Soil extracellular enzyme activities from a forest harvest and climate manipulation experiment in central Pennsylvania

We have previously shown that warming and wetting a post-harvest forest soil increased C and N mineralization rates. A series of interacting processes may lead to these observed patterns including shifts in microbial allocation among extracellular enzymes. We monitored soil extracellular enzyme activity (EEA) for 2.5 years in a post-harvest, climate manipulated field experiment. A 2 ha forest in Central Pennsylvania was whole-tree harvested in 2007 and climate manipulations of +2 °C (warmed), and +20% mean monthly historical precipitation (wetted) were added to plots and in combination in a factorial design. The following enzymes were quantified: β-1,4-glucosidase (BG), cellobiohydrolase (CBH), leucine aminopeptidase (LA), N-acetyglucosaminidase (NAG), peroxidase (PER), and polyphenol oxidase (PPO). 
Soil hydrolase enzymes (BG, CBH, LA, and NAG) all showed greater mean activities than those reported from other studies, including study sites with similar climatic and edaphic characteristics. This is indicative of high overall soil microbial activity after forest harvest. Only BG and NAG showed significant treatment effects with a two-factor interaction (warming+wetting) on BG (p = 0.029) and warmed effect on NAG (p = 0.007). Warming decreased BG relative to ambient and NAG relative to wetted plots. There was substantial variability in the season and time since harvest, but water-extractable organic C (WEOC), C:N, and total N best explained the variability of EEAs over the experiment (p’s < 0.05). Two exoenzyme ratios previously used in the literature, BG:(LA+NAG) and BG:PPO, did not vary among treatments. However, the ratio of nitrogen acquiring enzymes (LA+NAG) to the sum of all other enzymes was higher in single-factor treatments compared to the ambient treatment (p = 0.007). Extracellular enzyme activity in a post-harvest forest soil was affected more by increased temperature than by increased precipitation, warming decreasing BG and NAG. Warming has been shown to decrease EEA in other ecosystem warming studies, but this usually is accompanied by a decrease in soil C and N fluxes (also called “attenuation to heating”). Surprisingly the warming treatment had the opposite effect on soil C and N fluxes, increasing fluxes compared to ambient. This inconsistency indicates a physiological change, including a possible shift towards alternate C and N acquiring enzymes, and that disturbed soil EEAs may not respond to climate change in the same way as undisturbed systems