim of our project is to understand the interplay of abiotic FF properties with the microbial community composition under different P and temperature regimes. We hypothesize that the quality and quantity of soil organic matter and root exudates as well as nutrient stoichiometry (C, N, P, cations) drives the microbial potential to transform nitrogen and phosphorus and determines trophic interaction with other biota (fungi, fauna and trees). Moreover, spatial correlation of bacteria and nutrient hotspots of N and P are likely. To test these overarching hypotheses we will use a combination of experimental and analytical approaches. We will first identify key players and processes of microbial N and P turnover. Therefore, we will take samples in different FF layers and the mineral soil from 12 beech dominated sites displaying gradients of P concentrations and T and in the rhizosphere of beech, maple and spruce from three sites with contrasting P availability. To identify the long-term adaptation of microbial communities to FF properties and tree species we will combine metagenomics approaches for the identification of key processes and players and subsequent qPCR for the quantification of dominant key players under different settings. Moreover, we will investigate the colonization of roots by microbes involved in P turnover in FF and mineral soil by using CARD-FISH. Second, we will use different experimental approaches: (i) To identify the role of soil minerals for P availability and microbial P acquisition strategies in frame of a litter/OF bag experiment where additional primary and secondary minerals will be added to litter or OF material. (ii) To disentangle the interplay of FF fauna and microorganisms in frame of a fauna exclusion experiment. In addition to the developed quantification tools, we will also isolate bacteria involved in the solubilisation of mineral bound P of the litter/OF bag experiment to reconstruct operon structures of P genes. Based on the different approaches we will be able to identify major drivers of microbial community composition and derived services important for FF turnover under changing environmental conditions.