Effect of Rhizobial Strain and Host Plant on Nitrogen Isotopic Fractionation in Legumes

Abstract

Lotus pedculatus L., Medicago sativa L., Macroptilwum atropwrpureum, Glycine max, and Trifoliun repens L. were grown in a N-free medium and inoculated with one of ten Rhizobium strains. Dry matter, N content, and 8"N values were determined for various plant parts.Nodules, with the exception of those from lucerne, were enriched in 1"N relative to atmospheric N. Considerable variation was found in 81"N values of plant herbage (-4.5 to +0.8). The extent of isotopic discrimination was dependent on both the host plant and the infecting rhizobial strain. This further complicates, but does not invalidate, the use of small variations in the natural abundance of 1"N to estimate the contribution of symbiotically fixed N2 to the N in legume herbage. Some other implications of the observed differences are also discussed.Legumes obtain their N requirements from two sources: symbiotic N2 fixation and assimilation of soil mineral N. Although a quantitative partitioning of both sources is important in understanding the N nutrition of legume crops in mono-culture or in mixed species stands, there are relatively few reports in the literature of field measurements of symbiotic N2 fixation, largely because of the difficulties of making such measurements (14).Many investigators have reported small but measurable differences in the natural abundance of "N in N2-fixing and nonfixing plants growing in association, and some have suggested that these differences may be used to provide a quantitative integrated measurement over time ofthe contribution ofN2 to the N economy of the fixing plant (2,5,13,16,20 (22). This raises the question as to whether the isotopic composition of the host plant is influenced by environmental parameters or by the Rhizobium strain with which it is infected.Nodules of some N2-fixing plants are elevated in "N compared to other plant tissues (18) and it has been suggested that "N enrichment is a result of the denitrifying capability of some rhizobia (20) although this has subsequently been rejected (10). A more recent hypothesis is that "N enrichment in nodules is indicative of the predominant type of N compound transported from the nodule (18). Inasmuch as "N enrichment in nodules will affect the "N concentration in harvested plant material, it will influence estimates of N2 fixation based on variations in the natural abundance of "5N.

Comparison of15N natural abundance and difference methods for estimating N2fixation of soybeans grown in a tropical soil

“…Furthermore, the nodules were found to be strikingly enriched in 15N compared with other plant parts (SHEARER et al 1982;STEELE et al 1983;TURNER and BERGERSEN 1983;YONEYAMA et al 1984) and the enrichment increased with time during the growing season. This is probably the reason for the high ~SN value (about +7.3 0/o0) of the root+nodule component in our experiment.…”

Tansley Review No. 9515N natural abundance in soil‐plant systems

“…against ^''N (Table 1). Several authors have demonstrated that the fractionation during Nj-fixation is influenced by the bacterial strain (Steele et aL, 1983;Bergersen et aL, 1986;Ledgard, 1989), snd Ledgard (1989) provided evidence that nutrient supply and soil moisture also influence the fractionation factor.…”

Effects of Tillage and Crop Residue Application on Soybean Nitrogen Fixation in a Tropical Ferralsol

“…Such distribution was expected due to fractionation of 15 N and 14 N isotopes during transfer and reallocation of N-containing compounds to different plant parts in response to demand (see [17,18]). The relative difference in atom % 15 N excess, between the fixing and non-fixing (reference) plants, is used for determining the amount of fixed N. The atom % 15 N excess values in the reference plant were only 2% higher than in the fixing plant in the roots, but more than double for grain, husks, and stems and leaves.…”