Soil invertebrates increase soil organic carbon by shifting carbon fractions across China.

Soil invertebrates are widely recognized as ecosystem engineers that can influence soil organic carbon (SOC) dynamics, yet their effects on SOC fractions and the structural composition of SOC pools, as well as the environmental factors regulating these effects, have not been comprehensively synthesized. Here, we conducted a meta-analysis of 53 invertebrate manipulation studies across China, comprising 320 observations, to quantify changes in SOC, carbon fractions, and three functional carbon pools (active carbon, slow carbon, and recalcitrant carbon pools), and to evaluate how climate and soil properties moderate SOC responses. Overall, invertebrate manipulation was associated with significant increases in SOC and most measured fractions, with particularly strong positive effects on the active and recalcitrant carbon pools. However, effect sizes varied substantially among ecosystems, experimental settings, invertebrate groups, and manipulation durations, indicating pronounced context dependence. Joint-distribution analyses revealed heterogeneous relationships between SOC and the active, slow, and recalcitrant carbon pools, with no consistently uniform pattern across observations. Across environmental gradients, SOC responses showed statistically significant but weak positive relationships with mean annual temperature and precipitation, whereas they declined with increasing soil pH. Together, these results demonstrate that invertebrate effects on SOC should be interpreted in conjunction with changes in individual fractions and functional carbon pools, and that environmental conditions partially explain the variation in these responses.