Denitrification and Nitrate Reduction in Wisconsin Lake Sediments

The fate of 15 N‐labelled NO 3 ‐N in Wisconsin lake sediment‐water systems was investigated in laboratory and field experiments. In the laboratory, approximately 90% of the added NO 3 ‐N disappeared from a calcareous sediment (Lake Mendota) compared to about 40% from a noncalcareous sediment (Trout Lake) after 48 hours when incubated under He. About 10% (Mendota) and 5% (Trout) of the added NO 3 ‐N was recovered as organic‐N and NH 4 ‐N. The presence of significant amounts of 15 NH 4 ‐N, particularly in Lake Mendota sediment, indicated that immobilized N was subject to rapid mineralization. Nitrate‐ 15 N not accounted for was assumed lost through denitrification. Under aerobic conditions 10% (Mendota) and 2% (Trout) of the 15 N was recovered as organic N but little 15 N appeared in the NH 4 ‐N fraction. In a field investigation, 15 NO 3 ‐N, added to Lake Mendota sediment samples and returned to the lake bottom in wide‐mouth plastic bottles, disappeared within 4 days. After 4 days about 37% of the added NO 3 ‐N was in the organic and NH 4 ‐N fractions, while the remaining 63% was likely lost through denitrification. These data illustrate that denitrification and NO 3 ‐N reduction in sediments receiving NO 3 ‐N in ground waters must be evaluated in calculating nitrogen budgets of seepage lakes.