Effects of symbiosis with Frankia and arbuscular mycorrhizal fungus on thenatural abundance of N-15 in four species of Casuarina

The effect of interactions between Casuarina species, Frankia strains and A MF on nitrogen isotope fractionation within the plant were determined under conditions where changes in source nitrogen were minimized by growing plan ts in mineral nitrogen-deficient conditions and without added organic N. Ca suarina cunninghamiana, C. equisetifolia, C. glauca, and C. junghuniana wer e inoculated singly with three Frankia strains or were dual inoculated with Frankia and Glomus fasciculatum, The %N and delta(15)N of separated parts of plants inoculated with the three Frankia strains or with Frankia+Glomus were not significantly different within Casuarina species, However, the slo w-growing C. junghuniana differed in several variables from the other three species. There was a highly significant, linear relationship between the n atural logarithms of cladode N content and delta(15)N of plants of the four Casuarina species when inoculated with Frankia or with Frankia+Glomus, sho wing that nitrogen supply and the correlated variable, plant growth rate, w ere major determinants of delta(15)N. Provision of small quantities of (NH4 )(2)SO4 or KNO3 increased several-fold the growth of three of the Casuarina species when inoculated with Frankia alone or with Frankia+Glomus. Within species, mycorrhizal and non-mycorrhizal plants receiving supplementary sol uble phosphate were of similar dry weights at harvest. delta(15)N values fo r cladodes of C. cunninghamiana, C. equisetifolia and C. glauca were simila r, but values for the poor growing C. junghuniana were more variable and, w ith the exception of plants receiving KNO3, were lower than those of the ot her three species. Reduced growth due to suboptimal availability of N or P had a major influence on delta(15)N and, in these conditions where plants c ould not access significant amounts of organic N, outweighed any effects on cladode delta(15)N of colonization by Glomus. delta(15)N values of nodules were higher than other parts of Frankia or Frankia+Glomus inoculated Casua rinas, conceivably due to retention in nodules of fixed N, with delta(15)N close to zero.