Porosity and pore size distribution of Ultisols and correlations to soil iron oxides

• Pore microstructure of Ultisols was determined by mercury intrusion porosimetry.
• PSD of Ultisol had three peaks at the pore diameter of 0.01–0.05, 0.1–2, and > 70 μm.
• Significant correlation exists between < 0.1 μm pore volume and the soil Fed content.
• Ultramicropores (0.1–5 μm) and cryptopores (< 0.1 μm) were the main soil pore classes.
• PCA and heatmap analyzed the correlations between pore classes and soil properties.

The pore structure of soils greatly influences soil functions and processes. The Ultisols developed on Quaternary red clay in the subtropical China were collected to characterize the porosity and pore size distribution (PSD). Mercury intrusion porosimetry (MIP) and nitrogen adsorption/desorption (NAD) methods were used to quantitatively describe the PSD of soil in the range of equivalent pore diameter of 100–0.003 and 0.1–0.001 μm, respectively. The total MIP porosity of Ultisols ranged from 33.0% to 44.9% with a mean value of 36.5%. The soils exhibited three-modal PSDs with peaks at the pore diameter of about 0.01–0.05 μm, 0.1–2 μm, and > 70 μm, indicating the heterogeneous nature of the pore system. The ultramicropores (0.1–5 μm) and cryptopores (< 0.1 μm) were dominant pore classes representing an average of 35% and 30% of total pore fraction, respectively. Among the different land use types, bare land (BL) had significantly higher cryptopore (0.01–0.1 μm) volume compared with forest land (FL) and orchard land (OL). Higher volume of cryptopores under BL could be attributed to the higher iron oxide and low organic matter contents. The free iron oxide (Fed) and clay contents were positively and significantly correlated with crytoporosity (< 0.1 μm) while negatively correlated with ultramicroposity. The pores with 0.001–0.1 μm diameter were mainly attributed to the pores associated with the structure of clay and iron oxides. The ultramicroporosity (0.1–5 μm) increased with increasing soil aggregation index: mean weight diameter (MWD) and geometric mean diameter (GMD). No significant correlation was observed between soil porosity and soil organic matter and macro-, meso-, and microporosity were independent of soil components. Our data suggest that high Fed and clay contents lead to increase < 0.1 μm pore volume. The Fed and clay contents played an important role in determining the pore structure of Ultisols.