Structure turnover times of grassland soils under different moisture regimes

Abstract

Soil structure is a dynamic property of soils, which refers to temporal changes in the spatial arrangement of pores, organic matter, and minerals. Its turnover, i.e. the irreversible redistribution of soil constituents, determines essential soil functions including carbon storage. Structure turnover times and its response to biotic versus abiotic drivers have never been quantified directly under natural conditions. We used a novel combination of structure labelling with inert garnet particles and X-ray Computed Tomography to determine structure turnover times of two grassland topsoils with either access or exclusion of roots and fauna > 30 µm. Both, abiotic and biotic factors developed soil structure at a site-specific rate towards a dynamic equilibrium, at which bulk properties remain constant because creation and destruction of structural properties are in balance. Its turnover, however, was mainly determined by macrofaunal activity which varied with environmental conditions. Under dry conditions less favorable for bioturbation, the extrapolated structure turnover time was 33 ± 3 years, while being 16 ± 1 years under moist conditions. Previous studies on organic matter turnover determined in the vicinity of the experimental site showed similar turnover times for particulate organic matter. The similar turnover times suggest that the accessibility of particulate organic matter to decomposers is closely linked to structure turnover, thus highlighting the intimate nexus between structure evolution and carbon persistence in soil.