Rhizobium and Nitrogen Fixation
New Zealand Agriculture
Biological nitrogen fixation is of
particular importance to New Zealand agriculture, providing 1 million
tonnes of nitrogen annually. Compared to the 26,373 tonnes of
nitrogenous fertiliser used by New Zealand farmers this is more than
97% our annual requirements. Although this process is free,
self-sustaining and non-polluting, it does not necessarily operate with
Partners for Life
Nitrogen fixation is the process by which
atmospheric nitrogen gas is made available for incorporation into
organic compounds. Only certain bacteria are capable of carrying out
this process, the genus Rhizobium being the most common.
Members of this genus are Gram negative aerobic rods that occur
free-living in soil or as micro-symbionts in root
nodules of leguminous plants.
Rhizobia in root nodules are estimated to
carry out 50-70% of the world biological nitrogen fixation, reducing
approximately 20 million tonnes of atmospheric nitrogen to ammonia. It
has been demonstrated that one difficulty with relying on natural
populations of Rhizobium is that they can transfer nitrogen fixation genes to
other soil bacteria such as Caulobacter. These bacteria then
compete for host plants but may not in fact be able to fix nitrogen at
The process of biological nitrogen fixation
may be ideal for a medium to long term study with a senior class. There
are opportunities to demonstrate some of the microbiology ideas and
techniques in the Level 1 NCEA Science curriculum. Rhizobium is
also an ideal organism to demonstrate gene transfer with Year 12 and
Year 13 classes.
We have carried out some of this work with
the assistance of the L.A. Alexander Trust.