X-ray imaging demonstrates that air-filled porosity and its connectivity controls carbon mineralisation near saturation in intact soil cores.

One source of uncertainty in the prediction of soil carbon (C) dynamics is the regulation of microbial activity by soil moisture. This important factor regulates both the survival and the activity of the microbial community through the availability of water, air and substrates. The role of soil structure in the response of C mineralisation to soil moisture is not taken into account in models. We need to better understand how the heterogeneity of the soil pore space and changes in soil structure affect C mineralisation through the regulation of soil water retention and thus the distribution of air and water in the pore system. We hypothesized that soil structure has a predominant effect on the response curve close to saturation by affecting the amount and distribution of the air phase in soil, in which the diffusion of gases takes place: transport of air (O₂) to microbes and transport of the mineralisation product (CO₂) back to the atmosphere.

To obtain soils of contrasting structure, we sampled 8 intact cores (at a depth of 10-15 cm) from four blocks of an agricultural field experiment located in northern France ^a under conventional or no-till management. Each core (5 x 6.5 cm) was consecutively incubated over a period of one week after equilibration at water potentials of -2.5, -10, -20 and -30 cm and C mineralisation rates were estimated at day 1, 3 and 7. Air distributions in the soil pore networks were quantified by X-ray tomography between each equilibration/incubation period. Water retention curves, soil dry bulk density and porosity were estimated from water contents (weights) measured at each potential.

The estimated porosity varied from 0.40 to 0.52 and the Van Genuchten parameter alpha (estimated from water retention curves) varied from 0.05 to 0.09 cm^-1 and both were slightly smaller under no till compared to conventional management. Air contents varied from zero to 0.09 m³ m^-3 and were positively correlated to the C mineralisation rates, which varied from 18 μg CO₂ g^-1C h^-1 near saturation to 65 μg CO₂ g^-1C h^-1 at water potentials of -20 to -30 cm. X-ray analyses carried out at the four different water potentials further showed that C mineralisation rates were positively correlated to the volume fraction of the air-filled porosity connected to the upper surface of the cores.

These results confirm that soil structure is important in the C mineralisation response to soil moisture close to saturation by regulating the air content and its distribution in soil at water potentials ranging from 0 to -30 cm.