Drought effects on wet soils in inland wetlands and peatlands

doi:10.1016/j.earscirev.2020.103387

Earth-Science Reviews

Volume 210, November 2020, 103387

https://doi.org/10.1016/j.earscirev.2020.103387 Get rights and content

Highlights

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Droughts are increasingly impacting on wet soils and leading to their degradation.
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Increase of oxygen in wet soils increases oxidation of organic matter and reduced inorganic minerals.
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Soil drying and increased redox potential during drought alters greenhouse gas emissions.
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Severe soil changes initiated by drought may cause a site to develop an alternative stable state.
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Wet soils undergo sequential transformations before, during and after drought.

Abstract

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.

Graphical abstract

Introduction

The geochemistry of wet soils and their responses to drought are significant topics of investigation as these soils are important ecosystems with high biodiversity that perform critical ecosystem processes such as flood mitigation, food production, water quality improvement and carbon storage (Ramsar Convention on Wetlands, 2018). Wet soils associated with permanently and ephemerally freshwater wet environments cover an area greater than 12.1 million km² (Davidson et al., 2018). Of these areas, inland wetlands are estimated to deliver Int$27.0 trillion (2001 values) per year of tangible and intangible benefits (Davidson et al., 2019). Inland wetlands are dominated by peats (30%), marshes and swamps with most (90%) peats occurring in temperate and boreal regions (Davidson and Finlayson, 2018). As of 2009, at least one third of global wetland extent had been lost (Hu et al., 2017); these losses continue today as peatland area is decreasing globally at a rate of 1% per year while some categories of peatlands are declining much more rapidly (tropical peatlands: 20% per year) (Davidson and Finlayson, 2018). The prime drivers of inland wetland loss are water abstraction and drainage of wetlands for agricultural use leading to hydrological droughts (Moore, 2002; Hu et al., 2017). Future declines are likely to be exacerbated in many regions of the world by increasing frequency and intensity of meteorological drought (Dai, 2013). Wet soil geochemical responses to drought can be significant, and in some cases catastrophic.

The aim of this review is to synthesise recent literature on the geochemical effects of drought on wet soils that are normally permanently wet or ephemerally inundated. These wet soils include those associated with inland wetlands, peatlands and rice paddies. There appears to have been no specific reviews on this general topic previously even though these environments are being increasingly exposed to drought. Some previous specific reviews have considered certain aspects of drought on wetlands such as plant responses (McKee Jr. and McKevlin, 1993), inland acid sulfate soils in Australia (Fitzpatrick et al., 2008a; Fitzpatrick and Shand, 2008), acid sulfate soils globally (Lin, 2012; Fanning et al., 2017), surface water quality (Mosley, 2015), and groundwater (Taylor et al., 2013). Aspects considered in this review include the immediate and longer term effects of drought on inland freshwater wet soils; we also discuss remediation of degraded inland wet soils and the potential for stable alternative states.

In this review, drought is considered as a temporary period of water deficit experienced over a period of time ranging from months to decades (Mishra and Singh, 2010); it can occur in effectively all climatic zones with varying intensities. Drought is defined a number of ways, and includes meteorological drought (prolonged deficit in precipitation), agricultural drought (soil moisture deficit affecting crops), and hydrological drought (deficits in streamflow, lake and groundwater levels) (IPCC, 2014). Hydrological droughts can occur independently in space and/or time from meteorological droughts. Recent examples of these scenarios can be observed in India, where over extraction of groundwater during average to above average precipitation periods is causing hydrological drought, and California, where extraction of groundwater during meteorological drought is permanently reducing groundwater storage capacity (Mishra and Singh, 2010; Taylor et al., 2013; Rodell et al., 2018); confluence of the two drought types can also be seen in Australia's Murray-Darling Basin (Mosley et al., 2014a, Mosley et al., 2014b (Bond et al. 2008, 2008, Fitzpatrick et al. 2009, Leblanc et al. 2012, Mosley et al. 2012, Van Dijk et al. 2013). Although there has been an overall increase in permanent surface water since 1984 due to the development of new bodies of surface water (such as dams), significant decreases due to water abstraction and meteorological drought have been observed in the Middle East, Central Asia, Australia, and the United States (Pekel et al. 2016, Rodell et al. 2018). While groundwater discharge can support wetlands during periods of low or no rainfall, changes in irrigation demand during drought and changed groundwater recharge capacity means that groundwater needs to be carefully managed during and after drought as described above (Taylor et al. 2013). Reduction in irrigation allowances is a common response to drought, increasing drought stress on wetlands associated with irrigation areas (Mosley et al. 2017). Drought also affects water quality and alters the normal transport of sediment, organic matter (OM) and nutrients within a catchment (Mosley 2015; Biswas and Mosley, 2018).

