Sediment Nitrification, Denitrification, and Nitrous Oxide Production in a Deep Arctic Lake

Abstract

We used a combination of 15 N tracer methods and a C 2 H 2 blockage technique to determine the role of sediment nitrification and denitrification in a deep oligotrophic arctic lake. Inorganic nitrogen concentrations ranged between 40 and 600 nmol · cm −3 , increasing with depth below the sediment-water interface. Nitrate concentrations were at least 10 times lower, and nitrate was only detectable within the top 0 to 6 cm of sediment. Eh and pH profiles showed an oxidized surface zone underlain by more reduced conditions. The lake water never became anoxic. Sediment Eh values ranged from −7 to 484 mV, decreasing with depth, whereas pH ranged from 6.0 to 7.3, usually increasing with depth. The average nitrification rate (49 ng of N · cm −3 · day −1 ) was similar to the average denitrification rate (44 ng of N · cm −3 · day −1 ). In situ N 2 O production from nitrification and denitrification ranged from 0 to 25 ng of N · cm −3 · day −1 . Denitrification appears to depend on the supply of nitrate by nitrification, such that the two processes are coupled functionally in this sediment system. However, the low rates result in only a small nitrogen loss.