Revisiting the concept of 'enzymic latch' on carbon in peatlands

Peatlands are long-term sinks of atmospheric carbon (C) largely due to water-saturated soil conditions, decay-resistant plant litter, and the presence of biochemical compounds such as soluble phenolics. As phenolics are known inhibitors of microbial enzymes in soils, the concept of the 'enzymic latch' on peat C was introduced, assuming that phenolics accumulate in peat water due to protection from degradation by oxidative enzymes as a result of anoxia. However, their inhibitory role in peat has not been unambiguously confirmed. We aimed to verify whether peat phenolics inhibit microbial and enzyme activities in laboratory-incubated Sphagnum litter, and bog and fen peat. Soluble humic substances were extracted from bog water as a source of natural phenolics and separated into two molecular-weight fractions. We tested the effects of (1) phenolics concentration, (2) their molecular weight and (3) anoxia on the activity of hydrolytic and oxidative enzymes, and on microbial respiration rate. The added phenolics did not suppress hydrolytic enzyme activities nor microbial respiration. Quite the contrary, phenolics addition (up to 1000 mg L^-1) sometimes supported enzyme and microbial activities, indicating that phenolics (or another constituent of peat humic substances) served as a source of C. The activities of hydrolytic enzymes did not vary between oxic and anoxic peat but were double in oxic than anoxic conditions in Sphagnum litter. Differences in enzymatic and microbial activities were driven by peat type with about three times greater microbial respiration rates and enzyme activities in fen peats. Our results do not support the concept of the enzymic latch, particularly its key assumption that peat phenolics inhibit hydrolytic enzymes. While the concept was established on oceanic peatlands with low phenolic concentrations, the peat microbial community in our experiments seemed acclimated to the naturally high phenolic concentrations, characteristic for other, non-oceanic northern peatlands. Thus, the enzymic latch should not be considered as a determinative mechanism preserving the global C store in peatlands.