Oxidative enzymes underlie tree species effects on soil organic carbon stocks: a common garden test with eight tree species

Abstract

Tree species effects on soil organic carbon (SOC) stocks became a hot topic in soil science during the past decades. Increasing evidence has shown that tree species have contrasting effects on SOC stocks, yet the underlying mechanism remains incompletely understood. The decomposition control hypothesis states that tree species producing recalcitrant litter with low decomposability could facilitate SOC accumulation. In contrast, the formation control hypothesis argues that tree species producing labile litter, which can be efficiently transformed to soil organic matter by soil microbes, could increase SOC stocks. To unravel this controversy, we leveraged a 40-year-old common garden with replicated monoculture stands of eight tree species and examined relationships between leaf litter and fine root traits, soil bacterial and fungal community composition, four C-degrading enzymes, and SOC stocks. There was more than two-fold variation in SOC, particulate organic C (POC) and mineral-associated organic C (MAOC) concentrations among the eight tree species. Specific peroxidase and phenol oxidase activities explained more variation in POC and MAOC concentrations than leaf litter and fine root traits. Specific peroxidase activity was positively correlated with the relative abundance of fungi with genetic potential to produce peroxidase (Fungi_per) and acidobacteria, and specific phenol oxidase activity was positively correlated with relative abundance of actinobacteria. Tree species producing labile leaf and fine root litter characterized by rich nitrogen and poor lignin concentrations were associated with low relative abundance of Fungi_per and high relative abundance of actinobacteria. Collectively, our results suggest that oxidative enzymes that catalyze the decomposition of chemically recalcitrant compounds, and cause destabilization of mineral-bound organic matter, play critical roles in determining tree species effects on SOC stocks.