Drought effects on microbial community structure and function during Fagus grandifolia leaf litter decomposition
Microbial production of extracellular enzymes drives carbon and nutrient cycling through litter decomposition in temperate forests. The large energy and nitrogen investment required for enzyme production suggests a competitive advantage can be achieved by optimizing returns of carbon and nutrients. Because extracellular enzymes reach intended substrates through passive diffusion, moisture availability is expected to alter community function due to changes in enzyme production. Drought, nitrogen deposition (nitrate & ammonium) and labile carbon (glucose) effects on Fagus grandifolia leaf litter decomposition were investigated in an upland temperate forest in Northeast Ohio. Leaf litter bags were deployed beneath rain exclusion tents and amended for two years with artificial rainwater, glucose, and ammonium+nitrate in a fully factorial design. Bacterial and fungal communities were characterized by terminal restriction fragment length polymorphism (T-RFLP) and fungal and bacterial biomass was measured. The activity of eight extracellular enzymes was assayed for a measure of function, including: α-glucosidase (αGlu), β-glucosidase (βGlu), β-cellobiohydrolase (βCell), xylosidase (XYL), β-aspartyl-N-acetylglucosaminidase (NAG), phosphatase (PHOS), leucine aminopeptidase (LAP), and laccase (LAC).
Data were log transformed and analyzed with a mixed-effects linear model accounting for block as a random effect. A three-way repeated measures ANOVA revealed significant increases in βGlu (p<0.01), XYL (p<0.01), and βCell (p<0.01) activities in response to drought treatments. Interaction effects between all treatments (nitrogen, glucose, and moisture) were detected for XYL (p=0.02) and αGlu (p=0.055), and a marginally significant interaction effect between drought, nitrogen, and time was detected for NAG (p=0.057). Enzyme activities were not affected by nitrogen; labile carbon treatments only affected XYL activity (p<0.05). No treatment effects were observed for bacterial biomass or leaf litter mass loss. Strong drought effects on enzyme activity suggest substrate uptake through diffusion may be an important factor in regulating enzyme gene expression. These changes in enzyme activity are likely driven by the fungal community, as no changes in bacterial biomass were observed. Furthermore, lack of a drought effect on leaf litter mass loss may indicate that increased enzyme activity is an effective compensatory mechanism allowing the microbial community to continue activity despite reduced moisture and rates of enzyme and substrate diffusion.