Spatial and temporal heterogeneity in bog enzymes and their temperature responses

Background/Question/Methods

Enzymes activities are a useful metric of bog microbial community function because they are the direct agents of decomposition for specific substrates in peat. Enzymatic depolymerization like other biochemical reactions is sensitive to temperature, however not all extracellular enzymes have the same response to temperature and may vary due to environmental factors and with microbial community composition. The Spruce and Peatland Under Climate and Environmental Change (SPRUCE) project will be a temperature and CO₂manipulation experiment at Marcell Experimental Forest, MN. During enclosure construction we are assessing baseline enzyme activities and kinetics to understand how they vary across space and time to predict how enzymes might respond to the SPRUCE warming manipulation. We investigated the effect of temperature fluctuations, due to depth and season, on the temperature response of enzymes involved in carbon, nitrogen, and phosphorus cycling from peat.

Peat was collected from Marcell Experimental Forest, MN in February, July and August 2012. Three cores were extracted from 0-200cm below the bog surface and split into 10 or 25cm increments. Enzymes activities were assayed at multiple temperatures, from 2 to 65°C. The Arrhenius equation and Q₁₀were used to estimate the temperature response for each enzyme at each depth and season.

Results/Conclusions

All three enzyme metrics, activity, temperature sensitivity, and stoichiometry decreased with depth, but no response was noted by season. With increasing depth there was decreasing microbial biomass, stable year around temperatures, and smaller peat carbon:nitrogen ratios; all three of these variables may have affected our enzyme metrics. Microorganisms produce enzymes, so a reduction in biomass with depth would likely result in reduced potential enzyme activity. Temperatures are stable below 100cm, less than a 5°C change throughout the year, possibly resulting in communities and enzymes specialized to specific temperature ranges, thus lowering temperature sensitivity. Total nitrogen increases with depth whereas total carbon remains the same, resulting in a lowered peat C:N. This occurs with a concomitant decline in the enzyme C:N mineralization ratio (β-glucosidase : (N-acetyl glucosaminidase+leucine amino peptidase)). Enzyme type was another significant variable, with proteases having lower temperature responses compared to other hydrolytic enzymes. For proteases, E_a<20, whereas the other hydrolytic enzymes involved in C and P depolymerization were more responsive to temperature, with E_a = 20-60. The lack of a temperature response in protease activity indicates a possibility for decoupling of N cycling from C and P cycling in the bog with warming.