Eleven Facts about Soil Carbon with Professor Pete Smith

1. Soil carbon is only one aspect of soil health

Soil health refers to soil’s ability to support the ecosystem services and functions that it’s meant to provide, and the ability to continue to provide those into the future. To get a full picture of soil health, you need biological, chemical and physical indicators, at least one of each but preferably more. A key measure of soil chemistry is the soil organic matter content, which is made up of carbon, nitrogen, oxygen, and a few other things.

Generally speaking, soil organic matter contains about 58% carbon, so if you measure the carbon, you have a good idea of the organic matter content, which is a key aspect of soil chemistry, which itself is just one part of measuring soil health.

So soil carbon is certainly a headline indicator of soil health, but it isn’t the only thing that you need to consider- you also have to think about other components – does it support microorganisms and invertebrates? Does it also have a good structure?

To answer these questions you need to take a few measures together- for example consider a physical measure of soil structure, soil organic matter as the chemical, and for example, the presence or absence of earthworms as a simple biological indicator. These three aspects are generally measured in the top 15 or 30cm, looking at them together will probably give you a good idea of whether you’ve got healthy topsoil.

2. How you measure soil depends on what you want to know

Most farmers’ primary focus is on soil fertility, even more than soil health. They tend to look at pH and how much nitrogen, potassium, and phosphorus (NPK) there is in the soil, because that affects what you can grow, it tells the farmer which nutrients to add, and crucially that affects yield and their bottom line.

Meanwhile, collecting more detailed data on biological, chemical and physical measures to give an indication of overall soil health is valuable for a farmer or scientist who’s more interested in the long-term resilience of soil. If they want to get into carbon markets on the other hand, then they need only send off some samples to a lab to get a measure of soil carbon.

The old way of measuring carbon is called loss On Ignition (LOI) – you weigh your soil, burn it at a very high temperature, weigh it again and assume anything burned off is organic matter. More recently we’ve got access to dry combustion analysers, which is the way that most commercial labs will do it these days. These are machines that tell you the carbon and nitrogen content directly.

You need to dry the soil, grind it up and the machine tells you your percentage carbon. So that’s the way that most commercial labs do it these days – they’ll measure it with one of these carbon and nitrogen (C&N) analysers.

So, we’re in a situation where farmers have been thinking in terms of soil fertility, climate change researchers have been thinking in terms of soil carbon, and ecologists have been thinking in terms of soil health. They’re all related, but they’re not the same concept.

3. Models and protocols help understand changes in soil carbon

Because it takes so long to measure changes in soil carbon, we also use models that simulate changes into the future, which are useful if you want to show that a farmer or land manager is doing something beneficial to the soil. They can help you work out your trajectory, and they’re good for experimenting with different management interventions. Because they have built into them the impacts of ground cover, tillage practice, different crop types, or fertiliser type, you can model what would happen to your soil carbon if you did each one of these interventions.

Models that are developed for researchers are slightly different from farm level calculators; they are based on inputs of meteorological data, and simulate the turnover of carbon, how much is being lost to CO₂, and how much is being locked up longer term in the soil by becoming less decomposable, because the carbon is stuck to minerals or locked up is soil aggregates.

You should always bear in mind that models aren’t a replacement for measurements. They’re only as good as the measurements you use to initialise and parameterise them, but they can help you work out your trajectory.

Then you have different protocols, mostly linked to the carbon market, that take soil tests and models to demonstrate change and allow farmers to be paid for sequestration. And that’s just for soil carbon- there are protocols for measuring soil health which use simple biological, chemical and physical indicators for assessing the change over time.

As discussed, soil carbon is best thought about in the context of soil health. It would be really useful to have standard protocols for measuring soil health which use simple indicators for assessing change over time. With soil carbon, we are just talking about one element – carbon in the soil. With soil health, we’re talking about a whole range of physical, chemical and biological changes that can occur in soil. In terms of modelling, you’d need a slightly more complex model, one that would also respond to land use change and land management change in the same way as the carbon models do, and account for soil type. I think that’s a good challenge for the research community to come up with that.

4. Soil carbon levels and potential for sequestration vary across the UK

Cropland soils have lost between 40 and 60% of the original carbon they had as natural, native ecosystems (forestry systems, woodlands, and grasslands). Most of this change occurs over the first few decades. Current grassland and woodland soils tend to be closer to the natural system as they’ve generally had permanent ground cover, meaning things are growing there all year round. A lot of the soil carbon decline in croplands is because we’ve been constantly ploughing crop lands, without leaving them fallow, and constantly taking away biomass as grain or straw.

