Interaction between carbon exudation by tree roots and their mycorrhiza associations and forest floor thickness

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Different trees species exert a strong control on FF dynamics, as reflected by a steady state between litter input and decomposition. FF thickness increases from maple to beech to spruce (Vesterdal et al. 2008), the three tree species in the centre of our proposed RU. FF dynamics are controlled by leaf and root littler quality as well as as rhizodeposition that includes root exudation, boarder root cells and mucilage, which are carbon and nutrient sources for the fungi and microbes of the rhizosphere (Haichar et al. 2014). Roughly 30% of carbohydrates synthesised by photosynthesis are translocated belowground to the rhizosphere, i.e. to tree roots and their symbiotic mycorrhiza fungi and associated microbial community (Pausch and Kuzyakov, 2018, Pena et al., 2010). We hypothesize that tree nutrient use strategies and, in particular, the C composition of root exudates has a strong impact on the mycorrhiza, saprophytic and microbial community and activity that consequently affects FF nutrient availability. Our main research question examines how and to which extent the composition of roots exudates – i.e. C compound composition of sugars and organic acids – affects FF turnover by changing microbial and fungal composition and subsequent, SOM and litter decomposition. Hence, the role of P9 in the RU is to provide information on species and site-specific leaf and litter quality, nutrient use strategies, and in particular, the sugar and organic acid composition of root exudation from tree mycorrhiza association. Moreover, the lateral distribution of nutrients from leaf litter mediated by trees will be investigated within the joint 15N litter dispersal experiment (coordinated by P2). The outcome will be linked to microbial community (P7) and mycorrhiza types (P8), and will be related to nutrient uptake rates depending on nutrient availability provided by P4.