Positive effects of species mixing on soil carbon sequestration and water retention in global forest plantations

Abstract Mixed‐species plantations are increasingly promoted as a means to enhance soil functioning; yet the global magnitude and consistency of their effects on soil carbon and water dynamics remain insufficiently resolved. We compiled 6550 paired observations from 300 studies to quantify how species mixing influences soil carbon pools and hydrological properties across forest types, soil layers and environmental gradients. We found that species mixing increased soil carbon storage and water holding capacity by an average of 10.5%, with the strongest gains in conifer and broadleaved mixtures. These improvements were underpinned by coordinated shifts in soil physical structure, including lower bulk density, greater aeration, expanded capillary and non‐capillary porosity, and higher infiltration rates. Such structural modification facilitated substantial increases in soil organic carbon and water retention. Responses were most pronounced in topsoil but were also evident in subsoil, revealing depth‐dependent sensitivity and vertical heterogeneity in below‐ground processes. Climate and stand structure exerted strong modulation. Positive effects intensified with mean annual temperature and precipitation, peaked at intermediate elevation, and were greatest in stands with higher species richness and intermediate stand age and stand density. These patterns indicate that species interactions and functional complementarity become more influential in favourable or structurally balanced environments. Synthesis : Our findings show that species mixing enhances soil carbon sequestration and water regulation through integrated physical and biogeochemical pathways. These results highlight species diversity as an effective strategy for strengthening the resilience, productivity and multifunctionality of plantation forests under changing environmental conditions.