Leaf decomposition is driven by microbial production and secretion of extracellular enzymes. The high energy and nutrient costs of these enzymes suggests that tight regulation provides a competitive advantage under harsh environmental conditions. Extreme weather events such as drought are predicted to increase with global climate change. It is therefore important to understand how ecosystem processes such decomposition will respond, and the drivers behind these responses. The effects of drought, nitrogen deposition (ammonium+nitrate), and labile carbon amendment (glucose) on Fagus grandifolia leaf litter decomposition were investigated in an upland temperate forest in Northeast Ohio. F. grandifolia litterbags were deployed under precipitation exclusion tents and amended with artificial rainwater, glucose, and ammonium+nitrate treatments in a fully factorial, randomized block design. Potential activity of eight extracellular enzymes was assayed, 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). Terminal restriction fragment length polymorphism (T-RFLP) of 16S rRNA gene and ITS4 regions were used to assess bacterial and fungal communities, respectively. Bacterial biomass was measured using 4’,6-diamidino-2-phenylindole (DAPI) staining and microscopy.
Surprisingly, leaf litter mass loss was not affected by treatments (drought, nitrogen deposition, or labile carbon). Drought significantly increased activity of cellulose-degrading enzymes (βGlu and βCell). This suggests that up-regulation of cellulose-degrading enzyme production offset the limitations to litter breakdown imposed by drought conditions. Interaction effects between all treatments occurred with XYL (p=0.02), αGlu (p=0.055), and NAG (p=0.057) activities. XYL and αGlu activity increased under drought and nitrogen treatments, except when amended with glucose. Labile carbon amendments may have provided an alternative energy source that reduced the benefits of starch (αGlu) and hemi-cellulose (XYL) degrading enzyme production. Redundancy analysis revealed that treatments explained 15-22% of total variation in the bacterial community composition. However, treatments did not significantly impact bacterial biomass. Together, these results suggest the increase in enzyme activity under drought conditions observed in this study may be driven shifts in gene expression by the microbial community.