Soil provides essential nutrients to crops, as well as ecosystem services such as reducing flood risk, providing clean water, and absorbing carbon from the atmosphere, but intensive land management practises have degraded many arable soils in the UK. It is essential that we develop reliable indicators of soil health that can allow us to track the effectiveness of different restoration strategies or monitor the impact of changes in land management on soil recovery. In search of consistent and responsive indicators of soil health, Dr Rob Griffiths led a UK-wide study of soil characteristics and processes under different land-management regimes. They found that the intensity of land management influenced the carbon content, pH and microbial communities in the soil.

IntroductionHealthy soil provides nutrients for plants and crops, acts as a sponge to soak up heavy rainfall, and absorbs carbon dioxide from the atmosphere. Most of the biological and chemical activity occurs in the upper layer of soil, known as top soil (see Box 1). But research has shown that intensive land management practises slowly degrade top soil by depleting essential nutrients, damaging soil structure and harming soil critters.

It is important to develop bioindicators – microbial species, chemical, or visual traits – that can be used by land managers to monitor the quality of their soil and by policy-makers to evaluate the effectiveness of different restoration strategies. But finding traits that are consistent and responsive indicators of soil health across the diverse landscapes of the UK remains a challenge. For example, although the total organic content of soil is a reliable indicator of health, it develops very slowly with restoration, so the effect of a particular intervention may not be detectable for up to 5 years. In search of better bioindicators, a team of researchers led by Rob Griffiths (NERC Centre for Ecology and Hydrology) conducted a UK-wide survey of microbial communities and natural soil services under different land management types.

MethodologyThe team surveyed soils from 20 sites across the UK that varied in soil type and climate conditions and measured key properties such as carbon content and pH, as well as measuring processes like carbon cycling. They compared sites with the same geography and climate but that differed in the intensity of land management. The researchers sequenced DNA from microbes present in the soil and looked at the activity of genes known to be involved in nutrient cycling. They created mathematical models to predict how changes in microbial communities would influence ecosystem services such as carbon sequestration.

Surveying for practical indicators of soil health

The first 30 cm or so below our feet is known as ‘top soil’ and it is where most of the activity in the soil takes place. Top soil contains millions of bacteria, fungi and other soil microbes, which are involved in breaking down decaying matter and recycling nutrients.

Soil microbes store carbon in the soil, preventing it from reaching the atmosphere as the greenhouse gas carbon dioxide. In fact, soil contains around twice as much carbon as the air, and over half of that is found in the top soil.

•The type of land use is the main factor affecting soil microbial communities.

•Across soil types, land-use intensification reduced carbon content and increased pH.

•More intensively managed soils have less organic matter and their microbial communities are smaller and less active.

UK Farmland, Devon. Researchers found that the type of land use affected the carbon content, acidity, and microbial communities of the soil.

ResultsThey found that more intensively managed soils tended to contain less carbon and were more alkaline. However, the effect of intensification on how much of the soil carbon is consumed and absorbed by microbes – known as carbon use efficiency - depended on the geography and climate of the site.

For a given soil type, less intensively managed soils with a higher carbon content tended to have higher activity of key microbial enzymes involved in nutrient cycling and carbon absorption. Land-use intensification in pH neutral soils reduced the number of microbes in the soil as well as making them less efficient at storing carbon, whereas initially acidic soils actually benefited from the pH increases associated with land-use intensity.

The team identified key microbial groups that are good indicators of land-use intensity, but these varied between soil types. “You don’t see the same indicators in an upland system compared to a lowland [site], but there are commonalities within those habitats”, said Griffiths.

Land-use intensification also means different things in different places – intensive management in the Scottish uplands might mean draining land to graze sheep, while in the south of England it is more likely to refer to agriculture.

Contact Details: Dr Rob Griffiths (rig@ceh.ac.uk)

ReferencesMalik, A. A., Puissant, J., Buckeridge, K. M., Goodall, T., Jehmlich, N., Chowdhury,S., ... & Blaud, A. (2018). Land use driven change in soil pH affects microbial carbon cycling processes. Nature Communications, 9 (1), 3591.

Malik, A. A., Puissant, J., Goodall, T., Allison, S. D., & Griffiths, R. I. (2019). Soil microbial communities with greater investment in resource acquisition have lower growth yield. Soil Biology and Biochemistry, 132, 36-39.

The relationships between soil microbial communities in different soils. Land management left an imprint on the soil microbial community.

A collage of soil samples collected from across the UK. Intensively managed soils tend to contain less carbon and have a lower pH.

•Minimising the disturbance of soil is the most effective way to enhance soil organic content and biodiversity.

•Less intensive land management practices could boost ecosystem services provided by soil, such as carbon sequestration.

•To make information about key soil microbes more readily available, the team created a bioinformatics portal as a resource for academics, policy-makers and other stakeholder groups.

“The challenge now … is to put the microbial taxa, functional genes, and measured processes together to work out exactly how soils work, to allow us to devise new management approaches which optimise microbial processes” - Rob Griffiths

Grassland

Arable

GrasslandIntensive

Degraded Arable