Observation of Reoxidation and Denitrification Layers in Reduced Soil Under Nitrate Infiltration.

To clarify redox condition in paddy soil near the surface with water flux, we performed a column experiment on infiltration through reduced soil. Soil was corrected at experimental paddy of Mie University, Japan. The soil was air dried, mixed with substrate, uniformly packed into an acrylic column with an inner diameter of 7.7 cm and a height of 15 cm (1.4 g/cm³), saturated by distilled water, and settled at ambient temperature of 25 ^oC for 48 h with 1 cm ponding and no water flux. The column was instrumented with two Eh probes, two tensiometers and four four-electrode probes. Micro Eh and DO probes were also set at surface of the soil. Then, constant water flux was applied to the reduced soil with holding the ponding depth. During the experiment, the micro probes were vertically inserted to 5 cm depth at several times and measured the Eh and DO profiles with spatial resolution of 500 micrometer. When distilled water at a DO of 6.5 mg/L was applied to the top of the column with a flux of 10 cm/d, the Eh and DO near surface increased. After 11 days, 1.5-cm thick of surface reoxidation layer was developed. In the reoxidation layer, Eh became almost constant (Eh > 400 mV), while DO linearly decreased with depth. The thickness of the reoxidation layer depended on the water flux and substrate concentration. Meanwhile, during infiltration of 0.005 M nitrate, soil was stratified into three distinct layers with Eh > 400 mV, ca. 300 mV and < 100 mV, respectively. And, the thickness of each layer increased with time. These layers can be considered as layers in which reoxidation, denitrification and reduction were occurring.