Cover crops are often promoted for their benefits to soil health, water quality and erosion control, but what role can they play in reducing the carbon footprint of arable systems? CPM explores the latest ADAS research.
“Cover crops must be viewed as part of the whole farming system.” PETE BERRY
By Charlotte Cunningham
Cover crops have become an increasingly familiar feature of UK arable rotations, valued for protecting soils, improving structure and retaining nutrients over winter. But as carbon accounting becomes an increasing priority for growers and supply chains, attention is also turning to their role in reducing greenhouse gas emissions.
According to ADAS crop physiology consultant, Ben Hague, understanding where emissions originate within crop production is key. “Nitrous oxide and carbon dioxide are the main drivers of greenhouse gas emissions associated with crop production,” he explains. “Nitrous oxide has a warming impact around 265 times greater than carbon dioxide, so reducing nitrogen losses is a major opportunity for reducing the carbon footprint of arable crops.”
Data from ADAS’ YEN Zero network highlights the challenge, with Ben noting that carbon footprints range from around 750kg CO₂e/ha for pulse crops to more than 2500kg CO₂e/ha for high nitrogen-demanding crops such as winter wheat and winter oilseed rape – with nitrogen fertiliser accounting for much of those emissions. “As such, reducing the nitrogen input within crops is the big lever to reduce the overall carbon footprint.”
Cover crops can contribute by capturing nitrogen that might otherwise be lost through leaching, and according to ADAS’ head of crop physiology Pete Berry, this is where their greatest carbon benefit lies. “The majority of the benefit is going to come from reducing fertiliser use, and that’s the main route for reducing carbon emissions,” he explains.
ADAS research has shown a well-established cereal cover crop can typically take up around 45kgN/ha, while brassica and legume mixtures may capture as much as 90kgN/ha. However, success depends heavily on establishment timing. “We’re really looking at establishing before early September to take up these levels of nitrogen,” says Pete. “Anything later than mid-September significantly reduces nitrogen uptake and the potential benefits.”
The effect on nitrate leaching can also be dramatic, explains Pete. In one ADAS trial on a light, freely draining soil, nitrate losses from uncropped stubbles reached 90kgN/ha over winter. Where cover crops achieved 80-90% ground cover, losses fell to less than 10kgN/ha.
Another study found cover crops retained 70-80kgN/ha within crop biomass by spring, protecting nutrients until they could potentially be used by the following crop. “The cover crop has protected that nitrogen against nitrate leaching,” comments Pete.
Current UK guidance suggests a successful cover crop can justify reducing subsequent fertiliser applications by around 30kgN/ha. However, Pete stresses that this depends on mineralisation, with nitrogen release influenced by several environmental and management factors. “One of the most important things growers can do to maximise the chance of that nitrogen becoming available is to destroy the cover crop quite early,” he explains. “By quite early, we’ll be destroying it in January or February.”
Termination method also matters, with ADAS’ research suggesting herbicide destruction can improve nitrogen availability compared with some mechanical approaches. “We actually found that using glyphosate to destroy the crop is more effective at enabling the release of the nitrogen from the cover crop for the crop following it,” notes Pete.
Of course, cover crops aren’t without their own emissions costs. ADAS estimates greenhouse gas emissions associated with establishment and destruction range from around 20kg CO₂e/ha to more than 160kg CO₂e/ha, depending largely on species choice and management. “If you were simply broadcasting mustard seed and then using glyphosate herbicide to destroy it, then you’d be at the lower end of the range,” says Pete. “If you were using larger-seeded species such as peas or beans and mechanically destroying the crop by disking or chopping it, then you’re looking at more than 160kg CO₂e/ha.”
Even so, he argues these emissions remain relatively small when compared with the 700-2500kg CO₂e/ha carbon footprints typically associated with cash crops – particularly where cover crops allow nitrogen fertiliser savings.
However, cover crops don’t automatically improve following crop performance, and ADAS trials have recorded both yield increases and reductions, with management proving the deciding factor. “Where we’ve seen a yield reduction following a cover crop, it’s been largely due to late destruction, often using mechanical methods in wet conditions,” says Pete.
Heavy soils can be particularly challenging, as large amounts of residue combined with wet weather can delay drilling, encourage slugs and compromise crop establishment, he warns. “It’s especially important in these cases to have a decent long window between destruction of the cover crop and establishment of the following crop.”
Mixed farms can also gain additional value by grazing cover crops, although this also requires careful management. ADAS has observed increased nitrate leaching where crops are grazed too tightly or too early, as nitrogen captured within the crop is rapidly returned to the soil through dung and urine. “If the grazing occurs quite early on in the winter, then we’ve actually seen quite a lot of nitrate leaching,” comments Pete.
To minimise losses, ADAS recommends grazing as late as practical while retaining at least 20% ground cover. “Our advice is that if you’re grazing and you want to minimise those nitrate leaching losses, then you should graze as late as possible in winter and also try to leave a bit of crop cover,” he adds.
While reducing carbon emissions is becoming an increasingly important objective for growers, Pete believes it should be viewed as one of several benefits associated with cover crops rather than the sole driver for adoption.
Reduced nitrate losses, improved soil protection, enhanced biodiversity and opportunities for winter grazing all contribute value, while many of the wider soil benefits become more apparent over time. “Cover crops must be viewed as part of the whole farming system,” he says. “When they’re managed well, the benefits go far beyond carbon alone.”
This article was taken from the latest issue of CPM. Read the article in full here.
For more articles like this, subscribe here.
Sign up for Crop Production Magazine’s FREE e-newsletter here.
