Natural Nitrogen-15 Abundance of Soil and Plant Samples

Nitrogen isotope analysis of total soil N and soil-derived nitrate for nine selected Chernozemic and Luvisolic Ap horizons showed mean δ a 15 N values based on atmospheric abundance of 8.8. Luvisolic soils were characterized by a relatively low level of the heavier isotope. Isotope enrichment of the total N reached a maximum in the lower B horizon. Subsoil nitrate (180-cm depth) had a δ a 15 N value 1/3 that of the Ap horizon. The δ a 15 N of subsurface soil horizons containing residual fertilizer N were low (−5.2) compared to the surface horizon (7.0). The δ a 15 N of NH 4 -N in commercial fertilizers is close to that of atmospheric-N whereas the NO 3 -N has higher values. The data suggest that variations in δ 15 N abundance between horizons of the same soil, or between different soils, may be of real use in evaluating stresses which have been placed in the nitrogen cycle due to man's activities in the past. Similarly, differences in 15 N abundance of soil, legume and air samples may provide an integrated estimation of symbiotic nitrogen fixation under field conditions. More detailed understanding of biological and other processes which control the N isotope concentrations must be obtained before the data reported can be further interpreted.