ISOTOPIC LABELING OF SOIL NITRATE POOLS USING NITROGEN-15-NITRIC OXIDE GAS

In N cycling studies, soil NO3- pools are typically labeled by adding solutions containing (NO3-)-N-15. The associated increase in moisture may perturb the system and preclude measurement of N-cycle processes i n relatively dry soils. This study examined the feasibility of using N -15-NO gas to label soil NO3- pools without addition of water. Soil sa mples (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 q uantities of NO gas (0-600 mg NO-N kg(-1) soil). Maximum recovery of N O-N in soil NO2- and NO3- pools occurred after <2 h of exposure to the gas and ranged from approximate to 40% for moist soil samples to >90% in ashed soil. Relative increases in either NO2- or NO3- depended on the initial NO concentration and the soil treatment. In general, highe r soil moisture resulted in higher NO2-/NO3- ratios. Within 20 h of ad dition of (NO)-N-15, almost all of the NO2- was concerted to NO3-; how ever, a small amount was also converted to NH4+ and organic N. These r esults demonstrate that (NO)-N-15 can be used to label soil NO3- 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.