Isotopic Labeling of Soil Nitrate Pools Using 15N-Nitric Oxide Gas
Soil Science Society of America Journal
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.
Stark, J.M., and M.K. Firestone. 1995. Isotopic labeling of soil nitrate pools using 15N-nitric oxide gas. Soil Science Society of America J. 59:844-847.