THE CONTRIBUTION OF ASSOCIATIVE AND SYMBIOTIC NITROGEN-FIXATION TO THE NITROGEN NUTRITION OF NON-LEGUMES

During the past 10 years estimates of N2 fixation associated with sugar cane, forage grasses, cereals and actinorhizal plants grown in soil with and without addition of inoculum have been obtained using the N-15 isotope dilution technique. These experiments are reviewed in this paper with the aim of deterimining the proportional and absolute contribution of N2 fixation to the N nutrition of nonlegumes, and its role as a source of N in agriculture. The review also identifies deficiencies in both the totality of data which are currently available and the experimental approaches used to quantify N2 fixation associated with non-legumes. Field data indicate that associative N2 fixation can potentially contribute agronomically-significant amounts of N (> 30-40 kg N ha-1 y-1) to the N nutrition of plants of importance in tropical agriculture, including sugar cane (Saccharum sp.) and forage grasses (Panicum maximum, Brachiaria sp. and Leptochloa fusca) when grown in uninoculated, N-deficient soils. Marked variations in proportions of plant N derived from the atmosphere have been measured between species or cultivars within species. Limited pot-culture data indicate that rice can benefit naturally from associative N2 fixation, and that inoculation responses due to N2 fixation can occur. Wheat can also respond to inoculation but responses do not appear to be due to associative N2 fixation. N-15 dilution studies confirm that substantial amounts of N2 can be fixed by actinorhizal plants.