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

Abstract

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: δ18O and Δ17O); 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 δ18O-NO−3 variation, and summarize the uncertainties in using tissue NO−3 parameters to interpret plant NO−3 utilization.

A Non‐steady State Model Based on Dual Nitrogen and Oxygen Isotopes to Constrain Moss Nitrate Uptake and Reduction

“…Due to enrichments in 15 N and 18 O during partial NO 3 − reduction by NR (Ledgard et al, 1985; Tcherkez & Farquhar, 2006), the isotopic values of the NO 3 − remaining in the mosses will be higher than those of the initial mixture of atm‐NO 3 − and soil‐NO 3 − . Isotopic discriminations of NR in mosses ( 15 ∆ = 12.1‰ and 18 ∆ = 14.4‰ measured for Hypnum plumaeforme ; Liu, Koba, Yoh, & Liu, 2012) are generally similar to those measured for enzymatic NO 3 − reduction in vascular plants (Ledgard et al, 1985; Liu et al, 2014; Tcherkez & Farquhar, 2006). Accordingly, by considering the isotopic fractionation of NR in mosses, the proportional contributions of atm‐NO 3 − and soil‐NO 3 − to the total NO 3 − influx into mosses and the moss NO 3 − ‐reduction dynamics can be evaluated by combining the isotopic signatures of the moss‐tissue NO 3 − pool with the isotopic signatures of atm‐NO 3 − and soil‐NO 3 − .…”

“…However, NRA is inducible by NO 3 − , so NRA may be overestimated by experimental NO 3 − additions during NRA assays (Tischner, 2000). Moreover, NRA assays may also be influenced by the addition of ethanol, pH adjustment, vacuum infiltration, or plant pigments during colorimetrical measurements of NO 3 − and NO 2 − concentrations (Liu et al, 2014). Therefore, the real rates of moss NO 3 − reduction, the responses of moss NO 3 − reduction to NO 3 − uptake, and the importance of NO 3 − assimilation to the bulk N in moss biomass remain uncertain.…”

“…Stable isotopes can elucidate plant N uptake, primary metabolic processes, and the responses of these two processes to changes in N availability (Craine et al, 2015; Hobbie & Högberg, 2012; Tcherkez & Hodges, 2008). Recently, the conversion of NO 3 − in plant extracts to N 2 O by a denitrifier (Casciotti et al, 2002) was used to measure the concentrations and the natural N and O isotopes of NO 3 − in moss tissues, which provided new parameters for determining moss NO 3 − dynamics (Liu, Koba, Liu, et al, 2012; Liu, Koba, Takebayashi, et al, 2012; Liu, Koba, Yoh, et al, 2012; Liu et al, 2014, 2018). Compared with the NRA assay, the natural isotopic abundances of NO 3 − in plant tissues provide an improved estimate of in situ NO 3 − reduction in wild terrestrial plants (Bloom et al, 2014; Liu et al, 2014, 2018; Yoneyama & Tanaka, 1999).…”

“…Recently, the conversion of NO 3 − in plant extracts to N 2 O by a denitrifier (Casciotti et al, 2002) was used to measure the concentrations and the natural N and O isotopes of NO 3 − in moss tissues, which provided new parameters for determining moss NO 3 − dynamics (Liu, Koba, Liu, et al, 2012; Liu, Koba, Takebayashi, et al, 2012; Liu, Koba, Yoh, et al, 2012; Liu et al, 2014, 2018). Compared with the NRA assay, the natural isotopic abundances of NO 3 − in plant tissues provide an improved estimate of in situ NO 3 − reduction in wild terrestrial plants (Bloom et al, 2014; Liu et al, 2014, 2018; Yoneyama & Tanaka, 1999). Because mosses have relatively simple tissue structures (Raven et al, 1998), the entry of the source NO 3 − into moss cells may not have substantial isotopic effects (verified in phytoplanktons; Granger et al, 2004, 2010).…”

Relationships between leaf δ<sup>15</sup>N and leaf metallic nutrients

“…CC BY 4.0 License. nitrogen isotopic fractionation is associated with the process of plant N uptake and assimilation (Evans, 2001; Tcherkez and Hodges, 2008;Liu et al, 2014), we hypothesize that leaf δ 15 N relates to these leaf metallic nutrients. Thus, the aim of the current study was to confirm the hypothesis by measuring leaf δ 15 N and leaf K, Ca, Mg, Fe, Mn and Zn contents of more than 600 plant samples from mainland China.…”