Loosely in the soil like diamonds?

Since we started ploughing we’ve moved over 130 billion tonnes of soil carbon into the atmosphere as CO2. That’s a lot less than fossil fuels, but it’s still equivalent to about half the excess carbon now in the atmosphere compared to pre-industrial levels.

So getting some of that CO2 out of the atmosphere and back into the soil as stable carbon is a no-brainer. Most of the policy focus so far has been on trees, and on some soils trees will increase carbon storage above and below ground – with early gains for some species and larger long-term gains with others. 

However on many of Scotland’s soils trees lose more carbon below ground than they accumulate above ground, for years or decades. So it’s important to look at soil carbon in farms as well as forests.

Soil carbon comes in two forms – inorganic carbon locked up in minerals, and organic carbon. Organic carbon morphs between different forms – plant roots, micro-organisms, worms and spiders, leaf litter, dung, and stable aggregates bound into the soil, and even biochar. Converting grassland or woodland into cropland, draining peaty soils or allowing soil erosion can all release tons of soil carbon per hectare.

Some management practices can increase soil carbon over time – and this has wider benefits in terms of the soil’s resilience to droughts and flooding and capacity to hold nutrients. Fungi help plants extract nutrients from the soil and can protect them from pests.

Even in Scotland, where most farmed soils have relatively high carbon levels (partly because carbon cycles more slowly in our cold earth) it’s been estimated that we could lock up an extra 150-215 Mt (equivalent to four or five years of our total emissions) in our agricultural soils. 

At farm level, increasing soil carbon can also be seen as offsetting other farm emissions – especially methane from grazing cattle. Plugging even a minor gain in soil carbon into a carbon calculator can move the needle into the black. Some advocates of ‘carbon farming’ present cattle as part of the solution, producing ‘carbon positive’ beef – and there’s certainly scope globally to get millions of tonnes carbon back into degraded rangeland soils.

So since there’s already a global carbon market and plenty of firms wanting to offset their emissions, it seems a simple step to have farmers sell the extra carbon they lock up in their soil. Microsoft recently bought a slew of carbon credits in Australia, where there’s been a scheme in place for ten years, and the Biden administration has floated a plan to put billions into a carbon credit market to do likewise.

Not so fast. 

There are three main problems with cashing in on soil carbon.

First, making sure that what you buy is what you get – or more formally monitoring, reporting and verification (MRV). Carbon doesn’t just sit there in the soil waiting to be measured. It’s in constant flux with the atmosphere – the soil respiration cycle shifts many more tonnes of CO2 than all our fossil fuel emissions. Much of the carbon is held in the top 30cm, but it moves up and down, with significant amounts down to 100cm and beyond. It varies with seasons and can also vary greatly within a single field: so measuring change reliably means taking a large number of samples at a considerable depth and adjusting for any changes in soil density.

The EU’s recent technical note on carbon farming concludes that for grassland and arable farming “Costs and uncertainty of MRV for SOC undermine the cost- effectiveness of large scale result-based schemes” – in other words we’re not there yet.

Second, forever is a long time. It’s much much easier to lose soil carbon than to accumulate it – twenty years of gain can be wiped out in a couple of seasons. So schemes have to build in permanence – a commitment to keep the carbon in the ground for decades – which is not only a financial risk for current farmers but may make it harder to sell land with such long strings attached.

Third, you can only sell the family silver once. If the soil carbon is sold, increasing it no longer offsets other farm emissions – the claim to be ‘carbon neutral’ has been bought by the company or investment house which bought the credits and no longer belongs to the farm or its products.

But even if it’s early days for a soil carbon market, it’s vital that farmers keep the carbon they have and do what they can to lock up more. Cover crops, incorporating organic materials, using more diverse grass mixes and agroforestry can all help. The evidence is mixed on no till which in some cases moves the soil carbon nearer the surface but may not increase it overall. Minimising soil erosion is also essential – we’re still washing tonnes of carbon into the ditches.

By contrast the Woodland Carbon Code is well established, and it makes more sense to pull private investment into well-managed tree planting and use public investment and regulation to support farmers to maintain and increase farmland soil carbon. That’s a win-win for the farm and the planet, but it’s a long game – don’t expect results to show up in the inventory any time soon.

This article was first published in The Geographer (June’21 issue).