Sediment cores from an intertidal marine area were experimentally loaded with different amounts of organic material, in order to investigate regulation of the processes in the nitrogen cycle, and the fate of the inorganic and organic N (NH4+, No3-, DON, and urea) released through mneralization. The particulate orgamc matenal was added in amounts of 30 g dw m-2 (30MIX) and 100 g dw m 2 (100MIX) mixed into the sediment, and as 100 g dw rrr2 (100SURF) to the sediment top layer. Processes and pools were related to unamended controls (CTRL). The cores were incubated with low (0 to 2 pM) N O 3 in the overlymg water, and measurements were made after 1 wk. Total sediment respiration rate measured as 0; uptake and CO; release were 10 and 18 mmol m 2 d l , respectively, in the CTRL, gradually increasmg m the order of treatments (30MIX, IOOMIX, 100SURF) to 62 and 64 mrnol m-' d-' respectively 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 the sediment amounting to 12 to 28 % of the loading, with most increase where the organic matter was mixed into the sediment. Nitrification and denitrification rates were highest in the moderately loaded sediment (30MIX), followed by control cores and the 100MIX. There was no nitrification or denitrification activity in the 100SURF cores, and NO3" was completely absent in this sediment. Dissimilative N o 3 reduction to NH4+ was insignificant in all treatments. The experiment showed that moderate loading increased N removal through denitrification, while high loading decreased denitrification. The marked differences in the fate of nitrogen, due to organic matter distribution, demonstrated the importance of bioturbation and other physical mixing processes.
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