Soil microbial necromass as an essential phosphorus reservoir in forest nutrition

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Forest ecosystems successfully meet the challenge of low phosphorous (P) supply by exploiting minimally accessible inorganic and organic P forms and retaining P within ecosystems. Microorganisms efficiently mobilize plant inaccessible P but also compete with plants for P. Although microbial bio- and necromass can account for greater than 10% of total soil P, they rarely were considered as primary P contributors to forest P nutrition. This proposal seeks to unravel the relevance of microbial bio- and necromass as P pools for beech P nutrition by characterizing the spatial heterogeneity of P distribution in forest ecosystems and contrasting P cycling in acquiring versus recycling forest ecosystems. The role of individual microbial groups in P (im)mobilization, reduction of P losses via leaching and re-mobilization of microbial necromass P (Pnecro) will be analyzed depending on P availability and speciation in forest soils.Immobilization of rhizodeposit P and soil solution phosphate in microbial biomass will be quantified in mesocosms by labeling of soils with KH233PO4 and plants via the wick method. After simulating rainfall events, dissolved organic P and colloidal discharge will be quantified and characterized. Microbial P retention and microbial groups accounting for P immobilization will be determined by chloroform fumigation extraction (33P-CFE) and 33P phospholipid fatty acid (33P-PLFA) analysis. To determine sources of Pnecro, microbial P formation from low available P sources (33P apatite, Fe-33P-phosphate and 33P plant residues) will be targeted in a subsequent incubation experiment with 33P-CFE and 33P-PLFA analysis. Microbial dilution will reveal functional redundancy and specialization within microbial communities. Following identification of Pnecro sources, their functions as an intermediate P reservoir for forest nutrition will be assessed by application of 33P-labelled microbial necromass. A seasonally harvested rhizotron experiment with heterogeneously distributed Pnecro will enable visualization of P re-mobilization in beech rhizosphere by 33P imaging and phosphatase zymography. T-shaped mesocosms will allow quantitative determination of Pnecro contribution to beech nutrition with increasing distance to the stem, especially considering ectomycorrhiza as an alternative Pnecro uptake pathway. Within the central P x N application experiment, PLFA, CFE and phosphatase activity analyzed at increasing distance to the stem will allow transferring these results on the role of Pnecro to single tree influence circles at the field scale.Consequently, functions of microbial necromass as an intermediate P reservoir and its role in forest P nutrition will be assessed within the proposed project considering sites of contrasting P availability and speciation. Understanding spatial and seasonal dynamics of microbial necromass P in forest soils will fundamentally enhance our understanding of forest ecosystems capability to face low P supply.