DINITROGEN FIXATION OF LENTIL, FIELD PEA AND FABABEAN UNDER DRYLAND CONDITIONS

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Abstract
Annual grain legume production has increased substantially in Western Canada over the past 15 yr but more information on the N2-fixing potential of these crops is needed. 15N isotope dilution was used to determine N2 fixation of several grain legumes under dryland field conditions in Saskatchewan. Two cultivars of lentil (Lens culinaris Medik), pea (Pisum sativum L.), and fababean (Vicia faba L.) were grown at five locations in both 1984 and 1985, with all major soil zones represented by at least one location in each year. Drought stress was moderate to severe at all sites in 1984 and at sites in the Brown and Dark Brown soil zones in 1985. Barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) were nearly identical as non-N2-fixing reference crops, but their validity as reference crops for the grain legumes included in this study was not tested. Indigenous rhizobia were incapable of supporting adequate levels of N2 fixation at most sites in this study. Inoculation increased total dry matter, total N and N2 fixation of all grain legume cultivars tested. Proportion of N assimilated from the atmosphere declined with increasing soil nitrate levels and increasing drought stress. Annual rates of N2 fixation were as high as 75, 105 and 160 kg N ha−1 for lentil, pea and fababean, respectively, at sites in the Gray and Gray-Black soil zones in 1985, but declined by an average of 5.3, 7.6 and 10.5 kg N ha−1, respectively, for every cm reduction in moisture use. Maximum rates of N2 fixation in 1984 were about 80 kg ha−1. Fababean fixed the most N2 under wetter conditions, while pea and lentil fixed the most under drought stressed conditions. Key words: 15N isotope dilution, dinitrogen fixation, lentil, pea, fababean, drought stress