CONTRIBUTION TO THE N-STATUS OF SOIL BY FREE-LIVING N-2-FIXING BACTERIA IN A LUCERNE STAND

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.