Biological fixation of nitrogen by rhizobia

Root nodules are associated with the presence of nitrogen-fixing bacteria called rhizobia.

Many legume species develop these nodules and fix nitrogen from the atmosphere.

This nitrogen becomes available to other crops after the cover crop is incorporated in the soil and residues mineralise.

Nitrogen fixation and the residual effects of the cover crop are variable and affected by many factors, including species, cultivar, soil type, climate and management.

Key points to note

• Legume species and varieties vary in their ability to fix nitrogen, ranging from 15 to 325 kg N/ha each year
• Nitrogen fixation is most active between 7° and 20°C, meaning very little nitrogen is fixed over winter
• Nitrogen fixation is positively correlated with dry matter production
• Mixes of legumes and non-legumes can encourage greater overall nitrogen fixation and reduce nitrogen leaching compared with a straight legume cover crop
• Sufficient rhizobia must be present for the formation of effective root nodules
• If the same types of plants are regularly grown, then sufficient bacteria will usually be present
• Different bacterial species interact with different groups of legumes (e.g. clovers, lucerne and trefoils, lupins and beans)
• Visible nodules develop three to five weeks after crop emergence
• Sometimes, nitrogen fixation does not occur, even if the roots form a symbiosis with bacteria
• Some strains of rhizobia are not very effective, and some can even drain the plant of resources