Moisture and nitrogen effects on enzyme activity and microbial communities during Fagus grandifolia leaf litter decomposition

Background/Question/Methods

Extracellular enzymes produced by microbial communities catalyze the breakdown of complex leaf materials to acquire carbon, nitrogen, and phosphorus. Enzyme production requires a large investment of energy and nitrogen, suggesting tight regulation by substrate availability.  In addition, because microbes are dependent on water for substrate diffusion into the cell, moisture availability likely impacts this regulation.  We investigated the effects of nitrogen (nitrate & ammonium), labile carbon (glucose), and moisture availability on Fagus grandifolia leaf litter decomposition in an upland temperate forest in Northeast Ohio.  Leaf litter bags were deployed beneath rain tents and treated with artificial rainwater, glucose, and ammonium+nitrate amendments for two years. Measured variables included: bacterial and fungal biomass, abundance of bacterial and fungal ribosomal genes based on quantitative PCR, and bacterial and fungal community composition using terminal restriction fragment length polymorphism (T-RFLP) of these same genes.  In addition, eight extracellular enzymes were assayed: α-glucosidase (αGlu), β-glucosidase (βGlu), β-cellobiohydrolase (βCell), xylosidase (XYL), β-aspartyl-N-acetylglucosaminidase (NAG), phosphatase (PHOS), leucine aminopeptidase (LAP), and laccase (LAC). 

Results/Conclusions

Experimental treatments (normal vs. reduced moisture, normal vs. elevated N and normal vs. elevated labile C) did not result in differences in mass loss of leaves.  However, there were significant effects on extracellular enzyme activity, which increased in strength over time. No significant differences were observed after two months of decomposition. After seven months, elevated moisture significantly reduced (between 15% and 80%) activities of: αGlu (p=0.04), βGlu (p=0.03), βCell (p<0.01), NAG (p=0.03), PHOS (p=0.03), and XYL (p<0.01) relative to the negative control.  Ammonium+nitrate amendments significantly increased activities of several carbon-acquiring enzymes, including αGlu (p=0.04), βGlu (p=0.05), and βCell (p<0.01).  Glucose amendments only increased αGlu activity (p<0.01).  The increase in carbon and phosphorus acquiring enzymes in ammonium+nitrate amended communities suggests alleviation of nitrogen limitation.  The reduction in enzyme activity in artificial rainwater amended communities may indicate down-regulation of enzyme gene expression.  Microorganisms may be unable to detect target substrates because of increased diffusion away from the cell under high moisture conditions.  Alternatively, increased diffusivity may facilitate monomer uptake by microorganisms that did not produce these extracellular enzymes.  Our results emphasize the importance of nitrogen and moisture availability when examining carbon cycling in terrestrial microbial communities.