Review ArticleDrought effects on wet soils in inland wetlands and peatlands
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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