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Nitrate Transformations in a Column with a Controlled Water Table¹

ABSTRACT

The relation of dissolved oxygen, soluble carbon, and redox potential to the movement of nitrate in soil columns and into submerged tile lines was studied. Columns 300 cm long were drained at depths of 180, 240, or 300 cm with the water table in all columns maintained at 175 cm. After adding ammonium nitrate and irrigating, the peak in nitrate concentration moved downward to the 160-cm depth, but nitrate concentrations decreased at 180 cm and almost disappeared at the 240- and 300-cm depths. The soil solution at 180 cm contained an average of 5.2 ppm nitrate-nitrogen and a maximum of 16 ppm. In contrast the soil solution at 240 or 300 cm, which had passed through a submerged zone of 65 or 125 cm, averaged 0.5 ppm and had a maximum concentration of 1.1 ppm. Disappearance of nitrate was associated with decreases in redox potential, oxygen content of the soil solution and oxygen levels in the soil atmosphere, and with increases in soluble iron and manganese. Denitrification occurred without submergence if the soil redox potential was sufficiently low. Results of this experiment indicate that nitrate concentrations probably can be reduced in tile effluents by submerging the tile lines.

¹ Contribution from the Southwest Branch, Soil & Water Conservation Research Division, ARS, USDA.

² Soil Scientists and Agricultural Engineer, USDA, Southwestern Irrigation Field Station, Brawley, Calif.

Received for publication April 19, 1969. Accepted for publication October 22, 1969.