Spatial Variability of Nitrogen‐15 Natural Abundance

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Abstract
The primary objective of this study was to investigate the spatial variability of 15N natural abundance (δ15N) in a non‐N2‐fixing plant (Triticum aestivum L.) on two fields. Two center intersecting 50‐point perpendicular transects were sampled on two separate fields, on Klassen (Udic Boroll) and Kruger (Typic Boroll) soils in Saskatchewan, Canada. Autocorrelation and semivariograms were used to assess the spatial structure of δ15N. Data from the Klassen field indicated that variation in grain δ15N was random, with no spatial structure at the scale of investigation (2–50 m). The pooled mean grain δ15N value and 95% confidence band was 5.43 ± 0.10‰, with a coefficient of variation of 8.9%. Observed variability in δ15N was primarily accounted for by analytical error in the Klassen field. The range in δ15N values for Klassen was 2.80‰, and the maximum difference between samples separated by 2 m was 2.00‰. These data indicate that, even in a low‐variability field, site‐to‐site variation may be significant. Thus, N2‐fixing plants should be grown in close proximity to the reference crop for quantifying N2 fixation. Spatial variability in δ15N for the Kruger field was significantly greater than in the Klassen field. Anisotropy was noted in δ15N in the Kruger field, with a pure nugget effect observed for one transect; for the other, a spherical model described the semivariance pattern with a range for spatial dependence of 16 m. The coefficient of variation and the minimum sample size required for estimating δ15N were corrected for the influence of serial correlation. Mean grain δ15N values for the two Kruger transects were 4.45 ± 0.13‰, and 4.29 ± 0.11‰. It is hypothesized that the greater variability in grain δ15N in the Kruger field resulted from a shorter lag time between fertilizer‐N application and sampling, that is 1.3 yr at Kruger, and 4 yr at Klassen.

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