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.