Combination probe for nitrogen-15 soil labeling and sampling of soil atmosphere to measure subsurface denitrification activity

Investigations of subsoil denitrification have been hampered by the difficulty in adapting traditional methods for measuring denitrification in surface soils to subsoils. In this study, we present, and test, a combination probe constructed to detect denitrification in the vadose zone. The probe was designed to label a volume of the subsoil with (NO3-)-N-15 and subsequently to collect gas samples from which the denitrifying activity in the surrounding soil can be estimated by isotopic analysis of N-2. Gas samples were collected from the labeled soil volume by use of He-flushed silicone tubing having high permeability for gases. No mass now of gases through the soil was caused by the procedure. Nitrogen gas diffusing into the tubing can be collected after equilibration periods of 15 to 20 min. A combined tensiometer and sampler for dissolved ions was built into the probe to have direct estimates of soil water potential together with information about the N-15 atom fraction of the dissolved NO3- in the surrounding soil. The probe was tested under laboratory conditions. The probe introduced (NO3-)-N-15 into a roughly spherical volume of soil with a symmetric distribution of (NO3-)-N-15. Estimates of the N-15 atom fraction of the NO3- undergoing denitrification based on gas samples agreed favorably with destructive sampling of NO3-. The accumulation of N-2 derived from denitrification was consistent with expected trends and, along with computer simulations and N-15 mass balance methods, was used to calculate denitrification rates. These different estimates of denitrification agreed well. This combination probe shows promise for measuring denitrification in subsoil.