Objective To evaluate the effect of sludge application promoting soil aggregation by using Meta-analysis, and excavate the influenciny factors.

Method We fine screened 36 highly matched papers from 568 papers published in international and domestic journals between 1990 and 2023. The effects of sludge application on soil aggregates and soil properties were evaluated by Meta-analysis. The relationships between soil aggregates and soil properties under sludge application were further analyzed by linear fitting analysis and random-forest mode method.

Result Sludge application significantly increased the relative content of soil macroaggregate (particle diameter > 0.25 mm) and the mean weight diameter of aggregates. Compared with surface application (0.201), mixed application of sludge had a higher effect size (0.840) in the relative content of macroaggregate. Sludge application significantly increased the mean weight diameter of aggregates in both surface (0−20 cm) and deep ( 20−40 cm) soil layers, but only significantly increased the relative content of macroaggregate in the surface soil layer. The effect size of the relative content of macroaggregate in soils with clay content <15%, 15%−25%, and >25% under sludge application was 0.26, 0.13, and 0.05, respectively, indicating that the higher effect size occurred in soils with lower clay content. For different sludge application rates, the highest effect size (0.35) of macroaggregate relative content was found in rate of 100−200 t/hm². Linear fitting analysis showed that the relative content of macroaggregate had with the significant and positive correlations contents of soil organic matter, carbohydrate, total nitrogen, alkaline hydrolyzable nitrogen, total phosphorus, available phosphorus, and phosphatase activity, while having significant and negative correlations with soil exchangeable sodium percentage and electrical conductivity. Random-forest analysis further indicated that the increase of soil organic matter and carbohydrate contents by sludge application was the main reason for the improvement of soil aggregation. Meta-regression analysis showed that the increase of organic matter content could explain 93.79% of the effect size of macroaggregate relative content, and the increase of carbohydrate content could explain 76.30%.

Conclusion The optimal application condition of sludge is mixed in 0−20 cm depth soil with clay content <15% at the amount of 100~200 t/hm². The sludge application increases soil organic matter content and carbohydrate content, and then promotes soil aggregation.