Print PageA fundamental tenet of microbial ecology is that heterotrophic microbial activity responds positively to increasing water availability under oxic conditions. However, once released into the environment, extracellular enzymes- the proximate agent of decomposition- may act as independent biotic entities. Free enzymes are not governed by the same biological constraints as the living organisms that produce them in soils. While extreme desiccation may reversibly alter the conformation of enzymes such that their active site is inaccessible, for the most part, hydrolytic enzymes are insensitive to oxygen availability and can maintain activity across a wide range of soil moistures. The primary effect of soil moisture on enzyme activity is due to limitations on enzyme and substrate diffusion. These constraints result in moisture response curves that are similar to other biological processes. We hypothesized that alterations in precipitation regimes could alter the shape of these moisture response curves. We collected soils from the Boston-Area Climate Experiment, which subjects old fields to a factorial combination of warming and precipitation alteration (+/- 50% of ambient).
Results/Conclusions
Potential enzyme activity rates are primarily controlled by the concentration of enzymes in the soil environment. These enzyme pools are not neccasarily in equilibrium with soil microbial biomass or activity rates. Rather, they are controlled by the net balance of enzyme production and turnover. We found evidence that enzyme turnover can decrease under experimental drought, resulting in a net increase in potential enzyme activity when substrates become accessible during rewetting. We speculate that the pulse of DOC typically observed after rewetting events is partly attributed to the large reservoir of active enzymes that accumulate during drought. Following rewetting, microbial assimilation of DOC is constrained because of microbial stress responses, resulting in a brief period of elevated DOC pools.
We propose that enzymes must be viewed as independent biotic agents that react to soil moisture independently from active microbes. They represent a reservoir of potential activity that can instantly resume depolymerization of organic matter in soils as soon as water availability allows diffusion of substrates and enzymes. This has important consequences for the timing and magnitude of soil processes that are observed at the ecosystem scale.
