Nitrogen fixation in two annual Medicago legumes, as affected by inoculation and seed density

Abstract

Highly successful ley farming depends largely on the legume genotype and its ability to symbiose effectively with the native or introduced rhizobia. In our study in northern Syria using two medic species (Medicago rigidula L.) All., selection 716, and M. truncatula Gaertn., cultivar Jemalong), two Rhizobium inoculants (strains WSM244 and CC169) and the 15N-isotope-dilution method to estimate N2 fixed, we found large genotypic differences in herbage yield and in symbiotic nitrogen (N2) fixation between the two legume species. The herbage yield of the M. rigidula genotype was on average, about thrice that of the M. truncatula cultivar. Medicago rigidula was, in addition, a superior symbiont, deriving about 91%, or 85 kg N ha−1, of its N from N2 fixation, compared with the 64%, or 23 kg N ha−1, for the M. truncatula/R. meliloti symbiosis. The two genotypes differed in Rhizobium specificity. Medicago rigidula nodulated effectively with the naturally occurring rhizobia strain(s), and neither nodule numbers nor N2 fixation were affected by inoculation with the R. meliloti strains. In contrast, M. truncatula was poorly nodulated by the indigenous strains, and nodulation and total N2 fixed in this species were enhanced by inoculation with either WSM244 or CC169. While seeding density did not generally influence %N2 fixed, it had a highly significant effect on total N2 fixed, through the indirect effect of higher seeding rates increasing the dry matter and total N yields. Thus, high reserves of viable medic seeds in soil may greatly increase the contribution of N2 fixed in a ley-farming system.