Direct Measurement of Oxygen Profiles and Denitrification Rates in Soil Aggregates

Abstract An oxygen microelectrode was modified to measure O 2 concentrations in wet aggregates of a silt loam soil. The microelectrode tip had an O 2 ‐permeable membrane opening 3 µm in diameter, and O 2 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 O 2 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 O 2 profiles and had aerobic centers, apparently due to O 2 intrusion caused by old root channels. Oxygen profiles obtained in 45 degree increments around an aggregate circumference were used to construct contour maps of O 2 concentrations within the aggregate. Oxygen gradients were somewhat asymmetric, suggesting nonuniformly distributed sites of O 2 consumption. An average intra‐aggregate O 2 diffusion coefficient of 8.5 × 10 −6 cm 2 · 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 O 2 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.