Section snippets

Current and future drought

Across the globe, drought has increased in frequency or severity in north and central America (Cook et al. 2004, Mishra and Singh 2010), Europe (Stagge et al. 2017), China (Li et al. 2015), India (Rodell et al. 2018), West Africa (Mishra and Singh 2010), and Australia (Van Dijk et al. 2013, Rodell et al. 2018). As predicted by climate change models, precipitation is generally decreasing in mid-latitudes and increasing in low and high latitudes (Rodell et al. 2018); however, a lack of data means

Wet soils general characteristics and distribution

In this paper we use the general term “wet soils” and define them as being organic or mineral soils associated with wetland vegetation and saturated in the majority of the soil profile for a length of time that permits the development of reducing conditions; these conditions can be achieved over short or long terms. Wet soils can form naturally due to poor drainage or plentiful water inputs or can form due to human induced flooding (such as in paddy soils). As far as we are aware, the only

Drainage and aeration of wet soils

Wet soils can experience drought through a number of natural and human induced pathways. Drying of wet soils, whether partially or completely, allows the penetration of oxygen into previously anoxic sites leading to the oxidation of organic and reduced inorganic species. Except in cases where soils have collapsed or compressed, organic (or peat) horizons and soils are typically highly porous; oxygen is therefore able to penetrate deep into the profile during drought conditions (Estop-Aragonés

Effects of increasing soil temperatures

Increasing soil temperature during drought can have complex effects on wet soils that are likely to be intimately connected to water dynamics. Increased temperature in the absence of changes to soil water content and substrate availability is likely to increase OM decomposition due to the decreased effort required to achieve activation energy for chemical reactions and thus increased microbial activity (Moinet et al. 2018). Persistent high temperature has been shown to decrease peat

Soil biogeochemical changes during drought

The geochemistry of wet soils is typically dominated by redox reactions and OM dynamics, and mediated by abiotic geochemical factors such as salinity and pH. The development of anoxic zones in wet soils occurs due to the decomposition of OM consuming oxygen; this can be due to large inputs of OM from wetland plants and algae relative to decomposition capacity, or due to poor gas penetration into the profile, or both. Inundated soils with relatively little OM can become anoxic simply because

Drought alteration of greenhouse gas production and emissions

The emission of greenhouse gasses (GHG) such as carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) are controlled by the decomposition of OM, transformation of nitrogen species, and the pathways by which gasses formed in wet soils escape into the atmosphere. Production of these gasses is controlled by substrate availability, soil water content and by the spatial distribution of wet and dry areas in the soil (Harms et al. 2009, Xu et al. 2015). As GHGs are formed under differing soil

Post drought fluxes

Post drought fluxes and transitions of materials out of and into wetland soils is a complex relationship between the substrate, solutes and time. Resilient soils may apparently return to their pre-drought condition (Table 1, Table 2) while their more sensitive contemporaries continue to respond to the environmental perturbation. In oxidised soils with significant quantities of organic matter, outflow chemistry is typically strongly acid with high SO₄²⁻ and metal contents. Even in the absence of

Soil recovery after drought or alternative stable states?

