Nitrogen Availability Regulates Tree Mixture Effects on Soil Organic Carbon in Temperate Forests: Insights From a Meta-Analysis and Long-Term Experiment

Abstract

Aim: Forests are essential for maintaining global soil organic carbon (SOC) balance, especially in species-rich ecosystems. However, the effect of tree diversity on SOC accumulation among different biomes remains poorly understood, owing to complex biotic–abiotic interactions. Location: Global. Time Period: 1983–2021. Major Taxa Studied: Trees. Methods: A comprehensive meta-analysis was used to examine the effects of tree diversity on SOC across different forest biomes. In addition, a 34-year manipulation experiment was used to investigate the mechanisms driving diversity effects on SOC in temperate forests. Results: Tree mixtures increased SOC by 6.2% globally. The effects of tree mixtures on SOC showed significant biome-specific variation, with positive effects in tropical/subtropical forests and no significant effects in temperate/boreal forests. However, tree mixtures with nitrogen-fixing species significantly increased SOC by 13.2% in temperate forests. Furthermore, our temperate field experiment further suggested that increased tree diversity tends to negatively affect nitrogen availability, which may affect positive effect of tree mixtures on SOC. Main Conclusions: Our results provide strong evidence that increasing tree diversity enhances SOC storage, especially in tropical/subtropical forests, underscoring the importance of mixed plantations for climate change mitigation. However, in nitrogen-limited temperate forests, tree mixture effects are constrained by intensified plant-microbe competition for nitrogen. Introducing nitrogen-fixing tree species during afforestation or plantation management in temperate regions is expected to enhance SOC storage by alleviating nitrogen limitation.