Mm. Roper et al., CONTRIBUTION TO THE N-STATUS OF SOIL BY FREE-LIVING N-2-FIXING BACTERIA IN A LUCERNE STAND, Soil biology & biochemistry, 27(4-5), 1995, pp. 467-471
Asymbiotic N-2 fixation by free-living bacteria was determined as part
of an experiment examining potential sources of N for irrigated lucer
ne. A most probable number count indicated that asymbiotic N-2-fixing
bacteria occurred in the soil profile below lucerne, particularly in t
he root region. In situ C2H2 reduction assays to measure nitrogenase a
ctivity in the field indicated only small amounts of N-2 fixation (up
to 1 kg N ha(-1)) over a 35-day period between 26 February and 2 April
1990. However, it is likely that this was an underestimate of the pot
ential for asymbiotic N-2 fixation, because of falling temperatures la
te in the growing season (normally from early September to the end of
April) and an inability of the assay to measure nitrogenase activity a
t depth in the soil profile. Nitrogenase activity by free-living bacte
ria amounted to between 3-10% of the activity of whole root systems (a
symbiotic + nodules). Based on a C2H2-to-N-2 ratio of 3:1, this repres
ented 3-10 kg N ha(-1) in the 1989-1990 growing season in an establish
ed crop of lucerne.