Soils associated with wet and ephemerally wet environments, i.e. wet soils, cover an area greater than 12.1 million km2; 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.
When we think about soils, water is the key variable that lets chemistry and biology do their thing. Water is, indeed, the language of life. But most life needs oxygen, and even those that don’t need a suitable substitute. And when there is a lack of oxygen, soils can accumulate organic matter and minerals that if they stay wet are perfectly fine but if they dry out can cause all sorts of environmental headaches.
Wetlands come in a variety of types – cold peaty places to hot dry place that flood occasionally to everything in between. Humanity doesn’t like flooding in general though, so a lot of effort has been undertaken into flood minimisation. This means that some soils that would be seasonally wet and dry are now permanently wet or dry. But, as the Millennium Drought showed in Australia, and other droughts have shown around the word – ‘permanently’ wet is not a permanent as one might hope.
Based on these thoughts, we explored some questions around the biogeochemistry of wet soils in a world of drought.
Perhaps the most pressing finding from this review is that there are huge swaths of the world where there is no readily available published research on drought affected wet soils. Considering the Western-centric nature of scientific research until recently, this is perhaps unsurprising. The vast majority of papers in this review came from Europe and North America, and a substantial bulk of the Australian research comes from a single research area. How can interpolate future outcomes with so little data?
The second most pressing finding is that there is effectively no applied research into water management outcomes for wetlands and wetland soils. Water management is the most direct tool society has at controlling wetland health and productivity and yet all managers have to work with is common sense and history. But what happens when the environment changes outside of historical bounds? And what happens when common sense solutions make a problem worse?
At a purely scientific level, this review explores the role of water and oxygen in soil biogeochemical properties of wet soils under drought conditions. That is, what happens when normally wet soils dry out? There are a few possibilities depending on the type of soil, the materials that have accumulated during the wet period, the intensity of drying, and the history of drying episodes in the past. Counterintuitively, a wet soil progressing rapidly from wet to completely dry isn’t necessarily the worst case scenario. Very little chemistry or life processes happen in a completely dry soil, and what does happen happens very slowly. Instead, a still moist soil that is exposed to air gets the ‘best’ of both worlds (enough water and enough oxygen) for processes to move along rapidly. Microbial peat breakdown in alpine sites and acidification of acid sulfate soils both happen fastest when there is enough oxygen and enough water to keep microbes working.
What does this mean?
My hope from this work is to support proposals into wetland soil and water management research and to encourage people to reach out to institutions in underrepresented regions to collaborate on their wet soils and their drought experiences. Because fens, swamps, bogs, floodplains, springs and moorlands are some of the most productive places on the planet, and we’re losing them at an astonishing rate.