Cover crops can boost effectiveness of anaerobic digestate and reduce nitrate leaching

Cover crops could help farmers capture more nitrogen from anaerobic digestate (AD), according to the results of an Innovative Farmers field lab coordinated by Agri-Tech East. A follow-on field lab will be launched this autumn.

The farmer-led trial benefits from having the support of researchers at NIAB and Cranfield University and has already spent three years investigating how to use digestate effectively within government regulations. This included assessing whether cover crops could help improve soil health in combination with autumn applications of digestate – a by-product of anaerobic digestors.

Farmers from six farms took soil samples after separating their land into four different treatment areas – AD with cover crops, AD only, cover crops only, and a control section of neither. The soil samples were analysed at different depths across the four treatments in order to see how nitrogen levels are affected by the application of digestate and/or cover crops. Where nitrogen sinks to the lower levels of soil it potentially becomes unavailable to the next crop, and the risk of nitrate leaching into ground water increases.

The results:

Results found that often when cover crops were grown on soil after the AD was applied, the available nitrogen was significantly reduced in all levels of the soil, and more than halved in the lower levels, indicating that the cover crops had absorbed the excess nitrogen. This suggests cover crops can potentially improve soil health by increasing the effectiveness of digestate applications, reducing the risk of leaching, as well as making more nitrogen available for the farmer’s next crop.

As more trials are needed to test this theory Innovative Farmers and Agri-Tech East are launching a follow-on field lab this autumn with sponsorship from AHDB. The second phase will continue to investigate how best to stabilise nitrogen so that it is not released as GHG or leaches into water, but there is also interest from the farmers to explore how digestate affects soil bacteria.

Triallist Nick Sheppard, farm manager at Upton Suffolk Farms, said: “For me this is all about trying to increase the soil organic matter and safeguard my land for future generations. I am very encouraged by the trial – it’s a long-term journey but we’re seeing the benefits of cover crops in our soil and in our yields. Only time will tell if it saves me money, and it’s too early to tell on the organic matter levels, but we are on a fantastic learning curve and it’s going to be interesting to see what happens over the next few years.”

Researchers in the field lab are also providing training to help farmers carry out Visual Evaluation of Soil Structure (VESS). This is to see whether any of the different treatments have an impact on soil compaction, which would suggest an improvement in soil health. Worm counts and green area index measurements are also being done by triallists.

Dr Lydia Smith, Head of NIAB Innovation Farm and the Eastern Agri-Tech Innovation Hub, said: “The early results were very exciting – often the cover crops were doing exactly what we wanted in taking up that excess nitrogen from the soil. Going forward we’re keen to look more closely at the nitrogen availability and at worm counts, and to get the VESS training up to scratch so farmers can help in working out how and when is best to carry out these assessments.”

Helen Aldis, Innovative Farmers Development Manager, said: “Farmer-led research is critical to solving challenges in every day farming. It is encouraging that the farmers are continuing this field lab and getting help with training to get the best out of the trial. We’re really looking forward to seeing more results from the next phase of the research and finding out what the implications are for soil health.”

Want to get involved in next year's trial? 

If you have access to digestate, are currently growing cover crops and are interested in getting involved in this field lab please contact:

Please note: Due to funding limitations, additional farmers joining this trial will need to be self-funded. 

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