Organic-N loss by efflux and burial associated with a low efflux of inorganic N and with nitrate assimilation in Arctic sediments (Svalbard, Norway)

Abstract

S e d i m e n t ~ were sampled a t water depths from 170 to 2577 m at 17 stations adjacent to Svalbard. In general, with increasing water depth there was decreasing NH,' with increasing N O 1 in the sediment pore water, increasing depth of 0, penetration, decreasing NHI'-and increasing NO?-efflux rates, decreasing nitrification and dcnitrif~cation rates, and decreasing rates of organic nltrogen bunal. Most sediments had insignificant rates of nltrogen mineralisation (0 to 0.34 mm01 m-' d-'); there was a very high C:N ratio (mean 68) in the measured efflux products. Efflux and consunlption rates of NO,-, calculated from pore water profiles, were generally higher than the measured rates, but these calculated rates also predicted high C:N minerallsation ratios. The h ~g h rat~os demanded that the part~culate organic substrate must also have had a low nitrogen content. The high measured efflux of dissolved organic nitrogen (mean 0.93 mmol m ' d -' ) from the sediment suggested that fresh detritus (C:N 13) m ~g h t reach the sediment surface, and be hydrolysed with efflux loss of dissolved nitrogenrich organic matter (e.g. C.N 6) and with subsequent mineralisation ( C . 4 -68) or burial (C:N -10) of the transformed material. High C:N ratios in the products of sediment mineralisation are commonly reported, lndicatlng the prevalence of preferential nitrogen loss from detritus in the water column and probably also a t the sediment-water interface. The retention of nitrogen by the sediment can explain the discrepancy between measured and calculated NO.; fluxes: NO3-did not escape from the sediment to the water because it was assimilated by bacterla utilising high C:N substrate It is likely that some NO3which diffused downward was also assim~lated rather than denltrif~ed. Many of these sediments had a sub-surface zone of NH,' production associated with nitrification. Above and below this zone of net production were zones of NH,' and NO, disappearance.