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Denitrification in Subsoils of the North Carolina Coastal Plain as Affected by Soil Drainage¹

ABSTRACT

In a tiled moderately well-drained soil, over 200 kg NO₃-N/ha were generally found distributed throughout the top 3 m. However, in a poorly drained soil, relatively low levels of NO₃ were found in the top m and very little NO₃ persisted in the saturated zone beneath 1 m.

Oxidation-reduction potential (Eh) measurements in the tiled moderately well-drained soil indicated well-oxidized conditions (500–700 millivolts) to 3 m. The Eh measurements beneath 1 m in the poorly drained soil consistently indicated favorable conditions for denitrification. A marked decrease in the NO₃-N/Cl ratio with depth in the poorly drained soil supported the contention that denitrification was occurring in the soil.

The water-soluble C content of the subsurface soil water was 2–5 mg/liter in the moderately well-drained soil and 10–15 mg/liter in the poorly drained soil. The absence of saturated conditions for extended periods of time and lack of an adequate energy source apparently limited denitrification in the moderately well-drained soil and much of the fertilizer N not utilized by crops moved into shallow aquifers and surface water. However, sufficient available organic C was present in the subsoil of the saturated, poorly drained soil to bring about a reducing soil environment and subsequent denitrification of the residual NO₃. Thus, little NO₃ moved from this soil into surface waters.

Key Words: reduction-oxidation potential • water-soluble carbon

¹ Paper Number 4508 of the Journal series of the North Carolina Agric. Exp. Sta., Raleigh, NC 27607. The work upon which this publication is based was supported in part by funds provided by the Office of Water Resour. Res. Inst. of the Univ. of North Carolina as authorized under the Water Resources Research Act of 1964.

² Graduate Research Assistant, Associate Professor, and Professor, respectively, Soil Science Dep., North Carolina State University.

Received for publication October 16, 1974.

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