Determination of Atmospheric Volume for Direct Field Measurement of Denitrification in Soil Cores

Abstract

When denitrification is measured directly using 15 N, the emission of labeled N 2 and N 2 O is calculated from the volume of atmosphere confined within a closed chamber. This volume is readily estimated by measuring chamber height with a ruler; however, underestimation will occur with unsaturated soil because of air‐filled porosity. A much greater complication arises when plants are present, due to the volume occupied by the aboveground biomass and the internal volume available for gas exchange. A method was developed to measure atmospheric volume from dilution of a standard addition of Ne introduced prior to circulation of air within a closed‐chamber system. Ion‐current measurements to determine Ne, from which the volume was obtained by regression, were performed during mass spectrometric analysis for 15 N‐labeled N 2 Volume measurements by this method were accurate to within 3%, as compared with gravimetric measurements of air‐filled porosity for bare soil varying in moisture content. When introduction of Ne into a darkened chamber was delayed to ensure stomatal closure, volume measurements for a turfgrass system were accurate to within 2.5%, based on the difference obtained before and after connecting a mason jar to provide a known volume. A more accurate method of measuring atmospheric volume will improve the accuracy achieved in direct measurement of denitrification.