Dual N and O isotopes of nitrate in natural plants: first insights into individual variability and organ-specific patterns

Nitrate (NO3 −) is an important form of nitrogen (N) available to plants. The measurements of NO3 − concentration [NO3 −] and isotopes (δ15N and δ18O) in plants provide unique insights into ecosystem NO3 − availability and plant NO3 − dynamics. This work investigated the variability of these parameters in individuals of a broadleaved (Aucuba japonica) plant and a coniferous (Platycladus orientalis) plant, and explored the applicability of tissue NO3 − isotopes for deciphering plant NO3 − utilization mechanisms. The NO3 − in washed leaves showed concentration and isotopic ratios that were much lower than that in unwashed leaves, indicating a low contribution of atmospheric NO3 − to NO3 − in leaves. Current leaves showed higher [NO3 −] and isotopic ratios than mature leaves. Moreover, higher leaf [NO3 −] and isotopic enrichments (relative to soil NO3 −) were found under higher soil NO3 − availability for A. japonica. In contrast, leaves of P. orientalis showed low [NO3 −] and negligible isotopic enrichments despite high soil NO3 −. Higher [NO3 −] was found in both fine and coarse roots of the P. orientalis plant, but significant isotopic enrichment was found only in coarse roots. These results reflect that the NO3 − accumulation and isotopic effects decreased with leaf age, but increased with soil NO3 − supply. Leaves are therefore identified as a location of NO3 − reduction for A. japonica, while P. orientalis did not assimilate NO3 − in leaves but in coarse roots. This work provided the first organ-specific information on NO3 − isotopes in plant individuals, which will stimulate further studies of NO3 − dynamics in a broader spectrum of plant ecosystems.