Difference in δ15N signatures between nodulated roots and shoots of soybean is indicative of the contribution of symbiotic N2 fixation to plant N

Abstract

Symbiotic N2 fixation has a variable effect on the 15N abundance of different parts of legumes. Increases in fixation result in 15N enrichment of nodules, while decreases, in combination with an increased uptake of mineral N, result in 15N depletion of the root system. The difference between soybean shoot and below-ground δ15N (Δδ15N = δ15Nshoot-δ15N belowground) was assessed in hydroponic culture over a range of rates of supply of mineral N. The fractional contribution of N2 fixation to N uptake (%Ndfa) was determined using the natural abundance (NA) technique with ryegrass as a reference plant. ΔδN and %Ndfa were highly correlated, and the relationship was tested using the same soybean cultivar grown in pots in N-rich soil. Estimates of %Ndfa derived from the NA method and from the Δδ15N approach yielded near-identical values. A literature survey showed similar relationships between %Ndfa and Δδ15N with different growth stages of soybeans grown under glasshouse and field conditions, different cowpea (Vigna unguiculata) cultivars in the field, and tagasaste (Chamaecytisus proliferus) in hydroponic culture. Possible confounding and species-specific (either plant or Rhizobium spp.) influences are discussed. The difference in δ15N signatures between nodulated roots and shoots is confirmed as a robust means of quantifying %Ndfa: it is independent of reference plants and offers the possibility of estimating %Ndfa in soils where the isotope composition of mineral N closely matches that of atmospheric N2.