For most species of plants to grow, reactive forms of nitrogen must be available in the soil; a shortage of reactive nitrogen is the greatest limiting factor for growth and productivity of most arable plants.
Molecular di-nitrogen is the commonest form of nitrogen, but the only organisms on the planet that can use this form of nitrogen are bacteria. They have an enzyme, nitrogenase, that can convert di-nitrogen to ammonia, which can then be used in biological processes.
Legumes, such as peas and beans, get their nitrogen from a beneficial association with nitrogen-fixing bacteria and, therefore, do not need nitrogen fertiliser to be added to the soil. The plants form nodules on their roots that house the bacteria and provide the oxygen-regulated environment needed for nitrogen fixation to occur. It is a mutually beneficial process; the plant supports bacterial growth by delivering sugars from photosynthesis, and the bacteria supports plant growth by delivering ammonia.
Bacterial nitrogen fixation is the predominant natural way that reactive forms of nitrogen enter the environment. The nitrogenase enzyme is extremely sensitive to oxygen and can only function in an environment where oxygen is highly regulated, limiting the situations in which biological nitrogen fixation can occur.
To enable crops to utilise the full potential of fixed nitrogen, they also need to be able to access phosphorus and water. The ENSA team is working with mycorrhizal fungi, which form highly branched structures in root cells, called arbuscules, where the fungus delivers phosphorus, water and micronutrients to the plant. Mycorrhizal associations can form in most crop plants, but few crops fully benefit from this association in agriculture. This association can be improved to help deliver phosphorus, nitrogen, and water from the soil, as well as increasing resistance to plant diseases, and tolerances to many stresses, including drought and salt.