Glomalin-related soil protein and aggregate stability across a century-scale quercus liaotungensis restoration chronosequence on the loess plateau

Vegetation restoration can affect soil aggregate stability and organic carbon sequestration, but the role of glomalin-related soil protein (GRSP) within different aggregate fractions remains unclear, especially across century-scale restoration chronosequences. This study used a space-for-time chronosequence of Quercus liaotungensis forests (30, 60, 90, and 120 years) and abandoned farmland as the control (CK) on the Loess Plateau. Soil samples were collected at 0–20, 20–40, 40–60, and 60–80 cm depths. Wet sieving separated macroaggregates (> 250 μm), microaggregates (53–250 μm), and silt-clay particles (< 53 μm). Total GRSP (T-GRSP), easily extractable GRSP (EE-GRSP), and soil organic carbon (SOC) were measured in each fraction. Macroaggregate content and mean weight diameter (MWD) were generally higher in restored stands than in CK, with depth-specific peaks at intermediate restoration ages. SOC increased in all aggregate fractions after restoration relative to CK, especially in macroaggregates at 0–20 cm, and generally decreased with soil depth. T-GRSP generally accumulated with restoration age, whereas EE-GRSP showed a more pronounced peak at 60–90 years, particularly in macroaggregates. The EE-GRSP/T-GRSP ratio varied among fractions and depths and was highest at intermediate restoration stages. MWD was positively correlated with macroaggregate SOC at 0–20 cm and with macroaggregate EE-GRSP/T-GRSP at some deeper depths, but correlations were weak and based on a limited number of restoration-age groups. These results indicate that GRSP pools, especially in macroaggregates, are associated with aggregate stability during forest restoration, although causal relationships cannot be established from this observational chronosequence. The findings provide observational evidence for understanding soil structural recovery in semi-arid restored ecosystems, but broader extrapolation requires further validation across regions and soil types.