Microbial processes are critical to several aspects of wetland ecosystem structure and function. However, the factors that regulate the distribution and abundance of bacteria and fungi in these habitats are not well understood. This study explored microbial colonization and community development on leaf litter in a freshwater marsh, and further considered how moisture availability and leaf litter type may influence decomposition rates and microbial community structure. Within the wetland, three sites were selected to represent a range of hydrologic regimes; within each site, two sets of decomposition bags were deployed containing either (i) litter from vegetation native at the site or (ii) red maple leaves (common substrate for all sites). Bags were deployed in November 2007 and sampled at 3-month intervals for 1.5 years.To determine the relative importance of bacteria versus fungi to this decomposition, several assays were employed including: (i) qPCR to determine bacteria:fungi ratios, (ii) whole-community DNA fingerprinting via T-RFLP to examine bacterial and fungal community composition, and (iii) fungi and bacteria-specific respirations rates (¹⁴CO₂ production from heterotrophic uptake of ¹⁴C-labeled acetate).

Results/Conclusions

Decomposition rates were strongly influenced by moisture availability; litter type was also important, but to a lesser extent. The main controls on decomposition were redox status and leaching of dissolved organic carbon in more inundated sites. Differences in both group-specific respiration and bacteria:fungi ratios were significant between sites, and changed over the time course of the experiment. In general, microbial colonization and respiration was higher in the wetter sites, but the relative contribution of bacteria vs. fungi varied. For example, in the permanently inundated site, fungi were always more abundant and fungi-specific respiration was higher. This is in contrast with several previous studies that have suggested that fungi are more successful in drier habitats. T-RFLP community profiles of fungi indicated a clear successional pattern, wherein species were replaced with time but then did not reappear. In contrast, the bacterial community profiles showed a less defined pattern, with more stochastic appearance and disappearance of species. Collectively, these results demonstrate that both bacteria and fungi are important agents of decomposition in freshwater wetlands, and that microbial community diversity can depend greatly on organic matter substrate, frequency and duration of flooding, and time. In some cases, fungi are the initial colonizers of the OM, while bacteria predominate in early decomposition under other environmental regimes.