Meta-Analysis of Green Manure Incorporation Effects on Soil Aggregates and Organic Carbon

Abstract

Green manure incorporation is widely used to improve soil structure and enhance soil organic carbon (SOC) accumulation; however, the overall effects on soil aggregate distribution and carbon allocation among different aggregate fractions remain uncertain across diverse field conditions. Here, a multilevel meta-analysis was conducted using 603 effect-size estimates from 62 studies, including 66 experiments and 131 treatment–control comparisons. The logarithmic response ratio (lnRR) was used as the effect-size metric, and hierarchical dependencies among effect sizes were accounted for using a multilevel random-effects model with Study_ID/Experiment_ID/Pair_ID as random effects. The effects of green manure type, soil texture, incorporation duration, and mean annual precipitation (MAP) were further evaluated using subgroup analyses and multilevel meta-regression. Green manure incorporation significantly increased mean weight diameter (MWD) by 31.25% (lnRR = 0.272, 95% CI: 0.171–0.373), macroaggregate proportion by 27.98% (lnRR = 0.247, 95% CI: 0.148–0.346), and SOC by 16.76% (lnRR = 0.155, 95% CI: 0.091–0.219), while decreasing the silt–clay fraction by 22.35% (lnRR = −0.253, 95% CI: −0.342 to −0.164). In addition, macroaggregate-associated C, microaggregate-associated C, and silt–clay-associated C increased by 24.66%, 19.44%, and 13.26%, respectively, whereas the overall response of microaggregate proportion was not significant. No significant moderator effects were detected for green manure type, soil texture, incorporation duration, or MAP. Funnel plot diagnostics indicated potential small-study effects for several variables, but these were not interpreted as direct evidence of publication bias. Overall, these findings indicate that green manure incorporation can contribute to sustainable soil management by improving soil aggregate structure and promoting SOC accumulation, although its effectiveness should be evaluated in relation to local soil and management conditions.