Difference in δ15N Signatures among Plant Parts of PerennialSpecies Subjected to Drought Stress with Special Referenceto the Contribution of Symbiotic N2-fixation to Plant N

Abstract

The 15N natural abundance method has been widely used for evaluation of symbiotic N2-fixation. The method inevitably requires a reference plant that reflects soil derived δ15N similar to that in a N2-fixing target plant, for estimating the contribution of fixed N2. However, it is often difficult to select a suitable reference plant. Recently, an alternative method was proposed using the difference in δ15N values between shoots and nodulated roots, which did not require a reference plant per se. Whether this method is applicable to a wide range of N2-fixing plants having different growth habits and symbiosis types remains to be verified. To test the applicability of this method for perennial plants, we examined the difference in δ15N values between shoot and nodulated root (∆δ15Ns-nr), and that between shoot and root (∆δ15Ns-r) in 6-month-old plants grown in pots with different soil moisture regimes. The relationships between ∆δ15Ns-nr and the percentage of N derived from atmospheric N2 (%Ndfa) calculated from the conventional 15N natural abundance method, and between ∆δ15Ns-r and %Ndfa were analyzed in N2-fixing legume Lespedeza cuneata and N2-fixing non-legume Elaeagnus pungens and Myrica rubra. A close correlation was found between ∆δ15Ns-nr and %Ndfa as well as between ∆δ15Ns-r and %Ndfa in Lespedeza cuneata, while no correlation was found in N2-fixing non-legume species. The results indicated that ∆δ15N signatures could be useful for estimating %Ndfa for N2-fixing perennial legume (Lespedeza cuneata) in the first growth season but might not be applicable for N2-fixing actinorhizal plants.