Nodulation studies on legumes exotic to Australia: The influence of soil populations and inocula of Rhizobium leguminosarum bv viciae on nodulation and nitrogen fixation by field peas

Field investigations were conducted at two locations (soil pH 4.4-4.7, CaCl2, 0-10 cm) in the cereal belt of New South Wales to assist in devising strategies to maximize nitrogen fixation (N-2 fixation) by field peas (Pisum sativum L.) sown in acid soil. Counts of Rhizobium leguminosarum bv. viciae, the root-nodule bacteria for pea, already established in the soil showed that the populations at sites cropped with field peas one year earlier, 3 years earlier or never cropped with peas were 67, 9 or < 4 rhizobia per gram of soil, respectively. This confirmed previous findings that R. leguminosarum bv. viciae persisted poorly in acid soil. When field peas were appropriately inoculated, the amount of N-2 fixation, calculated with N-15 enrichment, was influenced by cultivar and time of sowing. Later sowing reduced N-2 fixation more severely than other indices of plant growth. Small populations of established rhizobia did not compete in nodule formation with the seed inocula at the sites where peas had grown 3 years before or had never grown. In contrast, at the site where peas had grown the previous year, the population of established rhizobia was large enough to compete with the inocula in forming nodules but did so less successfully against higher rates of inoculation. otherwise, increasing rates of inoculation above 65 000 rhizobia per seed did not further enhance nodulation, N-2 fixation or indices of yield. Regardless of any history of pea cultivation, it is concluded that it would be prudent practice, in order to ensure adequate nodulation and N-2 fixation, to inoculate field peas each time the crop were sown in acid soil.