What is the impact of drought on temperate soils?

The UK summer drought in 2022 produced significant speculation concerning how its termination could affect the national soil resource. It also highlighted a knowledge gap regarding the wider effects of drought on soil properties and functions in temperate soils. BGS scientists have contributed to a recently published review bringing relevant information together to address the knowledge gap and aid policymakers.

The paper focuses on agricultural and ecosystem drought in the UK, which is when soils experience dry periods that affect agriculture production and ecosystem function. However, each individual drought has its own characteristics with respect to length and intensity, with antecedent conditions particularly important to its overall impact.

Vegetation dieback is the most widely recognised effect of drought, often demonstrated in the media using satellite images. Questions frequently concentrate on crop yields, the impact of drought on food production and likely increases in retail prices. Another observable effect of drought in the UK (and globally) is that of soil cracking, which occurs when expansive clay minerals dehydrate and shrink, which may lead to undermining of foundations of houses and infrastructure. The process is of major economic consequence, with damage to infrastructure in the UK estimated at around £100 million a year, sometimes reaching £400 million in very dry years (Harrison et al., 2022).

Responses of soils and catchments to drought termination

Beyond the impact of drought on agricultural production and ecosystem function, a major concern is how the breakdown of soil may affect the soil resource in terms of runoff and potential erosion. This may influence surface-water quality through the transfer of sediment and nutrients. However, theoretically, dry soils should have the greatest potential for infiltration and, when the infiltration rate remains greater than the precipitation rate, erosion of the soil through the generation of runoff is less likely to occur. The response of both soils and catchments to drought termination in the short term will therefore initially be determined by the intensity and duration of precipitation, with intense storms more likely to generate conditions where rainfall exceeds infiltration capacity.

Impact of drought on soil properties

As we can have no long-term prior knowledge as to whether a drought will occur, evidence on how it affects soil properties is hard to obtain unless the drought coincides within the time frame of longer-term monitoring experiments of soil processes. However, experiments examining wetting and drying cycles provide some insight into the range of impacts on biological, chemical and physical processes in soils.

Infiltration depends heavily on soil structure, with many interactions occurring between the biological and physical components of the soil system, particularly in the production of sticky substances that help particles bind together in aggregates. The activity of bacterial and fungal communities in soil is generally negatively impacted by dry conditions and this may lead to some loss of soil structure, potentially affecting infiltration rates of precipitation. In addition, the activity of soil macroinvertebrates with body widths generally between 2 and 30 mm (such as earthworms, woodlice and millipedes) may decrease. These creatures are commonly seen as soil ecosystem engineers, as they create pathways for water drainage.

The biogeochemical cycles of major nutrients, including the production of greenhouse gases, may change due to the effects on the microbial communities that decompose organic matter. This can lead to flushes of nutrients and greenhouse gas emissions upon re-wetting.

Other effects may include:

• more pronounced shrink–swell behaviour than usual in soils containing expandable clays, leading to deep cracking and possible damage to infrastructure
• an increase in the water repellency of soils, particularly those soils high in organic matter, leading to greater surface runoff
• plant responses to drought that can severely reduce the plants’ protective effect, leaving soils exposed to erosion processes and degradation

Soil resilience to and recovery from drought

One focus of soil research in recent years has been exploring its resilience to and recovery from perturbations, of which drought is an obvious major one. ‘Resilience’ relates to the resistance (degree of change) coupled with the recovery (rate and extent) from a disturbance (Constanje et al., 2015).

The nature of precipitation, its intensity and frequency will help determine how soils initially respond to and recover after drought termination. It is likely that the physical, biological and chemical recovery from drought will happen over a variety of time scales, and some parts of the system may reflect an ongoing altered state.

The management of soil organic matter (SOM), a fundamental influence on soil moisture and structure, through cultivation practice and cropping will be important. Higher SOM concentrations offer greater resilience, at least in initial drought periods.  However, increased information is required regarding how biological soil communities, soil moisture dynamics and soil structure recover and how these affect biogeochemical cycles.

Conclusions

The paper reports on how the large number of interactions present between physical, chemical and biological soil properties helps explain soils’ response to drought. However, the results reviewed are drawn largely from experiments examining wetting and drying cycles.

Unlike UK ground and surface waters that have been continually monitored over historical periods, thus allowing assessment of the effects of droughts, soil data collected during actual drought periods from existing experiments are few. This means that knowledge relating to how key soil properties such as soil structure and biogeochemical cycling respond before, during and after a drought is needed for greater understanding. Collecting this data requires long-term experiments. The use of sensors, particularly to monitor soil moisture and shallow groundwater, along with the development of novel sensors could provide the basis of these experiments, allowing drought impacts to be placed into wider contexts.   

In addition, further gaps in our knowledge exist regarding soil water repellancy, the impact of wildfires on soils, multiple stressors (heat; moisture) and the effects of successive extreme events on soil systems, for example drought followed by flooding. On a planet that is experiencing more extreme climate events, addressing such questions will help identify actions that can be taken to build more resilient soil ecosystems.

The research paper, ‘Temperate soils exposed to drought — key processes, impacts, indicators and unknowns’, is now available to read in full online. 

About the author

Dr Andrew Tye

Process geochemist

BGS Keyworth

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