Direct Measurement of Oxygen Profiles and Denitrification Rates in Soil Aggregates

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Abstract
An oxygen microelectrode was modified to measure O2 concentrations in wet aggregates of a silt loam soil. The microelectrode tip had an O2‐permeable membrane opening 3 µm in diameter, and O2 measurements could be made in as little as 0.1‐mm increments to a depth of 12 mm. When aggregates were incubated in air, steep O2 gradients usually occurred over very small distances from the aggregate surface. The smallest aggregate exhibiting an anaerobic center had a radius of 4 mm, although small aggregates (radius ≤ 6 mm) were generally oxic. Larger aggregates (radius ≥ 10 mm) often had measureable anaerobic centers, with the exception of those from a native prairie soil which exhibited irregular O2 profiles and had aerobic centers, apparently due to O2 intrusion caused by old root channels. Oxygen profiles obtained in 45 degree increments around an aggregate circumference were used to construct contour maps of O2 concentrations within the aggregate. Oxygen gradients were somewhat asymmetric, suggesting nonuniformly distributed sites of O2 consumption. An average intra‐aggregate O2 diffusion coefficient of 8.5 × 10−6 cm2 · s−1 was measured for water‐saturated aggregates. The radii of anaerobic centers within several aggregates, measured directly with the electrode, correlated with those calculated from a model of radial diffusion using measured respiration rates and the intra‐aggregate O2 diffusion coefficient. Anaerobic centers were present in all aggregates that denitrified, but not all aggregates with anaerobic zones denitrified. The denitrification rate did not correlate with the size of the anaerobic zone, indicating that factors other than anaerobic volume contributed to the observed rates.
Funding Information
  • National Science Foundation (DEB‐80‐12168, DEB‐81‐09994)