Nitrogen mineralisation from cover crops

Bacteria and fungi mineralise nitrogen in cover crop residues from an organic form to a plant-available inorganic form. Most mineralisation occurs after cover crop incorporation with the soil.

How much nitrogen is mineralised?

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 Nutrient Management Guide (RB209) 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.”

Access the Nutrient Management Guide RB209

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’.

Further information

Find out more about the biological fixation of nitrogen by rhizobia

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