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

Sediments were sampled at water depths from 170 to 2577 m at 17 stations adjacent to Svalbard. In general with increasing water depth there was decreasing NH4+ with increasing NO3 in the sediment pore water, increasing depth ol O2 penetration, decreasing NH4+- and increasing NO3-efflux rates, decreasing nitrification and denitrification rates, and decreasing rates of organic nitrogen burial. Most sediments had insignificant rates of nitrogen mineralisation (0 to 0.34 mmol m-2 d-1); there was a very high C:N ratio (mean 68 in the measured efflux products. Efflux and consumption rates of NO3-, calculated from pore water profiles, were generally higher than the measured rates, but these calculated rates also predicted high C:N mineralisation ratios. The high ratios demanded that the particulate organic substrate must also have had a low nitrogen content. The high measured efflux of dissolved organic nitrogen (mean 0.93 mmol m-2 d-1) from the sediment suggested that fresh detritus (C:N 13) might reach the sediment surface, and be hydrolysed with efflux loss of dissolved nitrogen-rich organic matter (eg, C:N 6) and with subsequent mineralisation (C:N ~68) or burial (C:N ~10) of the transformed material. High C:N ratios in the products of sediment mineralisation are commonly reported, indicating the prevalence of preferential nitrogen loss from detritus in the water column and probably also at the sediment-water interface. The retention of nitrogen by the sediment can explain the discrepancy between measured and calculated NO3- fluxes: NO3- did not escape from the sediment to the water because it was assimilated by bacteria utilising high C:N substrate. It is likely that some NO3- which diffused downward was also assimilated rather than denitrified. Many of these sediments had a sub-surface zone of NH4+ production associated with nitrification. Above and below this zone of net production were zones of NH4+ and NO3- disappearance.