Spatial Variability of Nitrogen‐15 and Total Nitrogen in Some Virgin and Cultivated Soils

Abstract

Soil cores were taken from six locations representing three virgin and three cultivated soils in increments of 15 or 30 cm down to depths ranging from 120 to 300 cm. The number of samples of an individual soil profile varied from 20 to 96, though smaller numbers of some subsurface horizons were obtained at two locations where very rocky conditions were encountered. Frequency distribution analysis of δ¹⁵N values and of total N showed that they were log‐normally distributed in about half the groups of samples. Assumption of normal distribution in all samples would result in a maximum error of 37% in total N and 0.1% in ¹⁵N content if the real distribution were in fact log‐normal. Coefficients of variation of total N were somewhat lower in cultivated soils than in virgin soils, but C.V. values for atom %¹⁵N were comparable in the two groups of soils. It is concluded that the magnitude of natural variation in δ¹⁵N values at a given sampling site, both laterally and vertically, is so great as to preclude tracing biological events by means of natural abundances.