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Isotopic Labeling of Soil Nitrate Pools Using Nitrogen-15-Nitric Oxide Gas

Dep. of Biology and the Ecology Center, Utah State Univ., Logan, UT 84322-5500

Mary K. Firestone

Dep. of Soil Science, Univ. of California, Berkeley, CA 94720

*Corresponding author (jstark{at}cc.usu.edu).

ABSTRACT

In N cycling studies, soil NO^–₃ pools are typically labeled by adding solutions containing ¹⁵NO^–₃. The associated increase in moisture may perturb the system and preclude measurement of N-cycle processes in relatively dry soils. This study examined the feasibility of using ¹⁵N-NO gas to label soil NO^–₃ pools without addition of water. Soil samples (ashed, oven dry, or with soil water potentials ranging from –3.7 to –0.2 kJ kg^–1 [1 MPa = 1 kJ kg^–1]) were exposed to various quantities of NO gas (0–600 mg NO-N kg^–1 soil). Maximum recovery of NO-N in soil NO^–₂ and NO^–₃ pools occurred after <2 h of exposure to the gas and ranged from 40% for moist soil samples to >90% in ashed soil. Relative increases in either NO^–₂ or NO^–₃ depended on the initial NO concentration and the soil treatment. In general, higher soil moisture resulted in higher NO^–₂/NO^–₃ ratios. Within 20 h of addition of ¹⁵NO, almost all of the NO^–₂ was converted to NO^–₃; however, a small amount was also converted to NH⁺₄ and organic N. These results demonstrate that ¹⁵NO can be used to label soil NO^–₃ pools without addition of water. Depending on the objectives of the study, however, it may be necessary to monitor the enrichment of a variety of other soil N pools as well.

Received for publication February 22, 1995.

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