Wet soil recovery after drought is affected by soil properties such as pH, OM availability and soil texture, and by environmental conditions such as local hydrology. For example, recovery of acidified mineral soils may require both the re-establishment of reducing conditions and the flushing of undesirable solutes from the profile (Mosley et al. 2017); the relative importance of these two strategies is influenced by soil texture and structure as heavy textured, poorly structured soils are

Future research directions

It is clear from this review that there are a number of research gaps that need to be addressed. Firstly, the spatial distribution of drought studies has significant underrepresentation of a large number of regions, including south and central America, Africa, the Middle East, Asia and Oceania. As noted above many of these regions are predicted to be vulnerable to drought impacts due to climate change (Fig. 1). There is a notable lack of studies located in equatorial, tropical and subtropical

Summary and conclusions

Wet soils generally have horizons throughout with gleyed, mottled or redoximorphic features and are associated with important permanently and ephemerally wet ecosystems. They are naturally sensitive to meteorological and hydrological drought due to their close association with surface and ground waters; these soils are also under pressure from increased water abstraction for agricultural purposes and climate change. The primary risk associated with drought for wet soils is exposure of the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The funding support provided by the Australian Research Council (Discovery Project DP170104541) is gratefully acknowledged. We also thank the excellent and constructive input of two anonymous reviewers that enabled significant improvements to the structure and content of this review.

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Review ArticleDrought effects on wet soils in inland wetlands and peatlands

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Current and future drought

Wet soils general characteristics and distribution

Drainage and aeration of wet soils

Effects of increasing soil temperatures

Soil biogeochemical changes during drought

Drought alteration of greenhouse gas production and emissions

Post drought fluxes

Soil recovery after drought or alternative stable states?

Future research directions

Summary and conclusions

Declaration of Competing Interest

Acknowledgements

Geochim. Cosmochim. Acta

Sci. Total Environ.

Soil Biol. Biochem.

Soil Biol. Biochem.

Geochim. Cosmochim. Acta

Appl. Geochem.

Geochim. Cosmochim. Acta

Appl. Geochem.

J. Environ. Manag.

Sci. Total Environ.

Geochim. Cosmochim. Acta

Atmos. Environ.

Sci. Total Environ.

Mar. Pollut. Bull.

Chem. Geol.

Ecol. Eng.

Ecol. Indic.

Geoderma

Geoderma

Geoderma

Geoderma

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Geoderma Regional

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J. Hydrol.

Sci. Total Environ.

Chem. Geol.

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Geoderma

Geoderma

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Ionic concentrations in a blanket peat bog in northern England and correlations with deposition and climate variables

Eur. J. Soil Sci.

Climate change: Microbial mitigation

Nat. Geosci.

Acid Sulfate Soils Laboratory Methods Guidelines

Surface complexation of ferrous iron and carbonate on ferrihydrite and the mobilization of arsenic

Environ. Sci. Technol.

Reassessing the management of groundwater use from sandy aquifers: Acidification and base cation depletion exacerbated by drought and groundwater withdrawal on the Gnangara Mound, Western Australia

Hydrogeol. J.

Drought and saltwater incursion synergistically reduce dissolved organic carbon export from coastal freshwater wetlands

Biogeochemistry

Salinity effects on greenhouse gas emissions from wetland soils are contingent upon hydrologic setting: a microcosm experiment

Biogeochemistry

Pyrite oxidation in acid sulphate soils: the role of miroorganisms

Plant Soil

The role of microbial community composition in controlling soil respiration responses to temperature

PLoS One

The short-term effects of salinization on anaerobic nutrient cycling and microbial community structure in sediment from a freshwater wetland

Wetlands

Iron and aluminum hydroxysulfates from acid sulfate waters

Iron oxides

The effect of soil drying on humus decomposition and nitrogen availability

Plant Soil

From Mountain Ranges to Sweeping Plains, in Droughts and Flooding Rains; River Murray Water Quality over the Last Four Decades

Water Res. Manag.

Experimental inflow of groundwater induces a “biogeochemical regime shift” in iron-rich and acidic sediments

J. Geophys. Res. Biogeosci.

The impacts of drought on freshwater ecosystems: an Australian perspective

Hydrobiologia

Review Article
Drought effects on wet soils in inland wetlands and peatlands