Overview of change

In 2020, the Natural Capital Committee stated that available data suggest the state of soils in the UK is declining and a national survey is needed on the extent and condition of soils.1 This would provide a baseline and data for the soil health target proposed under the Environment Bill 2020.2 Soil health has been described as “the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans”.3 Appropriate metrics and soil health measures continue to be debated due to the complexity of soils and their biodiversity.4,5 UK soils are estimated to currently store about 10 billion tonnes of carbon, with peatland soils containing the most. Soil carbon varies widely between soil types; an organic carbon value of 1.5% is considered a lower limit for arable soils with 40% clay, but would be considered high in soils that have less than 10% clay in England and Wales.6 Most arable soils have lost 40–60% of their organic carbon, 2 million hectares of soil are at risk of erosion and 4 million hectares at risk of compaction.7 Without national monitoring of soil organic carbon, structure and biodiversity, it will be difficult to assess the status of the range of benefits supporting human well-being provided by soil. These include growing food, carbon storage, water quality and reducing flood risk. Good stewardship of soils could increase the provision of these benefits.8,9 Increased frequency of climate extremes, such as droughts, represent a significant future threat to soils and the benefits they provide.10

Challenges and opportunities

Erosion is the process by which top-soil is carried away by wind and water, which is accelerated by activities such as removal of vegetative cover, ploughing down sloping fields and overgrazing. Erosion is directly linked to the loss of soil carbon, which may have been stable for millennia.11,12,13 A global survey  found farmed soils to be losing more material to erosion than they are gaining.14,15 Modelling suggests there will be an increase in surface water run-off, from greater winter rainfall and more intense rainfall events from climate change, if soil conservation measures, such as cover crops and no till agriculture (not ploughing between crops), are not adopted.16 However, the effectiveness of no till agriculture depends on soil type and weather. No till agriculture may be beneficial for improving soil structure and some benefits that arise from the soil but it may reduce others,17 such as agricultural productivity.4,8,18 Not all farmers will adopt measures in the absence of incentives,19 such as payments for carbon storage.

Poor soil management can damage soil structure, which is not improved by shallow rooted crops that rely on fertilisers.20 Deep rooted crops and higher biodiversity grassland can increase fine roots, which bind soils and improve soil structure, as can crop diversification.21,22 Promoting a soil microbiome for higher plant productivity requires management of microbial and plant communities, and the processes they support.23,24 For example, adding nitrogen to soil over long time periods changes plant and microbe interactions decreasing diversity.25 Grassland plant communities shift towards fast-growing species with thinner larger leaves that rot faster.26 By contrast, relatively low nitrogen levels limit microbes’ ability to metabolise carbon compounds, so they excrete them as polymers that create an extensive network of pores for greater circulation of air, nutrients and retention of water.27 The ratio of carbon to nitrogen is also influenced by other additions to soil; for example, anaerobic digestion, an important source of carbon returned to land waste, is higher in nitrogen and lower in carbon raising concerns.28 Herbicides can also be used by soil microbes as carbon and/or nitrogen sources; their degradation can be enhanced through additions such as organic matter.29 Research on plant and soil microbe interactions also suggest additions, such as amino acids, may promote beneficial bacteria on plant roots.30

Soils and plant communities could be managed to maintain carbon storage and to be more resilient to climate change.31,32,33 For example, an assessment of 31 peatlands across England and Europe found that more peatlands became drier during the past 200 years than the previous 600 years and may become carbon emission sources without restoration and better management.34,35

Key unknowns

With climate change, temperate soils are forecast to experience a high degree of variability in moisture conditions due to periods of drought, flood and intense rainfall events. Increased wetting and drying sequences will lead to higher turnover of soil carbon by microbes, increased greenhouse gas emissions, and changes in soil microbial communities and the processes they support.36,37,38 Similarly, tropical soils may release more greenhouse gases as they become warmer,39 and areas in Russia and Canada may become suitable for cultivation that releases carbon.40 But uncertainties remain in the global soil carbon response to climate change.41

Restoring lost carbon through conventional approaches may take decades; research suggests faster restoration approaches, such as applying biochar, result in very variable soil responses.42,43,44,45,46,47 Novel approaches have also been suggested for improving soils, such as super moisture-absorbent gels or gene-edited bacteria, could be used to improve the water content or fertility of dryland soils.48,49.

Soil hydraulic properties determine the fraction of rainfall that infiltrates versus runs off and have been assumed to remain constant, but research shows increased precipitation may reduce infiltration rates.50 Climate change may also indirectly impact soil erosion through the use of different crops.51

Key questions for Parliament

  • What are the appropriate baselines and data needed for assessing the state of soil health nationally;1,4 such as a suite of soil ecological indicators based on essential biodiversity variables linked to targets?5
  • What are the appropriate frameworks and incentives for increasing stable carbon in soils, such as peatland restoration,52 and managing microbial communities and cropping systems for soil health?53
  • How to manage soils for mitigating and adapting to climate change and to sustain other benefits.54

Likelihood and impact

Continuing degradation of soil is likely globally with high impact,55 but managing soil carbon could also form part of net zero plans.

Research for Parliament 2021

Experts have helped us identify 30 areas of change to help the UK Parliament prepare for the future.

References

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Photo by Scott Goodwill on Unsplash

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