Application of earthworm cast improves soil aggregation and aggregate-associated carbon stability in typical soils from Loess Plateau

Earthworm casts exhibit remarkable fertility and have been widely used as an organic fertilizer. This study focused on the effects of earthworm cast application on soil aggregates and aggregate-associated carbon in typical soils from the Loess Plateau (China). Soil column experiments were conducted in the laboratory using cultivated loessial soil (CS), dark loessial soil (DS), and aeolian soil (AS). Application of earthworm casts significantly reduced the content of aggregates sized <0.5 mm but increased the content of water-stable aggregates. Compared to without-cast treatment, earthworm cast application increased the organic carbon content by 13.4-58.3%, 14.4-51.1%, 17.9-45.3%, 16.7-62.4%, 18.4-43.3%, and 19.8-62.9% in soil aggregate fractions of sizes 0.25, 0.25-0.5, 0.5-1, 1-2, 2-5, and 5 mm, respectively. The application of earthworm casts significantly increased heavy fraction organic carbon (HFOC), CaCO3, and exchangeable Ca contents in soil by 14.5-69.4%, 12.8-51.9%, and 33.3-63.2%, respectively. Compared with macroaggregates, microaggregates had higher CaCO3 contents but smaller light-fraction organic carbon (LFOC) to HFOC ratios, indicating that earthworm cast application improved the organic carbon stability more in microaggregates than macroaggregates. Comparison analysis of the three soils showed AS performed better in aggregation and aggregate-associated carbon stability than CS and DS after applying earthworm casts. The findings improve our understanding of the effects of earthworm cast application on soil aggregate distribution and aggregate-associated carbon stability, which will help improve the application efficiency of earthworm casts as an organic fertilizer in the Loess Plateau area.