Results published on how cover crops can refine nitrogen use and protect water quality
Study confirms that cover crops reduce nitrate leaching by up to 90%, with destruction by chemicals proving to be most effective for nitrogen availability to the following crop and for reducing weed burden.
The Nitrogen release from Cover Crops (NiCCs) project, funded by Affinity Water and Portsmouth Water and delivered by ADAS, evaluated the effect that cover crop species choice and different destruction methods had on nitrogen release into the following crop and beyond.
Trials were undertaken over two cropping seasons on commercial farms in Hertfordshire and West Sussex with two cover mixes and a weedy stubble control. The two cover crop mixes were:
- Mix 1: Phacelia (20%) & Oil Radish (80%) @ 15 kg/ha
- Mix 2: Japanese oats (45%), Buckwheat (45%) & Phacelia (10%) @ 10 kg/ha
The cover crops were then destroyed either mechanically by rolling on a frost, chopping and incorporating, or with chemicals.
Results
The results identified that cover crops reduced nitrate leaching losses by up to 90% when compared to the weedy stubble control. Depending on how well the cover crops established and the species mix, soil nitrogen supply to the following spring cereal crop also increased by up to 35 kg N/ha.
When it came to the subsequent spring cereal crop, yields following a cover crop were up by 0.2-1.0 t/ha when compared to yields following the weedy stubble control. On destruction, the cover crops released significantly more Nitrogen than the no cover/weedy stubble.
Moving to destruction methods, chemicals was found to have more benefits than mechanical. N mineral release, grain N offtake, and total crop N uptake were all higher with chemcial destruction. This lead to subsequent improvements in yield, and in the case of spring oats, grain weight.
The study did acknowledge that, without financial incentives, it was marginally more cost-efficient for farmers to not grow a cover crop and destroy weeds chemically. It emphasised that incentive schemes should support cover crop use so ‘harder to monetise’ benefits such as improved water quality, soil health and biodiversity can be realised.
Dr Anne Bhogal, Principal Soil Scientist, and project lead from ADAS said:
“Cover crops can capture significant quantities of nitrogen over winter, thereby protecting surface and ground waters. It has been great working with Affinity and Portsmouth water on this project which has given us new insights on how cover crop destruction method can impact the subsequent release of this nitrogen and the following crop performance.”
Simon Deacon, Catchment & Environment Manager from Portsmouth Water agreed:
“Cover crops have been proven to reduce nitrate leaching in to groundwater by up to 90% and therefore help toward improving water quality and supply of drinking water. This trial provides additional information that help toward improving cover crop management and improve water quality.”
Danny Coffey, Catchment Manager at Affinity Water added:
“We are working with farmers to find sustainable solutions which work for both crop and water production. The NiCCs trial has shown that cover crops are a vital measure for preventing nitrate leaching and demonstrate how farmers can potentially manage the destruction of cover crops to best utilise fixed nitrogen for subsequent crops. The data gathered through the NiCCs trial will feed into our schemes and guidance for farmers to encourage the best outcomes for both crops and water across catchments.”
Background to the NICCS project
Farmers can grow cover crops to ‘protect or improve’ soils between periods of regular crop production. They can be effective at improving soil functions by:
- increasing soil nutrient and water retention
- improving soil structure/quality
- reducing the risk of soil erosion, surface run-off and sediment,
- reducing nitrate and phosphorus losses to water by providing overwinter soil cover
Agriculture is responsible for an estimated 60% of nitrate losses to water. Some groundwater aquifers in the UK have high nitrate concentrations as a result of current and historic land-use and localised geology. Chalk aquifers, which both Affinity Water and Portsmouth Water rely on for public water supply, are particularly vulnerable to nitrate leaching. By working with farmers in vulnerable groundwater catchments both hope to mitigate diffuse nitrate leaching in their regions.