Drought effects on wet soils in inland wetlands and peatlands

Soils associated with wet and ephemerally wet environments, i.e. wet soils, cover an area greater than 12.1 million km(2); inland wetlands deliver at least Int$27.0 trillion in tangible and intangible benefits. However, due to their intimate association with wet environments, wet soils are at risk of degradation during drought events. This review investigates the effects of drought on wet soils, with particular attention to the changes in soil geochemistry and greenhouse gas emissions. It is clear from this review that drought poses a significant threat to wet soils, a threat which can be difficult to determine before an event but which poses a catastrophic risk to some sites. Drought causes oxygen penetration to increase in wet soils, leading to an increase in oxidation of organic matter and reduced inorganic species (e.g. sulfides). Oxidation of these materials can lead to soil acidification, metal mobilization and to negative impacts on water quality. Increased oxygen in the soil profile also affects biogeochemical cycling, with increased production of nitrous oxide and decreased production of methane. Effects of drought differ between peat and mineral soil types and subtypes. Wet soils undergo major chronological transformations and biogeochemical changes in the alteration of environments occurring before, during and after drought conditions. Water conditions (i.e. subaqueous, saturated, unsaturated and resaturated) also play a major role in chronological soil transformations. Soils may not easily recover between severe droughts and instead enter alternative stable states. This review highlights substantial gaps in our understanding of the effects of drought on wet soils and shows that previous studies overrepresent relatively small geographical regions.