Investigating the temporal impact of drainage and re-wetting on interactions between microbes, enzyme kinetics and dissolved organic compounds in peat

Peatland ecosystems contain one-third of the world's soil carbon store and are therefore a significant component of the global carbon cycle. Historically, peatlands have been drained for agricultural reclamation that has led to degradation and potential loss of a variety of ecosystem services including carbon sequestration and potable water quality via discoloration (i.e. high dissolved organic carbon export). The impact of rewetting by grip-blocking on dissolved organic carbon (DOC) export in peat bogs appears dependent on the length of time following rewetting but the mechanisms responsible are poorly understood. For long-term management of ecosystem services in peatlands, it is critical that the mechanisms responsible for DOC export can be proven empirically. Recent work shows that long-term restoration of a peatland watertable (>4 years) reduces DOC concentrations due to reduced decomposition after watertable recovery. However, short-term blocking of grips increases DOC and discolouration for up to 1 or 2 years following blocking of drains. Explanations for the short-term phenomenon include 1) flushing of elevated DOC that was produced by microbes during the drainage phase; 2) the persistence of enzymes that are no longer inhibited following drain-blocking; 3) a reduction of sulphate suppression in the soil solution leading to DOC release; and 4) the hydrophobic nature of peat delaying re-wetting and the release of DOC for 1 or 2 years. It is hypothesized that the long-term recovery of the watertable creates different biogeochemical conditions than short-term watertable elevation, i.e. there is no straight forward relationship between water saturation and DOC concentrations. Thus two distinct mechanisms are hypothesized to be responsible for DOC export over short (< 2 years) and long term (> 4 years) rewetting events. The role of the enzyme-latch mechanism (ELM) as a mechanism to explain the short-term flush of DOC following grip-blocking has not been fully evaluated in peatlands. In non-drained peatlands, enzyme activities are repressed due to recalcitrant phenolic compounds that persist in the anaerobic peat because the phenol oxidase enzyme responsible for their breakdown requires molecular oxygen as a co-factor. If oxygen in the peat mass increases because of drainage, phenol oxidase activity increases, reducing the concentration of phenolics and thus increasing the hydrolase enzyme activities promoting further microbial decomposition. However, restoration of the watertable level by grip-blocking may not restore the degradation-inhibiting phenolic compounds meaning that enzymic decomposition of the peat structure by hydrolase enzymes may continue and contribute to DOC flush. This study aims to build on research examining the relationships between key abiotic and biotic parameters associated with decomposition in peat that has been subjected to different hydrological regimes over the longer term (i.e. drained vs. grip-blocked) and will be subjected to short-term re-wetting/drying in the laboratory. The parameters to be studied include detailed enzyme kinetics of glucosidase and phenol oxidase, microbial activities and biomass, DOC, phenolics, ions and humic substances. This will permit detailed examination of the biotic mechanisms responsible for DOC release under different hydrological regimes. One important aspect is the effect of humic substances on the decomposition of OM to DOC and fundamental aspects of enzyme location and kinetics. This research is important for assessing the long-term impact of land management on the ecosystem services of carbon sequestration and water quality/provision.