FarmTalk #17 – Identifying herbicide impacts on nitrogen fixation of legumes

Fact Sheet #17 – January 2006

Dr Elizabeth Drew, Dr Gupta Vadakattu, David Roget CSIRO Land and Water, Waite Road Urrbrae, South Australia.

The Issue
Early observations from the Mallee Sustainable Farming Project identified a lack of rotational response in cereals following legumes in the Mallee farming regions of southern Australia. A number of herbicides recommended for use in legumes (peas, vetch and lentils) have been found to reduce nodulation of plants and nitrogen (N2) fixation in field trials.

With a shift towards more sustainable broad-acre farming practices, such as reduced tillage, direct drill sowing and intensive cropping, there has been a significant increase in the use of herbicides. In Australia alone, herbicide sales doubled between 1995 and 1998 from $450 million to over $800 million. While herbicide use is recognized as a necessity for both ecologically and economically sustainable agriculture, the impact of herbicides on grain legume production and N2 fixation is of increasing concern.

How legumes increase soil nitrogen
Legumes form a symbiotic relationship with soil bacteria called rhizobia. Rhizobia form nodules on the legume roots where N2 fixation occurs. The plant provides the nodules with energy to transform atmospheric nitrogen to a plant available form (Figure 1).
When legume residues breakdown, the nitrogen is released into the soil and becomes available to the following crop.