Variations in wetland soil labile organic carbon fractions and microbial characteristics along urban–rural gradients in Shenyang city

Soil labile organic carbon (LOC) fractions can respond rapidly to environmental changes and are crucial for soil carbon cycling. However, there are limited studies on the distribution of soil LOC fractions in urban wetlands along the urban-rural gradient. Understanding the dynamic changes in LOC fractions associated with microbial mechanisms in urban wetlands can inform management practices aimed at maximizing carbon storage in urban wetlands. In this study, using the urban-rural gradient approach, we investigated five typical wetlands in an urbanizing area of Shenyang City, China to examine changes in soil LOC fractions and associated microbial characteristics under the impacts of urbanization. The results showed that the average soil organic carbon (SOC) density of urban wetlands was approximately 23.16 kg/m². The proportions of soil light fraction organic carbon (LFOC), particulate organic carbon (POC), dissolved organic carbon (DOC) and microbial biomass carbon (MBC) to SOC content were 11.5-33.8%, 8.7-18.7%, 0.2-0.6% and 1.4-2.4%, respectively. Soil LOC fractions increased along urban-rural gradient, and decreased with soil depth increase. Soil LOC fractions in lake wetland were significantly higher than those in river wetland. The abundance of soil bacteria with 16S rRNA genes and carbon- fixing microorganisms with cbbL genes ranged from 2.79 × 10¹⁰ to 5.36 × 10¹⁰ copies/g dry soil and 3.48 × 10⁸ to 9.87 × 10⁸ copies/g dry soil, respectively. Urbanization significantly decreased the alpha diversity of cbbL-harboring microbes in the surface soil of wetlands (p < 0.05). Soil LOC fractions were significantly correlated with SOC across the five wetlands (p < 0.01), and positively correlated with bacteria possessing 16S rRNA and cbbL genes. The dominant microbial communities shifted from Gammaproteobacteria to Betaproteobacteria along urban-rural gradient. These results suggest that soil LOC fractions and cbbL-harboring microbial diversity and community composition in urban wetlands are significantly altered during urbanization. Urbanization and wetland type are important factors to consider for accurate carbon estimation in urban wetlands. Our study can provide valuable insights for decision-makers and urban planners to develop better management plans for urban wetlands.