EFFECT OF ORGANIC LOADING ON NITRIFICATION AND DENITRIFICATION IN A MARINE SEDIMENT MICROCOSM

The effects of organic additions on nitrification and denitrification were examined in sediment microcosms. The organic material, heat kille d yeast, had a C/N ratio of 7.5 and was added to sieved, homogenized s ediments. Four treatments were compared: no addition (control), 30 g d ry weight (dw) m-2 mixed throughout the 10 cm sediment column (30M), 1 00 g dw m-2 mixed throughout sediments (100M), and 100 g dw m-2 mixed into top 1 cm (100S). After the microcosms had been established for 7- 11 days, depth of O2 penetration, sediment-water fluxes and nitrificat ion rates were measured. Nitrification rates were measured using three different techniques: N-serve and acetylene inhibition in intact core s, and nitrification potentials in slurries. Increased organic additio ns decreased O2 penetration from 2.7 to 0.2 mm while increasing both O 2 consumption, from 30 to 70 mmol O2 m-2 d-1 and NO3- flux into sedime nts. Nitrification rates in intact cores were similar for the two meth ods. Highest rates occurred in the 30M treatment, while the lowest rat e was measured in the 100S treatment. Total denitrification rates (est imated from nitrification and nitrate fluxes) increased with increased organic addition, because of the high concentrations of NO3- (40 muM) in the overlying water. The ratio of nitrification: denitrification w as used as an indication of the importance of nitrification as the NO3 - supply for denitrification. This ratio decreased from 1.55 to 0.05 w ith increased organic addition.