Influence of Substrate Quality and Moisture Availability on Microbial Communities and Litter Decomposition

Abstract

The main source of carbon (C) to soil stocks is plant litter, the decomposition of which is controlled by a mixture of physical, chemical, and biological processes. Bacteria and fungi are the dominant biota responsible for decomposition, yet we know very little about their respective contributions or how community dynamics may be affected by litter quality. This study sought to gain a better understanding of the variable relationships between organic matter decomposition, litter quality, and microbial community composition, with a specific focus on distinguishing bacterial and fungal dynamics. Experiments were conducted under contrasting hydrological conditions, comparing a wetland with an upland forest environment. Decomposition of native vegetation was monitored in addition to breakdown of a common substrate (Acer rubrum (red maple) leaves) placed in both environments. In situ incubations lasted 16 months, and were sampled at ~3-month intervals. Regardless of site, maple litter decomposition proceeded at a similar rate, though we did observe differences in litter quality over time (C:N, %N, solubility of organic C). For the upland site, native litter decomposed more slowly than the maple did. At the wetland site, both litter types decomposed at a similar rate which, surprisingly, was faster than either litter type at the upland site. This finding could be attributed to water-limitation at the upland site and/or stimulation of decomposition at the wetland site due to allochthonous nutrient inputs or organic matter priming. Substrate induced respiration (SIR) was measured for native litter incubated at each sampling site, and the relative contributions of bacteria and fungi were compared. No consistent major differences were detected across these microbial groups, though we did observe much higher rates of SIR at the wetland site compared to the upland site. Community structure of each microbial group was examined via terminal restriction fragment length polymorphism (TRFLP), which revealed dramatic temporal shifts for both groups at both sites. In general, these results indicate a long-term effect of both litter type and environmental conditions (site) on the bacterial community, but show only environmental effects on the fungal communities. This suggests that different environmental conditions allow microbial communities to uniquely approach decomposition of leaf litter components.