Changes of soil labile organic carbon fractions and their relation to soil microbial characteristics in four typical wetlands of Sanjiang Plain, Northeast China

Soil labile organic carbon (LOC) fractions can response rapidly to environmental change and are essential to soil C cycling. There has been very little evaluation about the distribution differentiation of soil LOC factions in high-altitude wetlands. In this study, four typical wetland types were investigated in Sanjiang Plain of northeast China, including: (1) a mixed Calamagrostis angustifolia and Salix brachypoda (CSW), (2) Calamagrostis angustifolia wetland (CAW), (3) Carex lasiocarpa wetland (CLW) and (4) Phragmites australis wetland (PAW). The variation in soil LOC fractions of these wetland soils and their relationship with microbial characteristics were studied by analyzing the changes of dissolved organic carbon (DOC), microbial biomass carbon (MBC), easily oxidation carbon (EOC), soil microorganisms (bacteria, fungi, and actinomyces) and enzyme activities (invertase, cellulase and catalase). The results indicated that the contents of soil LOC fractions declined with increasing of soil depth in each wetland. At 0–30 cm depth, the highest mean DOC content in seasonal flooded CAW, while the greatest average content of both MBC and EOC was in the drained CSW. The proportions of soil DOC, MBC and EOC to SOC account for 0.27–0.63%, 1.27–5.94% and 19.63–41.25%, respectively. Compared with two long-term flooded CLW and PAW, CAW and CSW showed higher soil LOC concentrations and proportions. In terms of soil microbial flora, bacteria number was the most, followed by actinomycetes and fungi number was the least in each wetland. The three microbial species populations decreased with soil depth in the four wetlands. The average microbial total amount was ranked in the order of CAW > PAW > CSW > CLW at 0–30 cm depth. The higher soil activities of invertase, cellulase and catalase were observed in the surface soil layer for all wetlands. The three enzyme activities in CSW and CAW were significantly higher than that in CLW and PAW at 0–30 cm depth. Soil LOC fractions were significantly related to SOC in the four wetlands (p < 0.01), and positive correlation with microbial total number, bacteria, fungus and actinomyces in CSW and CAW (p < 0.01). Whereas no significantly correlated were observed between LOC fractions and fungus or actinomyces in CLW and PAW. Except for catalase activity of CAW, soil LOC fractions had significantly correlation with three enzyme activities in the four wetlands (p < 0.05). The results have important implications that microorganism and enzyme activities are good indicators for predicting minor changes of soil LOC fractions, especially in no-flooding CSW and CAW.