Influences of plant functional traits on soil organic carbon stocks: the roles of carbon input quality and diversity

Soil organic carbon (SOC) predominately originates from the decomposition of plant aboveground and belowground litter, which consists of diverse traits mixed in varying proportions. While numerous studies of litter decomposition have been conducted, the majority have primarily focused on litter mass loss rates. Our comprehension of how the quality and diversity of plant C inputs could impact SOC formation remains significantly constrained, largely due to the difficulty in their direct measurement at the ecosystem scale. Here we compiled a global dataset including community-weighted means and variances of plant aboveground structural and chemical traits, which serve as effective indicators of the quality and diversity of C inputs, respectively. We found that smaller community-weighted means of specific leaf area, signifying a lower quality of plant C inputs, could actually enhance SOC accumulation. This finding challenges the common notion that the high microbial carbon use efficiency of labile C inputs would ultimately benefit SOC sequestration. Our findings also showed that greater community-weighted variances of specific leaf area, reflecting more diverse plant C inputs, were positively associated with SOC stocks, likely due to the fact that higher C diversity can increase SOC persistence by increasing metabolism costs and diversifying organo-mineral bonds. On the contrary, community-weighted variances of leaf nitrogen content were negatively correlated with SOC stocks. This indicates that litter mixtures with diverse nutrient contents are prone to decomposition, possibly due to the complementary effects of nutrients. Notably, the contributions of C input quality and diversity to SOC stocks were more pronounced in colder or drier ecosystems, where the recalcitrance and diversity of C inputs exert a more substantial influence in limiting microbial decay of SOC. Our results point to the climate-dependent, yet important effects of plant C input quality and diversity on SOC stocks across the globe.