NITROGEN CYCLING IN SEDIMENTS WITH DIFFERENT ORGANIC LOADING

Sediment cores from an intertidal marine area were experimentally load ed with different amounts of organic material, in order to investigate regulation of the processes in the nitrogen cycle, and the fate of th e inorganic and organic N (NH4+, NO3-, DON, and urea) released through mineralization. The particulate organic material was added III amount s of 30 g dw m(-2) (30MIX) and 100 g dw m(-2) (100MIX) mixed into the sediment, and as 100 g dw m(-2) (100SURF) to the sediment top layer. P rocesses and pools were related to unamended controls (CTRL). The core s were incubated with low (0 to 2 mu M) NO3- in the overlying water, a nd measurements were made after 1 wk. Total sediment respiration rate measured as O-2 uptake and CO2 release were 10 and 18 mmol m(-2) d(-1) , respectively, in the CTRL, gradually increasing in the order of trea tments (30MIX, 100MIX, 100SURF) to 62 and 64 mmol m(-2) d(-1) respecti vely in 100SURF. Higher loading resulted in increasing effluxes of NH4 +. DON effluxes were quantitatively significant only from the 100SURF sediment cores. There was an accumulation of dissolved N-species in th e sediment amounting to 12 to 28 % of the loading, with most increase where the organic matter was mixed into the sediment. Nitrification an d denitrification rates were highest in the moderately loaded sediment (30MIX), followed by control cores and the 100MIX. There was no nitri fication or denitrification activity in the 100SURF cores, and NO3- wa s completely absent in this sediment. Dissimilative NO3- reduction to NH4+ was insignificant in all treatments. The experiment showed that m oderate loading increased N removal through denitrification, while hig h loading decreased denitrification. The marked differences in the fat e of nitrogen, due to organic matter distribution, demonstrated the im portance of bioturbation and other physical mixing processes.