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DIVISION S-8-NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

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

Dep. of Natural Resources and Environmental Sciences, 1102 S. Goodwin Ave., Univ. of Illinois, Urbana, IL 61801

Corresponding author (bhorgan{at}uiuc.edu)

When denitrification is measured directly using ¹⁵N, the emission of labeled N₂ and N₂O 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 ¹⁵N-labeled N₂. 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.

Abbreviations: D_b, bulk density • D_p, particle density • PVC, polyvinyl chloride

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