Relationship between soil carbon sequestration and the ability of soil aggregates to transport dissolved oxygen

•Aggregates taken from the longest experiments in the world were scanned using X-ray CT.•Transport ability of each aggregate was calculated from pore-scale simulations.•Soil organic carbon and bulk diffusion coefficient of the aggregates are positively correlated.•Bulk diffusion coefficient of the aggregates tends to plateau as soil organic carbon increases. A key finding in soil carbon studies over the past decade is that soil organic carbon (SOC) stabilization is not controlled by its molecular complexity or adsorption to clay, but by its physicochemical protection including occlusion in aggregates and sorption-precipitation with organo-mineral associations. The organo-mineral complexes and the adsorbed SOC can be dissolved microbially under anoxic conditions, which is an important pathway in carbon cycle but has been overlooked by most carbon models. As organo-mineral associations are reported to form in aerobic conditions and can be lost under anaerobic conditions, there should be a positive correlation between SOC and ability of the aggregates to transport dissolved oxygen. We develop a simulation model to test this using soil structural data from two long-term experiments which naturally created a SOC gradient: One is a winter wheat experiment established in 1843 to compare the effects of different fertilizations on the yield of winter wheat and the other one is a ley-arable experiment established in 1948 to investigate the consequence of cropping system changes for ecological yield. Aggregates from different treatments on the two experiments were scanned using X-ray Computed Tomography to simulate oxygen transport using a pore-scale model. We compared porosity and diffusion coefficient of all aggregates and linked them to SOC measured from the two experiments. The agronomic practice changes which occurred 67 or 172 years ago substantially reshaped the intra-aggregate structure (