Cations in soil solutions affect soil structural stability and thus soil quality and health. Ca2+ and Mg2+ could alleviate soil clay dispersion by replacing Na+, which was the main driver. However, Mg2+ could also cause soil disaggregation and weaken aggregate integrity. Currently, most studies on Ca2+ and Mg2+ mainly examine soil hydraulic characteristics and clay particle dispersion, rather than macropore geometry. We analyzed the impact of Ca2+ and Mg2+ on soil macropore morphology by measuring macropore length, aspect ratio, and area indices. An indoor soil column experiment was set up, and irrigation water was prepared with the same electrical conductivity (4 dS m(-1)) and different cation compositions (Na+-Ca2+ (NC), (Na)(+)-Ca2+-Mg2+ (NCM), Na+-Mg2+ (NM) and Na+-only (N) were added), and deionized water as the control (CK). The results indicated that N had the highest soil suspension turbidity among all treatments, with NM being higher than NC. The highest percentage of soil macropore aspect ratio 2.0. The macropore anisotropy of NM was closer to 1.0, and anisotropy of NC was closer to 0. The soil macropore morphology of NC developed towards a spherical shape, while the macropores of NM might extend along one or several similar directions. For Na+-Ca2+ dominated soil, Ca2+ mainly affected the macropore area. However, for Na+-Mg2+ dominated soil, Mg2+ primarily influenced the number of macropores. Ca2+ inhibited the negative effects of Na+, but Mg2+ promoted unidirectional extension of macropores, posing a risk of soil cracking. This study provided a better understanding to explore the differences in the effects of different cations on soil pore structure and helped to provide guidance for field water management.

Decreasing the void ratio of soil due to static compression causes soil structure changes and developing anisotropic structure. This phenomenon as a common result causes the development of anisotropic permeability ratio (kh/kv or rk). When the soil shows a high void ratio, it generally contains macropores that have the most effect on the permeability, soil structure changes, and rk evolution during compression. Thus, in this research, two high void ratio samples of clayey loess soil with a granular structure (containing macropores) were prepared to investigate the rk evolutions during one-dimensionally static compression. So, horizontal and vertical permeability of samples were measured at each new void ratio, from high to low values. The tests implemented by a 3D permeameter apparatus that was designed for this research. This apparatus was equipped with a camera to study the soil macrostructure changes during tests. The results show that rk have different trends during compression, so, three stages of permeability anisotropy variations recognized as A, B and C. At high void ratios (stage A), the connected macropores produce high pseudo anisotropic permeability that rapidly decreased during compression. At low void ratios, rk increased due to particle orientation. The Stage B that has minimal values of rk with low variations is the transition stage from A to C stage.

Purpose Thaw slumps are widely distributed in the Qinghai-Tibet Plateau (QTP) due to global warming and engineering constructions. However, an understanding of the effect of thaw slumps on the 3-D soil macropore networks is lacking. In this study, we aimed to quantify the responses of soil macropore structure to thaw slumps in QTP. Materials and methods Three stages were selected according to the intensities of thaw slumping, including the original grassland, collapsing areas, and collapsed areas. Nine undisturbed soil cores (0-30-cm deep) were collected in total with 3 replicates sampled at each stage, and they were scanned by X-ray computed tomography (CT). Results and discussion The results showed that collapsing areas had higher macroporosity, branch density, and node density than the original grassland and collapsed areas. The macropore networks in the collapsing areas had the highest connectivity among the three thaw slump stages. Macropores with volume > 10 mm(3) accounted for more than 50% of the total macropore volume in the original grassland, collapsing areas, and collapsed areas. We speculate that compared with the other two stages, the soil macropore structure in the collapsing areas is more conducive to water infiltration and lateral migration. The connectivity of macropore networks in the collapsed areas was the lowest among the three stages, which may result in water infiltration difficulties after thaw slumps. Conclusions Thaw slumps affected the soil macropore structure remarkably. The effects of thaw slumps on soil macropore network characteristics were more significantly than on the macropore size distribution.