Linking carbon-degrading enzyme activity to microbial carbon-use trophic strategy under salinization in a subtropical tidal wetland

Although increasing studies have reported that microorganisms play an important role in carbon (C)-degrading enzyme activities under salinization, there is little experimental evidence for the associations between C-degrading enzyme activities and microbial community structure. Here, we examined the C-degrading enzyme activities, bacterial and fungal community abundance and composition (16S and ITS rRNA gene sequencing), soil organic carbon (SOC) storage, carbon/nitrogen (C/N) ratios, labile organic carbon (LOC) levels, and LOC/SOC ratios along an estuarine salinity gradient [range: 0.1–2.2 part per thousand (ppt)] of a subtropical tidal wetland. To help isolate salinity effects, sampling sites were selected to be similar in plant community composition, soil texture, and tidal influence. Our results indicated that the SOC storage, LOC levels, and LOC/SOC ratios decreased, whereas C/N ratios increased with salinization. The activities of β-1,4-glucosidase, cellobiohydrolase, phenol oxidase, and peroxidase increased by 485%, 147%, 699%, and 868%, respectively, with increasing salinity. The α-diversities of bacterial and fungal communities did not change, whereas the abundances of bacterial and fungal communities decreased with increasing salinity. Both bacterial and fungal oligotrophs/copiotrophs ratios were positively associated with salinity, suggesting that bacterial and fungal communities shifted from being copiotroph-dominated to oligotroph-dominated as salinity increased. All C-degrading enzyme activities were correlated with the relative abundance of typical bacterial and fungal oligotrophs, e.g., α-Proteobacteria, δ-Proteobacteria, Chloroflexi, Acidobacteria, and Basidiomycota. Overall, the C-degrading enzyme activities were co-determined by the C/N and LOC/SOC ratios and the fungal and bacterial oligotrophs/copiotrophs ratios. Our observations indicated that salinization influenced C-degrading enzyme activities via modulating substrate quality and lability and causing shifts in microbial C-use trophic strategies. Our proposed oligotrophic–copiotrophic scheme could be a novel method for incorporating vast taxonomic data of microbial communities into enzyme activity prediction in an ecologically meaningful manner.