Nutrient dynamics along the litter – mineral soil continuum

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Slow turnover of the forest floor (FF) is often assumed to be related to immobilisation of nutrients within the organic matter. This is in contrast to the overall hypotheses of FOR 5315 that thick FF represents some kind of adaptation to low nutrient supply by mineral soils. With our project we investigate under which conditions slow turnover of FF material might be an advantage. Our objective is to determine which processes control the relevance of the FF for tree nutrition as compared to the mineral topsoil. We test the hypotheses that (1) at nutrient poor sites the turnover of C is lower than the turnover of P and N, (2) admixture of minerals to FF material and subsequent biotic formation of SOM-mineral associations decreases the pool of organically bound P+N and of easily mobilisable P+N, and (3) that in Norway spruce (Picea abies), FF will contribute more to N and P uptake than in maple (Acer pseudoplatanus) while beech (Fagus sylvatica) will be more plastic across site types: similar to maple on rich sites, and similar to spruce on poor sites. To test our hypotheses, we will study the mobilization kinetics of the macronutrients from FF and A horizons using soil dialysis, we will apply the novel approach of litter and OF bags with admixture of the respective minerals, and we will analyse the mobilization of N from labelled litter in the mesocosm approach for the three tree species. Installing labelled mesocosms at the controlled conditions of a tree species experiment will complement our analyses. In cooperation with the partners of the RU we will be able to explore the impact of the microbial community, root-mycorrhizal exudates, and soil faunal groups on the mobilisation of nutrients from the FF. Integration all our results, we will provide information on the role of FF for the nutrition of beech, maple and spruce at the different sites and how this is related to the FF morphology. With the synthesis of our project we will contribute to the RU cross cutting question on nutrients. We will know under which conditions and for which tree species longer residence time of nutrients in FF is of benefit.