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Nitrate Reduction in an Organic Soil-Water System¹

ABSTRACT

Flooded organic soil as a treatment system for nitrate-nitrogen (NO₃-N) removal from agricultural drainage water was evaluated under controlled conditions. Nitrate-N reduction rates in the flood water (agricultural drainage water) and in the underlying organic soil column were measured as a function of NO₃-N concentration (10, 25, and 50 µg NO₃-N/ml), energy source, and temperature (8, 18, and 28°C). Labeled NO₃-N was used to differentiate between denitrification and NO₃-N reduction to ammonium-N (NH₄-N) or organic N.

Nitrate-N reduction rates in the soil column incubated at 28°C with no excess flood water were best described by first-order kinetics with an average rate constant (k) of 0.751 day^–1. Flood water NO₃-N removal rates were also described by first-order kinetics. Nitrate removal rates from low oxygen-demand flood water (expressed as first-order rate constants) with an underlying soil column were 0.038, 0.750 day^–1 for an incubation temperature of 8, 18, and 28°C, respec- 28°C, respectively. In soil columns incubated with high oxygen-demand flood water, most of the added NO₃-N disappeared before it reached the underlying soil column, with k values of 0.172, 0.292, and 0.790 day–1 for an incubation temperature of 8, 18, and 28°C, respectively. In all treatments, denitrification was the dominant process in the removal of flood water NO₃-N. At low temperature (8°C), a greater portion of ¹⁵NO₃-N was reduced to ¹⁵NH₄-N and organic N. Most of the immobilized ¹⁵NO₃-N appeared as soluble organic N. At 8°C, flood water NO₃-N diffused deeper into the soil column as compared to the flooded soil columns incubated at 28°C.

The results obtained in this study indicate that flooded organic soil can function as an effective sink for reducing NO₃-N levels of agricultural drainage waters.

Key Words: flooded soil • NO₃-N diffusion • denitrification • first-order kinetics • waste waters • drainage waters

¹ Florida Agric. Exp. Stn. Journal Series no. 1979.

² Assistant Research Scientist and Graduate Student, Agricultural Research and Education Center, Sanford, FL 32771; and Associate Professor, Soils Dep., Univ. of Florida, Gainesville, FL 32611, respectively.

Received for publication October 20, 1979.