Grounded: Keeping the carbon beneath our feet

When we talk about carbon storage, we often talk about locking it away in forests or planting more trees in our cities. But did you know that around 80% of the carbon in land-based ecosystems is actually found in the ground? So addressing climate change also means talking about how to stop carbon being released from our soils.

Soils take up carbon mainly through plants, which get their energy from carbon dioxide via photosynthesis. The carbon becomes part of the plant material itself, such as the roots and leaves. When these die, the most readily decomposed parts, containing what is known as ‘labile carbon’, create a feeding frenzy among soil bacteria and fungi. Like all organisms, explains soil ecologist Sabine Reinsch from the UK Centre for Ecology & Hydrology (CEH) in Bangor, microbes need carbon for energy. ‘Microbes will eat these forms of carbon and then you have the carbon stored in the biomass of the microbes,’ she says. Less easily decomposed forms can become (chemically) bound to grains of soil and form part of bigger clumps called aggregates, where the carbon can be stored for years.

Can we stop carbon from escaping?

Because of the crucial role of plants in carbon cycling, keeping the ground covered with so-called ‘cover crops’ – even on fallow farmland – is one way to keep hold of soil’s organic content and prevent carbon from escaping into the atmosphere. ‘You avoid bare soil,’ says Sabine. ‘You grow plants all year round, even if you don’t have a crop.’

But how do we know how much carbon is actually in the soil? And how do different land uses and soil treatments compare in terms of helping the soil to absorb it? ‘We know pretty well from our long-term experiments, because they’ve been going on for 177 years,’ reveals Keith Goulding, a soil chemist at Rothamsted Research in Harpenden. Here, scientists have been growing wheat on the same field since 1843 and grass on the same parkland since 1856. As you might expect, establishing permanent grassland allows carbon to build up – but, ‘if you plough it, you lose it,’ says Keith.

Results from the wheat field experiment show that it’s possible to raise the carbon content by adding organic matter like manure, but the capacity of soil to soak up carbon eventually maxes out. Scientists working on this experiment test ‘cores’ of soil, from the top layer down to 23 centimetres deep. Since 1992, they’ve analysed soil organic carbon by burning soil and measuring the carbon content of the combustion gases. According to their findings, soil organic carbon has never risen more sharply than in the first 20 years of the experiment, when it averaged one tonne per hectare per year. By 2005, this average had plummeted to 0.2 tonnes over the same period.

What can we do?

Going deeper

Meanwhile, Sabine and her CEH colleagues have embarked on an in-depth study of carbon storage across different UK landscapes. She explains that most studies only consider how carbon is stored in the topsoil, up to around 30 centimetres. ‘But we know that carbon is also stored below that depth,’ she says. ‘So one aspect we are trying to investigate is how carbon is stored differently in those deeper layers – what are the processes and how are they different from the top layers?’ To understand how the carbon stores vary between landscapes, the team runs transects (sample lines) between grasslands and forests, for example, taking cores at regular intervals. They’re also looking at how carbon stores change over time due to changes in land use, such as when forest is converted to grassland.

Although Sabine and her team are still collecting data, they will eventually use the results to build a carbon risk map for the UK. According to Sabine, this could help show ‘how likely it is under certain conditions, such as a flood every year, that we would lose carbon from [certain] areas.’ The hope, she says, is that it will help policymakers decide how to better protect the precious carbon stores that we already have.