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

土壤与地下水的过渡区是土壤生物化学活动的重要热点。地下水位振荡改变了土壤的生物地球化学和微生物动力学，从而加剧了有机质的降解，并向大气释放CO2、CH4和N2O等气体。然而，地下水位波动对土壤生物地球化学过程的影响尚不清楚。在这里，我提出了一个实验，研究土壤在恒定和波动地下水位的影响下的初始发展，采用一种新颖的柱系统，其中地下水位可以自动调整。该项目的目的是阐明地下水位波动和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。主要目标是描述恒定的地下水位与循环的地下水位对土壤结构发展的影响，确定地下水位振荡是否会导致有机物的更高降解和不同微生物群落的建立，描述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再分配的速度和范围。并对孵育后形成的单个聚集体的微观形态和元素组成进行表征。用于实验的色谱柱配备了广泛的传感器监测系统和采样端口，以测量pH值，氧化还原电位，二氧化碳呼吸，水分含量和孔隙水组成。将使用人工土壤混合物来提供定义良好且均匀的初始材料。该混合物由石英、蒙脱石和针铁矿组成，以粪肥为有机质来源，接种从天然土壤中提取的微生物群落。它将在恒定或循环的地下水位中孵育6个月。孵育后，将以3厘米的深度分辨率对柱进行采样，并对有机物，可提取的氧化铁和微生物群落进行表征。铁在大团聚体中的空间分布将通过扫描电子显微镜-能量色散x射线能谱（SEM-EDX）和微x射线荧光（µXRF）对选定样品进行测定。这种新的自动化柱系统与定义明确的人工土壤相结合，为阐明地下水位波动对初始土壤发育的影响提供了独特的机会。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描述，该实验提供了重要的见解，有助于从机制上理解地下水位波动对土壤和水质的影响，以及潜在的二氧化碳储存或释放。

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.