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

不同树种对凋落物的动态具有较强的控制作用，表现为凋落物输入与分解之间的稳定状态。FF厚度从枫树到山毛榉再到云杉（Vesterdal et al. 2008），这是我们提出的RU中心的三种树种。FF动态受叶片和根小物质量以及根沉积（包括根渗出物、寄根细胞和粘液）控制，它们是根际真菌和微生物的碳和营养来源（Haichar et al. 2014）。光合作用合成的大约30%的碳水化合物被转移到地下根际，即树根及其共生菌根真菌和相关微生物群落（Pausch和Kuzyakov， 2018， Pena等，2010）。我们假设树木的养分利用策略，特别是根分泌物的C组成对菌根、腐生菌和微生物群落和活动有很强的影响，从而影响FF的养分有效性。我们的主要研究问题是考察根系分泌物的组成（即糖和有机酸的C化合物组成）如何以及在多大程度上通过改变微生物和真菌组成以及随后的SOM和凋落物分解来影响FF的更替。因此，P9在RU中的作用是提供物种和特定地点的叶片和凋落物质量，养分利用策略，特别是树木菌根结合根渗出物的糖和有机酸组成的信息。此外，在联合15N凋落物扩散试验中（由P2协调），将研究树木介导的凋落叶养分的横向分布。结果将与微生物群落（P7）和菌根类型（P8）有关，并将与养分吸收率有关，这取决于P4提供的养分有效性。

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.