Nitrogen mineralisation from cover crops

Mineralisation is influenced by many factors and affects how quickly nitrogen becomes available to crops. However, it is fastest in warm and moist soils.

Depending on the species and biomass content, destruction method and timing, 10–100 kg N/ha may be released in the first year of cash cropping following a cover crop. There could also be a residual benefit to the second-year cash crop.

However, in some cases (e.g. with rye), cover crops deplete available soil nitrogen and reduce the supply to the following crop.

The AHDB Nutrient Management Guide (RB209) provides limited guidance on how to account for nutrients from cover crops.

It states:

“Following destruction of the cover crop, [captured] nitrogen will be gradually mineralised over many years. However, the amount becoming available for uptake by the next crop is relatively small and difficult to predict. Where cover crops have been used regularly, a soil analysis can be a useful technique to help estimate the overall supply of soil mineral nitrogen.”

A summary of RB209 guidance (2023 edition)

Sustained use of cover crops can:

• Build soil fertility
• Reduce reliance on purchased inputs
• Improve soil organic matter content

Well-established winter cover crops can take up significant amounts of soil mineral nitrogen and reduce the risk of nitrate leaching over winter.

Generally, the earlier the cover crop is established, the greater the nitrogen uptake.

After destruction, cover crops can release useful quantities of nitrogen for the following spring cash crop. This may be sufficient to increase the soil nitrogen supply (SNS) estimates by up to two indices.

Only increase SNS if you are confident that the:

• Previous crop received the recommended rate of nitrogen
• Previous crop grew (and took up nitrogen) as expected*
• Cover crop established and grew well
• Cover crop was destroyed by the end of February**
• Nitrogen released was not at risk of leaching from the field

If nitrogen residues are likely to be lower than expected, it may be necessary to decrease SNS estimates.

Notes

*Applying starter nitrogen fertiliser to cover crops does not produce consistent establishment benefits, based on current evidence.

**Cover crops destroyed after February are associated with greater variability in the amount and timing of nitrogen release. For example, such crops are associated with a higher carbon-to-nitrogen ratio, which is likely to delay mineralisation.

Short-season cash crops reduce the opportunity for mineralised nitrogen to be taken up.

AHDB has commissioned research to improve information on nutrient release.

Carbon to nitrogen ratios

Besides climatic conditions, the carbon to nitrogen (C:N) ratio of residues is the main factor influencing mineralisation.

Vetch (a legume) has a high nitrogen content (3–4% of its dry weight) and is readily mineralised. It proliferates in the spring. In one trial, vetch accumulated 98 kg N/ha in late March, 132 kg/ha in mid-April and 206 kg/ha by early May.

Non-legumes (e.g. grazing rye or mustard) can add as much total nitrogen to the soil as legumes. However, nitrate availability is slower. This is because they have a lower nitrogen content (1–2%) and a higher C:N ratio.

Carbon form

The carbon form is also important. For example, lignin (found in woody materials) is more resistant to decomposition than cellulose.

Some plants also contain chemicals (e.g. polyphenols) that can inhibit microbial action.

Timing of incorporation

The nitrogen concentration of many non-legumes usually decreases as plants prepare to set seed. The timing of incorporation is, therefore, crucial.

An overwintered cover crop is more likely to supply nitrogen to the following crop if it is destroyed when it is still relatively green and fresh.

Rye incorporated at the end of April will release nitrogen into the soil, although not as much as vetch. If left until May, soil organisms will use any nitrogen available to break down the rye, effectively competing with plants for soil nitrate to break down the residues. This is sometimes called ‘nitrogen robbery’.