Biological Nitrogen Fixation by Pulse Crops

Leguminous pulse crops improve soil fertility by supporting bacteria that convert atmospheric nitrogen into a form plants can use. This process, called biological nitrogen fixation, occurs mainly through a mutually beneficial association between pulse roots and Rhizobium bacteria in specialised root nodules.

How nitrogen fixation works

Although nitrogen gas is abundant in the atmosphere, plants cannot absorb it directly. In the root nodules of pulses, bacteria make this nitrogen available through an enzyme-driven process.

• Compatible Rhizobium bacteria infect the roots and stimulate nodule formation; the plant supplies carbohydrates that support bacterial activity.
• The bacterial enzyme nitrogenase converts atmospheric nitrogen into ammonia using energy; the plant incorporates this nitrogen into amino acids and other nitrogen-containing compounds.
• Nitrogenase is sensitive to oxygen. Leghaemoglobin, a pigment in nodules, helps maintain the low free-oxygen conditions needed for fixation while supporting respiration.

How this improves soil fertility

Fixation brings additional nitrogen into the soil-plant system rather than merely drawing on existing soil nitrogen. Its benefit to subsequent crops depends substantially on how much nitrogen remains in the field after harvest.

• By obtaining part of their nitrogen from the atmosphere, pulses can reduce their dependence on soil nitrogen and externally supplied nitrogen fertilisers.
• Nitrogen contained in roots, nodules and retained crop residues enters the soil organic matter pool. Microbial decomposition and mineralisation release part of it into forms that later crops can absorb.
• Including pulses in crop rotations, such as cereal-pulse sequences, can therefore improve nitrogen availability for a succeeding cereal.
• Retaining pulse residues also adds organic material, which supports soil biological activity and helps maintain soil structure.

Conditions and limits of the benefit

Growing a pulse crop does not automatically produce a net gain in soil nitrogen. The outcome depends on successful fixation, crop growth and the removal or retention of plant material.

• Effective fixation requires compatible bacteria and suitable soil moisture, aeration and nutrient availability; Rhizobium inoculation can help where effective bacteria are insufficient.
• Harvested grain removes nitrogen from the field, and removing residues further reduces the amount returned to the soil.
• Pulses are not equivalent to green-manure crops incorporated into the soil, because a substantial part of their biomass may be harvested.
• Nitrogen fixation does not supply phosphorus, potassium or other nutrients, so balanced nutrient management remains necessary.