Differences in topsoil and subsoil root morphology and root functioning (water and nitrogen uptake, exudation) of European beech on various bedrock types

根系是土壤的重要组成部分，是土壤有机碳的主要来源，是影响根际化学和物理以及土壤容重的因素，也是将水分和养分从土壤输送到植物体内的植物器官。人们对深层土壤中树根的结构、动态和功能知之甚少。基于第一阶段细根生物量分布、14C根龄、细根轴向水力传输率和地下土壤原位渗出测量的结果，我们在这一阶段研究了不同土壤（贫碱性和富碱性）下6个山毛榉林的细根形态、根系动态和根系功能在地下和表层土壤中的差异。拟检验的主要假设为：(1)细根向地下分支的程度减小，细根比表面积（SRA）比表层土的小，根组织密度比表层土的大；(2)下层土的根对NO3-和NH4+的吸收能力比表层土的小；(4)容器特别大的底土根（高导电性根）比表土根具有更高的表面比吸水率。我们采用多种原位方法，利用15NO315NH4研究矿物氮的吸收能力（Geßler et al. 1998之后的方法），用微型量具测量根液通量，并将其与细根表面积联系起来（Senock & Leuschner 1999之后的方法），测量完整细根的渗出量（Meier et al. 2013之后的方法），用微型zotrons和图像分析量化细根更替，用14C-AMS分析估计根龄。研究结果表明，山毛榉细根系统在功能上存在表土根主要吸收氮（及其他营养物质）和底土根主要吸收和运输水分的功能分化。这应反映在根系的特定吸收能力上。我们提供根系动态数据，估算根系向地下土壤的碳输入，帮助参数化地下土壤碳周转模型，并接收与根系活动相关的土壤物理和化学条件数据。

Roots are an important component of the soil as a major source of SOC, as agents influencing rhizosphere chemistry and physics as well as the bulk density of the soil, and as plant organs that channel water and nutrients from the soil to the plant. Not much is known about the structure, dynamics and functioning of tree roots in deep soil horizons. Based on the results of the first phase concerning fine root biomass distribution, 14C root age, fine root axial hydraulic conductivity, and in situ exudation measurement in the subsoil, we address in this phase differences in fine root morphology, root dynamics and root functioning between subsoil and topsoil in six beech forests on different soil (acid base-poor to alkaline base-rich). The main hypotheses to be tested are (1) the degree of fine root branching decreases toward the subsoil, where specific fine root surface area (SRA) is lower and root tissue density higher than in topsoil roots, (2) subsoil roots have lower NO3- and NH4+ uptake capacities than topsoil roots, (3) specific root exudation rate decreases toward the subsoil, and (4) subsoil roots with particularly large vessels (high-conductivity roots) have higher surface-specific water uptake rates than topsoil roots. We employ a variety of in situ methods to study the uptake capacity for mineral N using 15NO315NH4 (method after Geßler et al. 1998), to measure root sap flux with miniature gauges and relate it to fine root surface area (method after Senock & Leuschner 1999), to measure the exudation of intact fine roots (method after Meier et al. 2013), to quantify fine root turnover with minirhizotrons and image analysis, and to estimate root age with 14C-AMS analysis. We search for evidence that the fine root system of beech trees is functionally differentiated in topsoil roots with mainly uptake function for N (and other nutrients) and subsoil roots mainly serving in water uptake and transport. This should be reflected in the specific uptake capacities of the roots. We deliver data on root dynamics and estimated root-borne C input into the subsoil and help parameterizing the subsoil C turnover model, and receive data on soil physical and chemical conditions relevant for root activity.