Soil aggregates, which are the basic units of soil structure, play an important role in the carbon cycle of ecosystems. The pore characteristics of aggregates influence soil organic carbon sequestration. However, studies on SOC mechanisms in aggregates have been limited to Mollisols. This study was conducted as a long-term experiment established in 2004 with a corn-soybean rotation in Mollisols. There are three treatments, including rotary tillage without straw return (conventional tillage, CT), subsoiling without straw return (reduced tillage, RT), and no tillage with straw return (NT). The soil pore size distribution, shape parameters, extracellular enzymes activity, and carbon mineralization were measured. The results showed that 15-year no tillage and reduced tillage increased the total porosity and proportion of larger pores, but significantly decreased the proportion of smaller pores in situ soil columns. Conventional tillage exhibited the most complex pores because of the highest pore fractal dimension (2.75-2.90), anisotropy (0.366-0.516), and the lowest sphericity (5.1-28.7). As for the soil columns filled with > 2 mm aggregates, reduced tillage significantly increased the pore connectivity by 3.02-3.62 %, whereas no tillage had no effect. The structural equation modelling indicated that in soil columns filled with > 2 mm aggregates, pore shape parameters, particularly connectivity and anisotropy, positively influenced the activities of beta-glucosidase and beta-xylosidase directly, and positively affected soil carbon mineralization by influencing extracellular enzymes activity indirectly. The findings emphasize the importance of pore shape parameters effect on soil carbon sequestration, and will be helpful in comprehending the microscopic mechanisms of soil carbon sequestration in > 2 mm aggregates.

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.