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

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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.