Interactive Effects of Tillage and Soil Depth on Soil Aggregate Stability in Relation to Soil Properties and Glomalin-Related Soil Proteins

Tillage practices influence soil structural stability by modifying the soil physicochemical environment and biological processes. However, the relationships among tillage, soil depth, Glomalin-related soil proteins (GRSP), and soil aggregate stability remain insufficiently understood, particularly in the Mollisols of Northeast China. A long-term field experiment was conducted to investigate the effects of three tillage practices, including no tillage (NT), deep tillage (DT), and rotary tillage (RT), across four soil depths (0–10, 10–20, 20–30, and 30–40 cm). Soil physicochemical properties, glomalin-related soil proteins (total glomalin, TTG; easily extractable glomalin, EEG), and soil aggregate stability indices, including mean weight diameter (MWD), geometric mean diameter (GMD), and the percentage of water-stable aggregates (R0.25), were determined. Pearson correlation analysis and structural equation modeling (SEM) were employed to elucidate the relationships among soil physicochemical properties, GRSP, and aggregate stability. Tillage practices significantly altered soil physicochemical properties, particularly soil moisture and electrical conductivity, with responses varying across soil depths. Deep tillage generally enhanced EEG accumulation and improved aggregate stability compared with rotary tillage, whereas TTG exhibited relatively smaller variations among treatments. Soil moisture was positively correlated with GRSP and aggregate stability, whereas electrical conductivity showed predominantly negative relationships with biological indicators. Correlation analysis showed significant associations among soil physicochemical properties, GRSP, and aggregate stability. In the SEM, soil physicochemical properties were positively associated with both GRSP (standardized path coefficient = 0.205, p < 0.05) and aggregate stability (standardized path coefficient = 0.416, p < 0.05), whereas the direct pathway from GRSP to aggregate stability was not statistically significant. Tillage and soil depth were significantly associated with variation in soil physicochemical properties and GRSP, while soil depth also showed a significant direct association with aggregate stability. These results indicate that the final SEM supported a stronger statistical association of soil physicochemical properties with aggregate stability than of GRSP with aggregate stability. Although GRSP was correlated with aggregate stability in the correlation analysis, its independent contribution to aggregate stability was not supported by the final SEM. These results provide new insights into the processes associated with soil structural stabilization and offer a scientific basis for optimizing tillage management to improve soil quality and promote sustainable agricultural development in the black soil region of Northeast China.