Targeting to soil type is critical, and we can further target by understanding which land uses offer the greatest potential for soil carbon. Here there are three main considerations:

Firstly, we must preserve what we already have: Some land use types already have very large carbon stocks. These include peatlands, forests and natural grasslands. The best thing we can do here is to protect them – leave them alone, not plough them up and not convert them to cropland or attempt to get other uses out of them. So, this should absolutely be the primary focus: let’s protect what we’ve got first, because it’s much easier to protect what you’ve got and stop it being lost than it is to try and scramble to get it all back again.

The second is restore, especially where you’ve got a degraded system that ought naturally to store significant soil carbon. So, if you’ve got a degraded woodland, or you’ve got a degraded peatland – and 80% of UK peatlands are degraded to some extent! – that’s where we can and should restore soil carbon. And critically here we can immediately switch off the very large, ongoing losses of carbon from those soils, because degraded soils that would be naturally high in soil carbon will be releasing that carbon back into the atmosphere. And if we focus efforts on restoration, we can eventually convert these back to storing more carbon again.

And thirdly, being targeted about efforts to increase farm soil carbon: Because the UK has such a vast area of grazing land, a lot of the attention for soil carbon sequestration has been focused on the grasslands. But grasslands tend already to be fairly high in soil carbon stock, or even close equilibrium point (where carbon inputs equal carbon outputs), meaning there may not be very much capacity to increase soil carbon in the grasslands. Cropland soils however are the ones with a really large potential for soil carbon increases, which may be achieved through a change in management, because they’ve lost so much. So, it’s croplands – especially arable systems, that should be the focus of our attention for increasing carbon in the soil. It won’t be easy, but that is where the potential is, so that is where we should focus.

5. More soil carbon is generally better for soil health

Things that increase soil organic matter tend to improve soil health as well, although there are a few exceptions to this rule. For example, if you put contaminated sludge onto the soil, you’re adding organic matter which increases carbon content, but it would also suppress all your microbial activity, and so the carbon wouldn’t be broken down. If you were just relying on carbon as an indicator of soil health, it would look good, but by destroying your biological component you’ve destroyed the soil health at the same time.

If you want to increase soil carbon and improve soil health, there are many practices that help both:

keeping the ground covered, having living roots in the system, trying to add back as much organic matter as you can, keeping the ground covered as much as you can. When you’re taking grain away, that’s carbon and nutrients that you’re taking away from the system, so trying to put back composts, municipal wastes, and other forms of mulches and such like, are a great way of getting carbon back into the soil.

Also important is trying to disturb soil as little as possible. So, reducing the intensity of the tillage will generally increase your soil organic carbon, because you’re not breaking up the aggregates. And the more you leave those intact, then the more carbon you leave locked away from the microbes that break it down.

6. You can lose soil carbon as well as build it

The good thing about soil carbon is that you can increase it, but the bad thing about soil carbon is that it’s very easily reversed. So, if a land manager were to stop good practices (covering the soil, keeping living roots in there, returning organic matter to the soil), carbon starts running down again. And, in fact, you can often lose it more quickly than you can gain it. So, if you’re going to engage on this process of improving your soil management, it has to be a permanent thing, and you have to persist into the future.

This is particularly relevant to ploughing- the top 30cm of the soil usually holds around half of the total carbon down to a metre, which makes it more vulnerable to loss. Once ploughed, if you have wind or water erosion, or if you have fires that sweep through and burn the surface you can rapidly lose your carbon. Add to that climate change- we are expecting a greater frequency and magnitude of extreme events- large storms, rainfall events, floods, all of which can threaten future soil carbon stocks; indeed all biological and carbon stocks are at greater risk of being lost as the climate changes.

If all that carbon is then going up into the atmosphere, either through increased oxidation, increased decomposition, or if it’s eroded, and doesn’t get trapped in sediment, if it’s then mixed up with stream water and then lost as carbon dioxide, it’s gone back into the atmosphere- and that contributes to climate change.

So all the effort a land manager makes to put carbon into the soil over 20 years could be lost in 1 extreme event. That’s the reason why increasing carbon is great, it’s got many co-benefits, but we shouldn’t rely on it as a solution to climate change- we shouldn’t put all our eggs in one basket. That’s why we have to make emissions reduction across sectors of the economy first, then think about soil carbon as adding that little bit, giving the carbon sink can offset some of the remaining emissions that we have in the system when we have reduced emissions as much as we can.

