Contribution to the N status of soil by free-living N2-fixing bacteria in a lucerne stand

Asymbiotic N2 fixation by free-living bacteria was determined as part of an experiment examining potential sources of N for irrigated lucerne. A most probable number count indicated that asymbiotic N2-fixing bacteria occurred in the soil profile below lucerne, particularly in the root region. In situ C2H2 reduction assays to measure nitrogenase activity in the field indicated only small amounts of N2 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 potential for asymbiotic N2 fixation, because of falling temperatures late in the growing season (normally from early September to the end of April) and an inability of the assay to measure nitrogenase activity at depth in the soil profile. Nitrogenase activity by free-living bacteria amounted to between 3–10% of the activity of whole root systems (asymbiotic+nodules). Based on a C2H2-to-N2 ratio of 3:1, this represented 3–10 kg N ha−1 in the 1989–1990 growing season in an established crop of lucerne.