Nitrate dynamics in natural plants: insights based on the concentration and natural isotope abundances of tissue nitrate

The dynamics of nitrate (NO(−)(3)), a major nitrogen (N) source for natural plants, has been studied mostly through experimental N addition, enzymatic assay, isotope labeling, and genetic expression. However, artificial N supply may not reasonably reflect the N strategies in natural plants because NO(−)(3) uptake and reduction may vary with external N availability. Due to abrupt application and short operation time, field N addition, and isotopic labeling hinder the elucidation of in situ NO(−)(3)-use mechanisms. The concentration and natural isotopes of tissue NO(−)(3) can offer insights into the plant NO(−)(3) sources and dynamics in a natural context. Furthermore, they facilitate the exploration of plant NO(−)(3) utilization and its interaction with N pollution and ecosystem N cycles without disturbing the N pools. The present study was conducted to review the application of the denitrifier method for concentration and isotope analyses of NO(−)(3) in plants. Moreover, this study highlights the utility and advantages of these parameters in interpreting NO(−)(3) sources and dynamics in natural plants. We summarize the major sources and reduction processes of NO(−)(3) in plants, and discuss the implications of NO(−)(3) concentration in plant tissues based on existing data. Particular emphasis was laid on the regulation of soil NO(−)(3) and plant ecophysiological functions in interspecific and intra-plant NO(−)(3) variations. We introduce N and O isotope systematics of NO(−)(3) in plants and discuss the principles and feasibilities of using isotopic enrichment and fractionation factors; the correlation between concentration and isotopes (N and O isotopes: δ(18)O and Δ(17)O); and isotope mass-balance calculations to constrain sources and reduction of NO(−)(3) in possible scenarios for natural plants are deliberated. Finally, we offer a preliminary framework of intraplant δ(18)O-NO(−)(3) variation, and summarize the uncertainties in using tissue NO(−)(3) parameters to interpret plant NO(−)(3) utilization.