New concepts for assessing soil structure turnover by structure labeling and analyses of biochemical gradients

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Soil structure is the manifestation of the interactions of many biotic and abiotic agents in soil and controls many soil functions such as matter turnover, water retention, and the production of biomass. Soil structure has been identified to govern long-term carbon sequestration in soil via physical protection of soil organic matter against decay, therewith possessing a central role in the global carbon cycle. Soil structure is often considered static but actually changes due to bioturbation, wetting/drying, freezing/thawing and tillage activities. Yet, conceptual approaches to link soil structure turnover to organic matter decomposition are still in their infancy, mainly due to methodological shortcomings that impair meaningful estimates of soil structure turnover rates. The main objective of this project is to establish novel conceptual approaches to measure soil structure turnover under natural conditions. The first is based on structure labeling where soil aggregates are coated with small inert garnet particles and their fate is studied using X-ray microtomography (µCT). The speed at which randomization with respect to particle‒pore distances is achieved will be interpreted as turnover rate. The second is based on the detection of microscopic biochemical gradients. They are expected to form when soil structure turnover is slow, whereas fast soil structure turnover continuously changes diffusion pathways and redistributes constituents, thus preventing the formation of biogeochemical gradients. In this project we focus on imaging methods that provide a comprehensive in-situ view to undisturbed soil structure. Two-dimensional microscopic and microspectroscopic data (XPS, SEM-EDS, LA-IRMS) are merged with three-dimensional physical structure (µCT) through 2D-3D image registration in order to truly link 3D diffusion pathways to spatial gradients in element ratios, carbon oxidation states, and carbon isotope ratios. The proposed approaches will be tested in laboratory and field experiments to identify abiotic (wetting, freezing) and biotic (microbial activity, bioturbation) drivers of soil structure turnover. Long-term vegetation change experiments with known carbon turnover rates are revisited to estimate soil structure turnover from the magnitude of biochemical gradients. With this project we expect novel insights into the mechanisms of soil structure formation under natural conditions and how altered pore size domains and biochemical gradients are linked to the cycling of soil organic matter.