Dual-scale micro-CT assessment of soil structure in a long-term fertilization experiment

- SOC affects the soil structure at both micro- and macroscale.
- Pore connectivity was the best index to distinguish the soil management effects.
- Pore size distribution was partially able to discriminate between treatments.
- Micro-CT must be combined with mercury porosimetry to quantify the soil structure.

In this study we used X-ray microtomography (micro-CT) to evaluate the 3D pore size distribution and morphology of both undisturbed soil cores (5 cm diameter, 6 cm length) and aggregates (sieved at 5-6 mm). Samples were collected in a long-term fertilization experiment, established in 1962 in northeastern Italy, which considers organic (farmyard and liquid manure) and mineral treatments. The aim of the work was to assess the dual-scale effect of soil organic carbon (SOC) to influence the pore network in the range 6.25-2000 μm and to test the sensitivity of morphological features (connectivity, degree of anisotropy, fractal dimension) as indices of soil quality to management practices. Results on pore morphology suggested that connectivity was a sensitive index to differentiate the effects of management practices. Soil structure was multifractal and did not highlight any difference between treatments. Pore size distribution was affected by SOC as it increased the pores > 560 μm of soil cores and, at the same time, decreased the pores in the range 80-320 μm. By contrast aggregate porosity, in the range 12.5-200 μm, was weakly affected by the SOC and management practices.Most likely the low instrumental resolution would mask their effects since previous studies conducted on the same soils showed a significant effect of the management practices on soil microporosity (0.0074-100 μm). Therefore the integration of these data, with those obtained in the same plots in previous studies, highlighted the dual effect of SOC as it affected the soil structure at both micro- and macroscale.