Nitrogen and drought effects on the tree-soil interaction of ECM and AM temperate trees

树木与土壤环境的相互作用不仅通过吸收水分和养分，还通过根系分泌物的释放，增加矿质养分的溶解度，为土壤生物群提供基质，并刺激土壤有机质中养分的释放。树木和土壤之间的这种反馈是由共生根关联介导的。中欧森林中两种主要的根关联类型是丛枝菌根和外生菌根，它们改变了重要的根功能和根际过程，如资源获取、凋落物质量和周转、根际微生物的启动和森林土壤的碳固存。虽然单一的、易于培养的根系对磷吸收的影响已被广泛接受，但微生物群落对生态系统水平的影响仍未得到充分研究，但可能是必不可少的，因为它们驱动森林土壤的碳保留和土壤有机质（SOM）的损失。了解根际和根际土壤有机质动态对于预测中欧重要树种下森林土壤碳储量如何应对氮沉降上升和夏季干旱至关重要。该研究项目旨在研究共生菌根对根系主要功能的影响，应用一种新的原位分泌物测量技术、微型根管成像技术和15n标记技术。选择4种外生菌根（Fagus sylvatica, Quercus robur, Tilia cordata, Carpinus betulus）和4种丛枝菌根（Fraxinus excelsior, pseudoplatanus, platanoides, Prunus avium）作为温带树木区系的样本。我们将调查受本地森林微生物感染的树木，以研究不同的微生物群落如何影响根系寿命、渗出和吸收过程。我们通过以下三种假设进行了验证：(i)在夏季自然发生干旱的情况下，对德国中部混合林中8种树种的成熟树的根际动态进行了实地调查；（ii）在Göttingen Rhizolab和相关的溶渗仪中，对这8种树种在受控干旱胁迫下的幼树的根功能和根际动态的本地微生物群落影响进行了实验分析。在主要的全球变化因子中，我们将重点关注氮有效性和夏季干旱强度的增加对共生根关联介导树土反馈的影响。

Trees interact with their soil environment not only by the uptake of water and nutrients, but also by the release of root exudates, which increase the solubility of mineral nutrients, provide substrates for soil biota and stimulate the release of nutrients from soil organic matter. This feedback between trees and soil is mediated by symbiotic root associations. Among the two main types of root associations in Central European forests are arbuscular mycorrhizae and ectomycorrhizae, which alter important root functions and rhizosphere processes like resource acquisition, litter quality and turnover, priming of rhizosphere microbes, and carbon sequestration in forest soils. While the influence of single, easy to culture root associates on P uptake has been widely accepted, ecosystem-level influences of microbial communities are still understudied, but may be essential, since they drive carbon retention and loss of soil organic matter (SOM) from forest soils. Understanding rhizo- and hyphosphere SOM dynamics will be critical to predicting how the carbon storage in forest soils under important Central European tree species will respond to rising N deposition and summer droughts.The proposed research project investigates the effect of symbiotic mycorrhizal root associations on major root functions, applying a novel technique for in situ exudation measurement, mini-rhizotron imaging and 15N-labeling. Four ectomycorrhizal (Fagus sylvatica, Quercus robur, Tilia cordata, Carpinus betulus) and four arbuscular mycorrhizal (Fraxinus excelsior, Acer pseudoplatanus, Acer platanoides, Prunus avium) tree species will be selected as examples from the temperate tree flora. We will investigate trees infected by indigenous forest microbes in order to examine how diverse microbial communities influence root longevity, exudation and absorption processes. We test three hypotheses with a combination of (i) field investigations of the rhizosphere dynamics in mature trees of the eight tree species in a mixed stand in Central Germany with natural occurrence of summer droughts and (ii) an experimental analysis on the influence of indigenous microbial communities for root functions and rhizosphere dynamics in young trees of these eight species under controlled drought stress in the Göttingen Rhizolab and associated lysimeters. Among the major global change factors we will focus on the influence of increasing N availability and intensity of summer droughts for the mediation of the tree-soil feedback by symbiotic root associations.