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

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 and clay content but by its physicochemical protections including occlusion in aggregates and sorption/precipitation with organo-mineral associations. The organo-mineral complexes and the adsorbed SOC could be dissolved microbially under anoxic conditions, which is an important pathway in the carbon cycle but has been overlooked by most carbon models. As it is reported that organo-mineral associations are formed in aerobic conditions and could be lost under anaerobic conditions, there should be a positive correlation between SOC and ability of the aggregates to transport dissolved oxygen. We test this using two long-term experiments with a SOC gradient at Rothamsted Research in the UK: One experiment compares the effects of different fertilizations on yield of winter wheat and the other experiment aims to study the consequence of cropping system change for SOC dynamics. Aggregates in samples taken from plots under different treatments on the two experiments were scanned using X-ray computed tomography at 1.5 μm resolution; the ability of each aggregate to transport oxygen was calculated based on the pore-scale lattice Boltzmann simulation assuming that the aggregate is saturated as this is the most anaerobic scenario. We compared porosity and diffusion coefficient of all aggregates and link them to soil carbon measured from different treatments on the two experiments. The results showed that the agronomic practice changes occurring 67 and 172 years ago substantially reshaped the intra-aggregate structure, and that the accrual of SOC is positively correlated with diffusion coefficient of the aggregates to transport oxygen. However, the diffusion coefficient increases with SOC asymptotically, plateauing when SOC exceeds a threshold value. We also found that diffusion coefficient of the aggregates in cropped soils chemically fertilized trended with their porosity approximately in the same way, deviating from those for other non-cropped treatments or fertilized with farmyard manure.