Investigating intra-aggregate microstructure characteristics and influencing factors of six soil types along a climatic gradient

Pore structure can reflect the soil aggregation process, determine various physical, chemical and biological processes in aggregates. This study investigates variations in the intra-aggregate microstructure (porosity, pore size distribution, and pore shape) of six different weathered soils across temperate (Haplic Phaeozems, HP; Haplic Luvisols, HL; Calcic Luvisols, CL), subtropical (Ferric Luvisols, FL; Plinthic Alisols, PA) and tropical (Acric Ferralsols, AF) climates. For this purpose, the three-dimensional microstructure of the soil aggregates using X-ray micro-computed tomography was quantified. Significant differences were found in the intra-aggregate microstructure of different soil types. The porosity of different effective pore diameters inside soil aggregates showed a U-shaped pattern of change, which gradually weakened with the increase in soil profile depth. HL, PA, and AF had higher total porosity and pore number, with a more uniform pore distribution, due to high content of SOC (soil organic carbon) or Fe–Al oxides. Large and long-shaped pores were found in the intra-aggregates of FL. The dominant soil intra-aggregate pores were mesopores (the porosity of effective pore size between 30 and 75 µm, P30-75) and the soil aggregate microstructure exhibited an elongated pore morphology. SOC and Al oxides were two determinants of the intra-aggregate microstructure with a significant positive effect, whereas Fe oxides’ negative effect was observed. These results have shed light on the main reasons for the differences in the microstructure of different weathered zonal soil intra-aggregates and the relationship between aggregate microstructure and cementing substances.