The problem

High inputs of chemical fertilisers, particularly containing nitrogen and phosphorus compounds, have increased crop yields dramatically over the past fifty years. However, it has been clear for a number of years that intensive arable cultivation of this kind is no longer sustainable.

Firstly, nitrogen fertilisers contribute to a number of environmental problems. Run-off from agricultural land causes pollution of rivers and lakes, and nitrate levels must be carefully monitored to ensure the quality of drinking water. Nitrogen oxides which are emitted during the application of nitrogen fertilisers are implicated in poor air quality at ground level and also nitrous oxide is a highly potent greenhouse gas. Furthermore, the production of chemical fertiliser are produced from natural gas or oil, and supplies of these commodities will become increasingly more expensive in the coming decades.

Plant growth promoting rhizobacteria

The European Union is aiming through its Framework 6 programme to support research projects which pave the way for reductions in inputs of chemical fertiliser. One approach, to be investigated in RHIBAC, is to introduce plant growth promoting rhizobacteria (PGPR) to the root systems of crops. Unlike symbiotic Rhizobia which invade the plant tissues of legumes, PGPR have a more associative interaction with the plant. The best known example is Azospirillum brasilense, which has been isolated in sugar cane in Brazil, and is thought to be responsible for the low requirement of this crop for applied fertilisers.

Like Rhizobia, Azospirillum and other PGPR fix nitrogen from the air, but the plant growth promotion effect is thought to be due mainly to other factors, including excretion of plant hormones which increase the efficiency of crop root systems, and increased mobilisation of soil nutrients.

For over 30 years, researchers have been investigating the application of PGPR such as Azospirillum in inocula to promote the growth of key arable crops such as wheat and maize. Results of greenhouse and field trials have been variable, and much remains to understand concerning the plant-microbe interactions.