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Isotopic Tracer Techniques for Identification of Sources of Nitrate Pollution¹

ABSTRACT

A comparison was made of the use of natural variations in ¹⁵N-abundance vs. ¹⁵N-enriched and ¹⁵N-depleted fertilizers in determining the contribution of inorganic ammonium fertilizers to the NO₃-N extracted from a Webster soil following a 30C aerobic incubation for 2.5 weeks. The natural abundance approach was unsatisfactory even in this simplified system and its lack of success is shown to be due in part to replicate variability. The fluctuations in ¹⁵N-content of the nitrate from carefully replicated samples of each fertilizer treatment were of the same order of magnitude as the mean difference between the ¹⁵N-contents of natural fertilizer-N and the soil-derived NO₃-N. Therefore, the use of labeled fertilizers with ¹⁵N-contents substantially higher or lower than the natural abundance figure is the only valid approach to the measurement of fertilizer contribution to the NO₃-N appearing in tile drains under field condition.

Key Words: natural isotopic abundances • ¹⁵N-depleted • ¹⁵N-enriched • ¹⁴N • ¹⁵N

¹ Contribution from the Soils and Fertilizer Research Branch, Div. of Agr. Dev., Tennessee Valley Authority, Muscle Shoals, Ala. 35660.

² Research Chemist (Soils).

Received for publication August 25, 1972.

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