Evaluating the effect of sludge application on soil aggregates based on Meta-analysis

Objective To evaluate the effect and mechanism of sludge application on soil aggregates by using Meta-analysis.

Method We fine screened 36 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 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), sludge mixed application 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 0.257, 0.127, and 0.047, respectively, indicating that the higher effect size occurred in soils with lower clay content. For different sludge application rates, the highest effect size was found in rate of 100~200 t/hm². Linear fitting analysis showed a significant and positive correlation between the relative content of macroaggregate and the contents of soil organic carbon, carbohydrate, total nitrogen, alkaline hydrolyzable nitrogen, total phosphorus, available phosphorus, or phosphatase activity, while a significant and negative correlation between the relative content of macroaggregate and soil exchange sodium percentage or electrical conductivity. Random-forest analysis further indicated that the increase of soil organic carbon and carbohydrate content by sludge application was the main reason for the improvement of soil aggregation. Meta-regression analysis showed that the increase of organic matter could explain 93.79% of the effect size of macroaggregate, and the increase of carbohydrate could explain 76.30% of the effect size of macroaggregate.

Conclusion The sludge application increases soil organic carbon content and carbohydrate content, and then promotes soil aggregation.