7. Carbon isn’t the only greenhouse gas important in soil systems

Building soil carbon by increasing organic matter will generally make soil healthier, but that doesn’t necessarily mean that you impact emissions too much. There are two other main greenhouse gases that are emitted from soils- nitrous oxide, which is about 300 times more potent than carbon dioxide, and methane, which is 25 to 30 times more potent than carbon dioxide when compared for their impact on the climate over a 100-year time frame.

It wouldn’t be useful from a climate perspective to increase the soil carbon if you were inadvertently increasing emissions of these other greenhouse gases. For example, adding more fertiliser to soils to increase the amount of biomass growth could increase the amount of soil organic carbon, but you would also be increasing nitrous oxide emissions from adding the nitrogen fertiliser.

Likewise, adding ruminant animals into your system because you’re trying to increase the amount of manure (i.e. organic matter) going onto the soil, you may be increasing soil carbon levels, but you blow all the benefits out of the water by the additional methane emissions from the ruminants.

So, it’s really important to take a full greenhouse gas balance, for whatever intervention you’re planning, to make sure that you’re not doing something that’s going to increase either the methane emissions or the nitrous oxide emissions, which could totally negate any net zero benefit that you get in terms of increased soil carbon.

8. It might be more effective to focus on soil health instead of soil carbon

I think that you can either look at these challenges from a carbon perspective, and everything else is the co-benefit, or you can take it from the other point of view – the farmers point of view. They are thinking ‘if we’ve got healthy soils, we’ve got higher productivity, more fertile soils, more resilient soils to pass on to the next generation of farmers, that’s perhaps more a useful way of looking at it. Then by having soil health as your priority, the co-benefit is that you get some carbon sequestration.

If you’re increasing your soil carbon, then generally you’re making things better, but I think there’s a danger of over focusing on soil carbon, because it’s all linked up with the carbon markets, and the question of trying to make money from it. The danger of going down that route is that you forget the real reason that you’re doing it, which is to improve soil health.

9. It matters what depth you measure to

It is important to consider the depth at which you’re measuring to gauge a true reflection of soil carbon levels.

The majority of soil carbon is stored in the first 30cm of soil, which is where most plant roots grow and is where most of the action is. Measuring deeper is backbreaking work, and it’s therefore much less well known how much carbon is stored in subsoils.

However, as carbon levels are more stable lower down, and less susceptible to loss through ploughing, wind, flooding etc. it is important to think about what’s going on deeper down too. If all efforts are focused on improving soil carbon levels in the top 30 centimetres, it will take centuries for any improvement to be made to the soil further down – which will be too late for even the least ambitious net zero goals. It is possible to get carbon into deeper soil, for example through deep rooted plants, or crops with more recalcitrant roots which decompose less quickly.

10. Soil type is an influencing factor for carbon storage in soils

It’s very important to understand your soils to start with so you can be realistic about what you’re looking to achieve.

There are many different soil types across the country and different soil types are capable of storing different levels of carbon. Consider soil type and understanding optimal thresholds for carbon content for different soils is helpful when thinking about how much – or where – you can increase soil carbon.

The UK has very good datasets for soil across the devolved nations including good soil-type maps. Soil maps give information about the organic matter content, bulk density and clay, and classify these characteristics into soil types. You can interrogate these databases for this soil type and land use to understand the gap that you could potentially fill by improving the management to increase soil carbon.

At the individual farm or site level, you could just take a sample in the cropland and in the headlands or hedge that has not been cropped. This will tell you for that same soil type, how much carbon you would expect was in there before you started ploughing it up. And that gives you a good idea of how much you could potentially get back in.

11. More evidence and research into soil carbon is needed

We still need better systems for monitoring, reporting and verification, beyond just for the carbon markets. We’ve got all the components: good measurements of soil carbon, great databases, repeat samples in certain places,  good models in the research community, good on farm data that the farmers collect, but nobody’s ever really brought all this together. If we were able to bring all those strands together, from the farm level up to the national level, we’d have a great opportunity to really get a good platform to monitor, report and verify the soil carbon- that would be a game changer. That would allow individual farmers to plug in their location, plug in what they’re doing, and all the other underlying data would be in the system.

And then for the voluntary carbon markets that would provide something that could give confidence to investors that the soil carbon is changing, and is being sequestered in a sustainable way, as well as people looking at the national inventory. We don’t have that yet, and it will require a little investment, but it’s not miles away because I think we have the individual components, and farmers are meanwhile desperately asking for information.

Ultimately you need harmonisation, because you’ve got different ways of measuring, for example different depths to measure, between the systems and the different protocols. So, the first thing we need to do is look into translation factors that allow you to convert one measurement system to another, in a way that everyone can understand. That’s what I’m hoping we can start to do, as part of the soil TAG under the LUNZ Hub.