Effects of Rhizobia Strain and Growing Temperature on the B-value of Three Forage Legumes Commonly Included in Uruguayan Mixed Pastures

The natural abundance technique is used to estimate the amount of N fixed by a legume, but the B value of this particular species has to be known first. This value, however, varies with the rhizobium strains and climatic conditions. To assess this variation range for forage legumes growing under Uruguayan conditions, seeds of Lotus corniculatus L., Trifolium repens L. and Trifolium pratense L. were sown in pots containing vermiculite-sand and inoculated with a commercial or a promissory rhizobium strain, and grown in a greenhouse during the winter and spring, or in a growth chamber that simulated summer temperatures. At harvest, B values were determined by analyzing δ15N in shoots, and their proportion of N derived from air (%Ndfa) were also estimated. All B values were within the reported ranges. The rhizobium strain affected B only in L. corniculatus, resulting in a 0.55‰ shift between the commercial and the research strains. The growing seasonal effect, however, was important across the three species, with B values becoming more negatives as the temperature and the isotopic fractionation increased. However, the impact of these B value changes on the associated %Ndfa values were not always agronomically relevant, but depended on the particular legume–rhizobia combination.