Determination of atmospheric volume for direct field measurement of denitrification in soil cores

When denitrification is measured directly using N-15, the emission of label ed N-2 and N2O is calculated from the volume of atmosphere confined within a closed chamber. This volume is readily estimated by measuring chamber hei ght with a ruler; however, underestimation will occur with unsaturated soil because of air-filled porosity, A much greater complication arises when pl ants 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 intro duced prior to circulation of air within a closed-chamber system. Ion-curre nt measurements to determine Ne, from which the volume was obtained by regr ession, were performed during mass spectrometric analysis for N-15-labeled N-2, Volume measurements by this method were accurate to within 3%, as comp ared with gravimetric measurements of air-filled porosity for bare soil var ying in moisture content, When introduction of Ne into a darkened chamber w as delayed to ensure stomatal closure, volume measurements for a turfgrass system were accurate to within 2.5%, based on the difference obtained befor e and after connecting a mason jar to provide a known volume. A more accura te method of measuring atmospheric volume will improve the accuracy achieve d in direct measurement of denitrification.