Abstract
Vegetation growing in urban ecosystems is frequently exposed to an environment with high atmospheric nitrogen (N) pollution. We systematically investigated the free amino acid concentrations in moss samples and Cinnamomum camphora leaf (new, middle-aged and old leaves), branch phloem, trunk phloem, and bark samples from field sites (Guiyang City, SW China) with different N deposition levels. The responses of the free amino acids to N deposition were analysed in the abovementioned plant tissues to determine whether plant free amino acids could act as biomarkers of the external N supply; moreover, differences in the N metabolism of these tissues under varying N deposition conditions were revealed by the compositions and sizes of their free amino acid pools. In particular, we reported the significant accumulation of arginine with increased N deposition in bark samples (while the arginine concentrations in the branch phloem, trunk phloem, and leaves remained low), which may indicate a long-term or historic external environment with a high N availability; additionally, the noticeable dominance and fluctuation of γ-aminobutyric acid in response to varied N deposition levels occurred in both the branch phloem and the trunk phloem, suggesting that the γ-aminobutyric acid transported in the phloem may be used as an important signal reflecting increases in the atmospheric N input. We conclude that the free amino acid concentrations in moss and camphor leaf, phloem and bark tissues are more sensitive to N deposition compared to their N concentrations and that tissue glutamine/glutamate and arginine/γ-aminobutyric acid ratios may serve as better biomarkers reflecting the tissue N accumulation status associated with increased N deposition. Therefore, free amino acid concentration analyses of different plant parts may provide a means to gain a more in-depth understanding of the impacts of atmospheric N pollution on plant physiology and N cycles.
Published Version
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