Objective The aims were to identify the effects of different sludge application methods and quantities on soil aggregation, and to explore rational sludge application approaches for forest soils.

Method Sludge was applied to forest soil at varying amounts (3 kg m⁻², 6 kg m⁻², 9 kg m⁻², and 12 kg m⁻² for mixed application) and by different application methods (hole application, mixed application, and broadcast application at 6 kg m⁻²), no sludge application was as the control. Soil aggregates and their stability were analyzed at different depths (0 ~ 10 cm, 10 ~ 20 cm, and 20 ~ 30 cm) under these treatments.

Result Compared to the control, sludge application significantly increased the proportion of large aggregates (> 0.25 mm) and aggregate stability. However, with increasing sludge application rates, both the proportion of large aggregates and aggregate stability initially increased before decreasing. Additionally, at 3 kg m⁻², the proportion of large aggregates and aggregate stability in the 0 ~ 10 cm and 10 ~ 20 cm soil layers reached their peak, while in the 20 ~ 30 cm soil layer, they peaked at 9 kg m⁻². Compared to the control, both broadcast and mixed applications significantly improved the soil's mean weight diameter and geometric mean diameter, reduced aggregate breakdown rate, and increased the proportion of water-stable large aggregates, thereby enhancing aggregate stability. Among these, broadcast application was more effective in improving aggregate stability in the 0 ~ 10 cm and 10 ~ 20 cm soil layers, while mixed application was more effective in the 20 ~ 30 cm soil layer.

Conclusion For increasing the proportion of large soil aggregates and enhancing aggregate stability, broadcasting 3 kg m⁻² of sludge proved to be the most effective treatment.