Mixed plantations enhance soil aggregation and carbon storage: A global meta-analysis

Afforestation serves as a crucial climate mitigation strategy by sequestering atmospheric CO2 through soil carbon (C) storage. Multispecies plantations demonstrate superior soil carbon accumulation compared to monocultures, potentially through synergistic soil aggregation processes. But the specific mechanisms elucidating soil aggregates and soil C dynamics respond to mixed plantations are not fully understood. Here, we conducted a global meta-analysis of 2507 observations from 255 studies to explore the impact of mixed plantations on soil aggregation and C sequestration. Our results showed that, on average, species mixing significantly increased the proportion of macroaggregates and mean weight diameter by 5.0 % and 8.1 %, respectively, compared to monoculture plantations. A higher response ratio of macroaggregate C (+20.5 %) was observed in mixed plantations compared to monoculture plantations, then followed by bulk soil C (+18.1 %), and microaggregate C (+15.0 %). The magnitude of the mixing effect on soil aggregation and C pools was contingent on functional traits, climatic variables, soil properties, and planting density. Multispecies plantations comprising both broadleaved and needle-leaved trees can achieve greater bulk soil C stock benefits compared to those with only broadleaved species. The positive mixing effects on macroaggregate C and bulk soil C stock demonstrated a unimodal pattern with increasing planting density. Moreover, the proportion of macroaggregates had positive correlations with the responses of actinomycetes biomass, bacterial biomass, and hydrolytic enzymes, but no significant relationship with fungal biomass. Overall, our meta-analysis suggests that mixed plantations promote soil aggregation and C accumulation by improving soil nutrient cycles. Our research highlights the important role of tree species combinations and planting density in optimizing soil aggregation and its contribution to soil C sequestration in mixed plantations.