The influence of water table position on soil microbial processes and carbon mineralization in a mid latitude spruce peatland

Background/Question/Methods

High latitude forest and peatland soils represent a major global carbon store sensitive to the impacts of global climate change_. While increased temperatures may impact rates of microbial enzyme activity and greenhouse gas release from peat soils, the interaction between increased temperatures and changing precipitation patterns is projected to simultaneously reduce peat moisture and water table (WT) height in high latitude peatlands. WT reduction increases oxygen diffusion within the peat profile and potentially impacts (1) microbial activity and enzyme production, and (2) the rate of carbon mineralization and greenhouse gas emission. We performed an experiment investigating the influence temperature and available oxygen on rates of microbial enzyme activity and carbon mineralization across a 0-40 cm depth-to-water-table gradient in Caribou Bog, Orono, ME. We incubated peat samples at four temperatures and three oxygen concentrations in order to investigate the temperature and oxygen sensitivity of extracellular enzyme activity and carbon gas emission. We assayed rates of four hydrolytic and two oxidative exoenzymes that depolymerize carbon (C), nitrogen (N), or phosphorus (P) and compared enzymatic activity to rates of carbon mineralization and CO₂ production in incubated samples. 

Results/Conclusions

CO₂ production increased five-fold across the 4°C and 26°C incubation temperature range and increased by a factor of 1/3 between low oxygen (1%) and ambient (21%) incubation atmospheres. Phosphate-mineralizing acid phosphatase and cellulose-depolymerizing beta-glucosidase activity displayed exponential temperature response functions, while nitrogen-releasing B-N-acetyl-glucosaminidase and cellobiohydrolase increased exponentially until 21°C, after which point activity declined. Activity of all enzymes decreased with sample depth. Oxidative exoenzyme (phenol oxidase and peroxidase) activity increased significantly with depth to water table, while rates of hydrolytic enzyme activity demonstrated less strength of relationship.  As oxidative exoenzymes degrade polyphenolic compounds such as lignins and tannins, the results suggest that water table decline associated with increased temperatures may simultaneously increase rate of hydrolytic enzyme activity through temperature effects, as well as stimulate decomposition and release of recalcitrant carbon deep in the peat profile through increased activity of oxidative enzymes.