Compound-specific δ15N composition of free amino acids in moss as indicators of atmospheric nitrogen sources

Abstract

Haplocladium microphyllum moss samples were collected in Nanchang, China. Free amino acid (FAA) concentrations and N isotope compositions (δ¹⁵N_FAA) in the samples were determined and compared with the bulk N concentrations and δ¹⁵N_bulk values. The aim was to determine whether δ¹⁵N_FAA values in moss (which are very variable) indicate the sources of atmospheric N. The δ¹⁵N_FAA values among individual FAA varied widely (from -19.3‰ to +16.1‰), possibly because of the different sources of N and isotope fractionation in amino acids metabolic pathways. Total ¹⁵N-enrichment for the individual FAAs was equal to total ¹⁵N-depletion relative to δ¹⁵N_bulk. The concentration-weighted mean δ¹⁵N value for total FAAs (TFAA) (δ¹⁵N_TFAA) was -3.1‰ ± 3.2‰, which was similar to δ¹⁵N_bulk (-4.0‰ ± 2.9‰). We concluded that a N isotope balance occurred during amino acid metabolism and that little isotope disparity occurred between the concentration-weighted TFAA and bulk N. We concluded that δ¹⁵N_TFAA ≈ δ¹⁵N_bulk ≈ δ¹⁵N_source. The mean δ¹⁵N_alanine (-4.1‰), δ¹⁵N_glutamate (-4.2‰), and δ¹⁵N_lysine (-4.0‰) were similar to the mean δ¹⁵N_bulk, which we attributed to little isotope fractionation occurring during their in situ the metabolic pathways. This suggests that δ¹⁵N_alanine, δ¹⁵N_glutamate, and δ¹⁵N_lysine in moss can be used to indicate the sources of atmospheric N deposition.

Figure 1

Concentrations of TN, TFAA, Gln, Asn, Glu and Arg in moss from Suburban, Zoo, Airport, Landfill and Urban in Nanchang city: (a) TN, (b) TFAA, (c) Gln, (d) Asn, (e) Glu and (f) Arg. Bars represent mean values ± standard deviations. Significantly different mean values (HSD Tukey’s, p < 0.05) of TN and FAA from different sampling sites are indicated with superscript letters ‘A’ and ‘B’.

Figure 2

Moss δ¹⁵N of individual FAAs in NanChang city. The vertical lines represent standard deviations. Moss δ¹⁵N_TFAA and δ¹⁵N_TN were showd in box plot. The box encloses 50% of the data, the whiskers 90% of the data, the solid lines is the median, the dashed line is the mean, solid circles are outliers. The δ¹⁵N ranges of the potential N sources are also included in the figure. The date of NH_x δ¹⁵N values from excretory wastes is cited from Freyer; Heaton and Moore. NH_x δ¹⁵N values from agricultural source is referenced from Xiao et al.. The δ¹⁵N value of NO_x is cited from Freyer and Saurer et al..

Figure 3

Relationships between concentrations of individual FAA (expressed as N concentrations) in moss and estimated total atmospheric N deposition. C_FAA-N calculated by: C_FAA-N = C_FAA∙n∙14. C_FAA is the molar concentration of each amino acid; n is the nitrogen atoms contained in each AA; 14 is the relative molecular mass of nitrogen atom. Total atmospheric N deposition (x) at each sampling sites was estimated using the linear correlation equation (y = 0.052x + 0.73, R² = 0.70, P < 0.001; Xiao et al.) between atmospheric N deposition (x) values and the corresponding moss TN concentrations (y) from the Yangtze River drainage basin.

Figure 4

The δ¹⁵N values of free amino acids (‰) vs. the concentrations of free amino acids (expressed as N concentrations, μg/g) in mosses. δ¹⁵N_TFAA = −3.1‰ is concentration-weighted average nitrogen isotope of free amino acids calculated by Rayleigh equilibrium equation.

Figure 5

Relationship between δ¹⁵N_source, δ¹⁵N_TFAA and δ¹⁵N_{bulk N}. Total ¹⁵N-enrichment for individual FAA and total ¹⁵N-depletion vs. δ¹⁵N_bulk (−4.0‰) are also showed.

Figure 6

The locations of moss sampling sites in Nanchang city. The locational map was modified from Google Earth 7.1.5.1557 (http://earth.google.com).