The Science of Soil Carbon

You probably know the Dovu mission: to heal the planet through soil carbon capture powered by a global community of farmers and crypto fans. But what about the science? We decided it was time to get down and dirty, and set out the facts.

So, we enlisted the help of Dr Luke Ridley, a UK-based expert in biogeochemistry and all matters soil, land and climate change, to explain it all from the ground up.

DOVU: What is soil carbon?

Dr Ridley: When people talk about soil carbon, they’re referring to carbon which is locked up in soil organic matter. Soil is made up of an incredibly diverse mix of organic compounds, minerals, biota (living organisms from tiny bacteria to larger animals like worms, ants and moles) and also contains pores or gaps which hold either water or air. 

DOVU: How does carbon enter soils?

Dr Ridley: Carbon gets into soils via organic matter from the soil surface. When plants grow, they capture carbon via photosynthesis, using sunlight to convert it into a range of organic compounds such as sugars and starches, which are then used as building materials by the plant. When the plants die, that organic matter is left on the soil surface and as it decomposes it becomes part of the top layer of the soil (the organic horizon). The movement of earthworms and other processes in healthy soils help to distribute the organic matter deeper into soils. Soil carbon can also enter soils through the roots of plants, and through dung and manure from grazing animals. 

DOVU: What happens to soil carbon with poor land management? 

Dr Ridley: Poor land management can lead to increased risk of erosion, either by water running over bare soils and washing them away, or through wind blowing away fine dry soil particles. When the top layer of soil is lost, the carbon that it was holding is also lost from the soil profile. 

Peatlands are another special case, where the water-logged soils hold huge amounts of organic carbon. When these peat soils are drained for agriculture, oxygen can get to the organic matter which has been stored away for thousands of years, and microbes within the soil quickly start to use the carbon as an energy source, releasing the trapped carbon back to the atmosphere as carbon dioxide or methane. 

DOVU: How does soil carbon help farmers?

Dr Ridley: Having high levels of organic matter in soil can deliver a range of benefits to farmers including better water retention so soils don’t dry out so quickly and plants don’t suffer as much in dry periods. Soils which have more organic carbon may also hold onto nutrients better so that farmers don’t have to use as much fertiliser to promote plant growth, saving money by reducing their costs and reducing runoff which can pollute nearby rivers and lakes. Soils which are depleted in organic matter can have reduced fertility making it more difficult to grow crops and can be depleted in soil biodiversity. 

DOVU: How can farmers increase soil carbon? 

Dr Ridley: The amount of carbon stored in soils controlled by the balance of inputs and outputs – carbon goes into soil through additions of organic matter (for example, crop residues after harvesting) and carbon leaves soils when it’s broken down by microbes and bacteria in the drained soil profile, and through loss by erosion. Farmers can increase soil carbon through changing both parts of that balance: by increasing carbon inputs and decreasing losses. Simple things like using cover crops (crops which aren’t harvested for food, but are there to protect bare soil over winter), adding the right amount of organic manures or crop residues and reducing the amount that soils are dug and turned over during cultivation (minimum-tillage) can have significant benefits for soil carbon levels. 

DOVU: How does this help fight climate change? 

Dr Ridley: Where soils aren’t managed well, they can be sources of carbon dioxide to the atmosphere. On the other hand, a sustainably managed soil can sequester carbon from the atmosphere, taking carbon captured by plants during photosynthesis and locking this away, thereby reducing the amount of greenhouse gases in the atmosphere. We need to both decrease the emissions of greenhouse gases (from things like burning fossil fuels) and to increase the amount of carbon that the land is capturing, through tree planting, peatland restoration and increasing soil carbon. 

DOVU: How do you measure how much carbon is in soils and how much it can store?

Dr Ridley: Soil carbon levels can take a long time to build up, and so changes might be seen slowly, but the good news is that measuring soil carbon isn’t particularly difficult or expensive. The carbon balance or carbon accounts of the soils on a particular farm could be measured in two general ways, and different projects may use one or a mixture of these for assessing progress and validating success: 

• Soil samples can be collected (sometimes from different depths in the soil profile) and sent to labs to measure the amount of organic carbon through a technique called “loss on ignition”. This gives you a cheap and accurate idea of how much carbon was in that particular soil sample, but it can be more difficult to spot trends which may take a long time and require lots of samples as soil carbon will vary from place to place over one field
• Carbon fluxes in and out of a field can be measured either continuously through expensive monitoring equipment such as eddy covariance towers or through using static chambers and gas analysers. These scientific techniques let you understand the carbon moving into and out of the soils under different climate and weather conditions, and build up a picture of the carbon balance much more quickly than through traditional sampling, but they can be expensive and labour intensive.  

DOVU: Are there plants or crops that are particularly good at accelerating carbon capture?

Dr Ridley: As mentioned previously, it can take a long time to see significant increases in soil carbon (but the total amount stored can also be very large). Although there isn’t one answer for all soils and all farms, there are a range of management practices which farmers can use to increase soil carbon, whether they grow crops or have grazing systems. Planting leguminous crops such as clover and beans as either cover crops or part of a crop rotation may deliver both soil carbon benefits and additional nitrogen capture, increasing the levels of nutrients in the soil without the need for additional fertilisers. 

So there you go. We’re all now a little bit wiser and suitably informed. The answer’s in the soil. You just have to dig deep (figuratively speaking).

Look out for more of these DOVU Ask The Expert blogposts over the coming weeks and months. Action happens through inspiration and education. Let’s heal the planet.