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A Comparison of the Effectiveness of ¹⁵N-Enriched vs. Unenriched Fertilizer Nitrogen as a Tracer in Laboratory Experiments¹

ABSTRACT

The experiments reported in this paper were designed (i) to determine whether it is possible to detect the presence of fertilizer nitrogen in nitrate produced by four central Illinois soils during laboratory incubation by using variations in the natural abundance of ¹⁵N, (ii) to use such variations to estimate the fractional contribution of fertilizer nitrogen to the nitrate produced, and (iii) to compare these estimates with those based on the use of fertilizer nitrogen which was artificially enriched with ¹⁵N. The mean ¹⁵N per mill excess) value of nitrate produced during incubation of the soils which had received unlabeled (NH₄)₂SO₄ was significantly lower in 11 out of 12 experiments compared with that produced by unfertilized soils. As expected, estimates of percent fertilizer derived nitrogen based on variations in natural abundance of ¹⁵N were less precise than those based on the use of (NH₄)₂SO₄ artificially enriched with ¹⁵N. The use of natural variations in ¹⁵N abundance to compute the percent fertilizer derived nitrate resulted in a lower estimate than the estimate based on ¹⁵N enriched material in seven experiments. This underestimation appeared to be related to the so called "priming effect" and/or the exchange of fertilizer and soil nitrogen.

Key Words: N isotope abundance • ¹⁵N • nitrate • fertilizer derived nitrogen • soil derived nitrogen

¹ The work is supported by the National Science Foundation under the Project Numbers GI-29925 X1 and GI-39009.

² Research Associate, Center for the Biology of Natural Systems and Soil Scientist, Institute of Soils and Water, Agr. Res. Org., Volcani Center, Bet-Dagan, Israel; Associate, Center for the Biology of Natural Systems; Research Assistant, Center for the Biology of Natural Systems; Associate Professor of Biology, Washington University and Senior Fellow, Center for the Biology of Natural Systems; Research Associate, Center for the Biology of Natural Systems; Director, Center for the Biology of Natural Systems, Washington University, St. Louis, Mo., respectively.

Received for publication February 19, 1974.