Analysis of the intra-aggregate pore structures in three soil types using X-ray computed tomography

The pore structure of soils can control important physical and biological processes in soil-plant-microbial systems. However, quantifying the intra-aggregate pore structure is challenging. This study investigated the soil pore structure in aggregates from three typical zonal soil types (alpine meadow soil, chestnut soil and red soil) from cold, temperate and subtropical regions of China, respectively, using X-ray microscopic computed tomography (CT) with a resolution of 7 mu m. The results indicated that the chestnut soil aggregates had higher porosity than the alpine meadow and red soil aggregates due to the presence of grass/shrub roots. The soil pores of the chestnut soil aggregates were mostly small, relatively regular and densely distributed. Alpine meadow soil aggregates had many large, elongated pores, which was attributed to the freeze-thaw cycles in alpine regions. The alpine meadow aggregate pores had the largest mean equivalent diameter, mean surface area and mean volume of the evaluated soil types. Red soil aggregates had low porosity and poor connectivity, which were attributed to the high temperatures and abundant precipitation in subtropical regions. Moreover, the soil aggregate pores of the three soil types were found to be dominated by micropores, especially the chestnut soil (90.39% of the number of pores). Soil texture significantly influences the total porosity of soil aggregates. Higher soil organic matter content may result in aggregate pores that are more irregular in shape. A soil pore architecture with high continuity likely facilitated greater water flow to the deeper soil layers in the alpine meadow soil than in the red and chestnut soils. However, in the chestnut soil aggregates, the water could be unavailable for uptake by plants due to the relatively poor soil pore structure. The results for the red soil aggregates are indicative of the poor structure and tendency toward rapid erosion of red soils.