Warren A. Kaplan

Transformations of fixed nitrogen and N2O in the Cariaco Trench

The cycle of nitrogen in the Cariaco Trench was studied using stable isotope techniques with vertical profiles of nutrients, oxygen, and N2O. The rate of nitrification was found to be zero order with respect to the concentration of NH4+ at levels above 0.1 μM. The associated half-saturation constant is about 0.15 μM, much lower than in laboratory cultures of nitrifying bacteria. The result could explain efficient oxidation of NH4+ in the deep sea, despite low numbers of nitrifying bacteria and low concentrations of NH4+. Nitrification ceased in the trench for O2 concentrations below 10 to 15 μM, coincident with the appearance of S2−, possibly reflecting inhibition by reduced sulfur compounds. Nitrous oxide is a by-product of nitrification and is depleted where O2 is < 65 μM, by probably denitrification. Denitrification and net loss of fixed N affected the concentrations of oxidized species (NO3−, N2O) at O2 levels between about 10 and 65 μM.

Nitrous oxide in fresh water systems - An estimate for the yield of atmospheric N2O associated with disposal of human waste

The N2O content of waters in the Potomac and Merrimack Rivers was measured on a number of occasions over the period April to July 1977. The concentrations of dissolved N2O exceed those which would apply in equilibrium with air by factors ranging from about 46 in the Potomac to 1.2 in the Merrimack. Highest concentrations of dissolved N2O were associated with sewage discharges from the vicinity of Washington, D.C. and analysis indicates a relatively high yield, 1.3 to 11 percent, for prompt conversion of waste nitrogen to N2O. The yield could be even higher if bubbles originating in sediments should contain as little as 0.3 percent N2O. Measurements of dissolved N2O in fresh water ponds near Boston demonstrate that aquatic systems may provide both strong sources and sinks for atmospheric N2O.

Nitrification and Production of N2O in the Potomac: Evidence for Variability

Extensive measurements were carried out during the summers of 1977 and 1978 to define concentrations of inorganic nitrogen, O2 and N2O in the Potomac River. The chemistry of the river varied significantly between 1977 and 1978, with nitrification rates slower near the city of Washington D.C. by more than a factor of 10 in 1978. The nitrification rate was inversely correlated with the rate of fresh water flow into the estuary. It appears that production of N2O in 1978 occurred mainly as a by-product of nitrification. The quantity of N2O released to the atmosphere represented approximately 0.3 percent of sewage nitrogen. Conversion was more efficient in the summer of 1977, about 1–5 percent, reflecting either additional mechanisms for production of N2O or larger yields for gas production in nitrification.