Combination Probe for Nitrogen‐15 Soil Labeling and Sampling of Soil Atmosphere to Measure Subsurface Denitrification Activity

Abstract

AbstractInvestigations 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 15NO‐3 and subsequently to collect gas samples from which the denitrifying activity in the surrounding soil can be estimated by isotopic analysis of N2. Gas samples were collected from the labeled soil volume by use of He‐flushed silicone tubing having high permeability for gases. No mass flow 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 15N atom fraction of the dissolved NO‐3 in the surrounding soil. The probe was tested under laboratory conditions. The probe introduced 15NO‐3 into a roughly spherical volume of soil with a symmetric distribution of 15NO‐3. Estimates of the 15N atom fraction of the NO‐3 undergoing denitrification based on gas samples agreed favorably with destructive sampling of NO‐3. The accumulation of N2 derived from denitrification was consistent with expected trends and, along with computer simulations and 15N 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.