Linking aggregate stability to the characteristics of pore structure in different soil types along a climatic gradient in China

Abstract: Aggregate structure and stability are related to a series of soil processes. However, it is still lacking in the microstructure and aggregate stability for the different types of zonal soil aggregates. In order to explore the changes of aggregate stability and pore structure for the different types of zonal soil aggregates and their relationship, this study aims to explore aggregate stability and its relevance to the pore structure characteristics in the different types of zonal soil. Five types of typical soils (Black, Brown, Cinnamon, Yellow-cinnamon, and Red soil) were selected as the research objects using geogenesis. The soil samples were collected separately from the Hailun (Heilongjiang), Shenyang (Liaoning), Luoyang (Henan), Xiangyang (Hubei), Changsha (Hunan), and Haikou (Hainan), according to the latitudinal direction zonality of soil distribution. The pore structure and MWD of aggregates were quantified using the CT scanning, wet sieving, and Le Bissonnais method..The results indicated that the water stability of aggregates was affected by the soil type and soil depth, and the values from low to high were Yellow-cinnamon soil, Cinnamon soil, Brown soil, Black soil and Red soil. The eluvial horizon in the Red soil was the highest (1.49 mm), and the parent material horizon in Yellow-cinnamon soil was the lowest (0.19 mm). The average mass diameter of the aggregates measured by LB method was ranked as MWDsw (slow wetting) >MWDws (shaking) >MWDfw (fast wetting), indicating that the dissipation and external mechanical failure were the main fragmentation mechanisms of aggregates. The resistance of soil aggregates to dissipation and fragmentation gradually decreased and then increased from north to south. Significant differences were found in the aggregate microstructure of different soil types. The porosity of the five zonal soil aggregates with different equivalent diameters showed a U-shaped variation from north to south, and the degree of variation decreased with the increase of soil depth. The total porosity and pore number were the highest for the Black soil aggregates and the lowest for the Yellow-cinnamon soil aggregates. The pore sizes in most aggregates were observed to be 30-<75 μm. However, the pores larger than 100 μm were dominated in the Yellow-cinnamon soil, which was connected with the original particle arrangement and the low content of cementing material in the soil. The aggregate pore morphology was dominated by elongated pores with a few regular and irregular pores. The elongated pores decreased first and then increased from north to south. By contrast, an opposite trend was found in the irregular and regular pores. The aggregates stability showed significant positive correlations with the total porosity, total pore numbers, elongated porosity, <30, 30-75, 75-100, and >100 μm porosity, while inversely correlated with the regular and irregular porosity (P<0.01). Partial least squares regression (PLSR) showed that the water stability of aggregates was significantly correlated with the regular porosity,,mean pore shape factor, 75-100 μm porosity, and elongated porosity. The regular porosity, elongated porosity, 75-100 μm porosity and >100 μm porosity were proved to be the main controlling factors of MWDfw, MWDws and MWDsw. These results will help to deepen the understanding of the relationship between soil aggregates and pore characteristics, and better reveal the mechanism of soil processes.