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

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.