Linking Water Table Dynamics, Soil Structure and Carbon Cycling: A Novel Experimental Approach Using an Artificial Soil System

土壤与地下水之间的过渡带是土壤生物化学活动的重要热点。地下水位波动改变了土壤的生物地球化学和微生物动力学，这可能导致有机物的加速降解和向大气中释放二氧化碳、CH4和N2O等气体。然而，地下水位波动对土壤中生物地球化学过程的影响仍不是很清楚。在这里，我提出了一个实验来研究土壤在恒定和波动的地下水位影响下的初始发展，使用了一种新型的可自动调节地下水位的柱子系统。该项目的目的是阐明地下水位波动以及随后的循环氧化还原条件对由清洁模型材料组成的土壤系统的初步发展的影响。主要目的是描述恒定和循环地下水位对土壤结构发展的影响，确定地下水位波动是否会导致有机质的更高降解和不同微生物群落的建立，表征由地下水位波动引起的循环氧化还原条件导致铁从最初均匀分布的模型材料转化和空间重新分布的速度和广度，以及表征培养后建立的单个团聚体的微尺度形态和元素组成。实验所使用的柱子配备了广泛的监测系统，包括传感器和采样端口，以测量pH、氧化还原电位、二氧化碳呼吸、水分含量和孔隙水组成。人工土壤混合物将被用来提供定义明确和均匀的初始材料。该混合物由石英、蒙脱石和针铁矿组成，以粪便为有机质来源，接种从自然土壤中提取的微生物群落。它将在恒定或循环地下水位下孵化6个月。培养后，柱子将以3厘米的深度分辨率采样，并将表征有机质、可提取铁氧化物和微生物群落。铁在大团聚体中的空间分布将使用选定样品的扫描电子显微镜-能量色散X射线光谱(SEM-EDX)和微型X射线荧光(µXRF)来确定。这种新的自动化柱子系统与定义良好的人造土壤相结合，为阐明地下水位波动对土壤初始发育的影响提供了一个独特的机会。通过对这些模型系统的生物地球化学、结构和矿物发育以及微生物学的综合表征，该实验提供了重要的见解，有助于从机械上理解地下水位波动对土壤和水质量的影响，以及二氧化碳的潜在储存或释放。

The transition zone between soil and groundwater forms an important hotspot of biological and chemical activity in soils. Water table oscillations change the biogeochemical and microbial dynamics of soils, which can cause enhanced degradation of organic matter and release of gasses like CO2, CH4 and N2O to the atmosphere. However, the influence of water table fluctuations on biogeochemical processes in soil is still not well understood. Here, I propose an experiment to study the initial development of soils under the influence of constant and fluctuating water table, using a novel column system in which the water table can be adjusted automatically. The aim of this project is to elucidate the effect of water table fluctuations, and the consequent cycling redox conditions, on the initial development of a soil system composed of clean model materials. Main objectives are to delineate the effect of a constant versus a cycling water table on soil structure development, to determine whether water table oscillations induce higher degradation of organic matter and establishment of distinct microbial communities, to characterise how fast and how extensively the cycling redox conditions caused by water table fluctuations lead to the transformation and spatial redistribution of iron from initially homogeneously distributed model materials, and to characterise the established micro-scale morphology and elemental composition of single aggregates after incubation.The columns used for the experiment are fitted with an extensive monitoring system of sensors and sampling ports to measure pH, redox potential, CO2 respiration, moisture content and pore-water composition. An artificial soil mixture will be used to provide a well-defined and homogeneous initial material. The mixture is composed of quartz, montmorillonite and goethite, with manure as organic matter source, and inoculated with a microbial community extracted from a natural soil. It will be incubated for 6 months with either a constant or cycling water table. After incubation, the columns will be sampled with a depth resolution of 3 cm, and the organic matter, extractable iron oxides and microbial community will be characterised. The spatial distribution of iron in macroaggregates will be determined using scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) and micro X-ray fluorescence (µXRF) on selected samples. The combination of this new automated column system with well-defined artificial soil provides a unique opportunity to elucidate the effect of water table fluctuation on initial soil development. By the combined characterisation of the biogeochemistry, structure and mineral development, and microbiology of these model systems, this experiment provides vital insights that work towards a mechanistic understanding of the consequences of water table fluctuations for soil and water quality, and the potential storage or release of CO2.