Stoichiometric homeostasis of soil microorganisms as a driver of element cycling in grasslands

土壤微生物在生物量化学计量方面是稳态的，即它们在一个狭窄的范围内保持其生物量化学计量相对恒定，而不受其环境中元素比例的影响。到目前为止，还不知道土壤微生物群落如何维持其生物量化学计量，以及这如何影响生态系统水平的元素循环。了解这一过程对于预测大气氮沉降和其他环境变化对元素循环的影响至关重要。因此，该项目的目的是建立一个土壤微生物化学计量动态平衡的概念框架，作为草地土壤元素循环的驱动因素。为此，我将研究土壤微生物群落用来维持其生物量化学计量的两种类型的过程；一方面是使微生物生物量中的元素循环适应元素可用性比例的过程，另一方面是有助于元素获取的过程。这些过程包括(i)微生物生物量对碳(C)、氮(N)和磷(P)的吸收，（ii）微生物对碳的利用效率，（iii）微生物生物量中单一元素的周转，（iv）有机碳、氮和磷的矿化，(v)结合的无机磷的增溶，以及（vi）氮的固定。为了研究这些过程，我将使用创新的方法，如基于DNA的180标记来确定微生物C的利用效率，通过DNA的同位素标记来评估微生物生物量中单个元素的平均停留时间，以及通过土壤酶谱分析地下外酶活性的分布。拟议的项目将以位于三大洲的草原为基础，并进行相同的全因子养分操纵实验（养分网络）。通过在全球范围内进行协调实验，我希望这项研究的结果具有较大的推断范围。该项目的主要价值在于，它提供了土壤微生物作为元素循环驱动因素的化学计量动态平衡的概念框架，它捕获了土壤微生物维持其生物量化学计量的过程的相对重要性，以及这些过程对生态系统水平元素循环的影响。

Soil microorganisms are homeostatic with respect to their biomass stoichiometry, i.e., they maintain their biomass stoichiometry relatively constant within a narrow range independently of the element ratios in their environment. So far, it is not known how soil microbial communities maintain their biomass stoichiometry, and how this affects ecosystem-level element cycling. Understanding this process is crucial for predicting impacts of atmospheric nitrogen (N) deposition and of other environmental changes on element cycling. Therefore, the aim of the proposed project is to develop a conceptual framework of stoichiometric homeostasis of soil microorganisms as a driver of element cycling in grassland soils. For this purpose, I will study two types of processes that soil microbial communities use to maintain their biomass stoichiometry; on the one hand processes that adapt element cycling in the microbial biomass to ratios of element availabilities, and on the other hand processes that contribute to element acquisition. Among these processes are (i) uptake of carbon (C), N and phosphorus (P) in the microbial biomass, (ii) the microbial C use efficiency, (iii) the turnover of single elements in the microbial biomass, (iv) mineralization of organic C, N and P, (v) solubilization of bound inorganic P, and (vi) fixation of N2. To investigate these processes, I will use innovative methods such as the determination of microbial C use efficiency based on 18O-labeling of DNA, the assessment of the mean residence time of single elements in the microbial biomass by isotopic labeling of DNA, and the analysis of the distribution of belowground exoenzyme activities by soil zymography. The proposed project will be based on grasslands that are located on three continents and that are subject to the same full factorial nutrient manipulation experiment (Nutrient Network). By working on a globally coordinated experiment, I expect the results of this research to have a large scope of inference. The main value of the project will be that it provides a conceptual framework of stoichiometric homeostasis of soil microorganisms as a driver of element cycling, which captures the relative importance of the processes by which soil microorganisms maintain their biomass stoichiometry as well as the impact of these processes on ecosystem-level element cycling.