Carbonate removal by acidification causes loss of nitrogenous compounds in continental margin sediments

This study reports on measurements of organic carbon (C_org) and total nitrogen (N_tot) in surface sediments originating from 6 transects along the northwest European continental margin. After elimination of carbonates by an acidification technique using sulphurous acid, both elements were analysed in the same sediment sample using an elemental analyser. C_org and N_tot in the sample were comparatively low, ranging between 1 and 10 mg C and 0.2 to 1 mg N g⁻¹ dry sediment. In a second analysis, the samples were analysed without acid addition, resulting in N_tot concentrations of 0–50% higher compared to their acidified counterparts. As a consequence, molar C/N ratios derived from the analysis of C_org and N_tot in the acidified sample ranged between 6 to 11, while the N_tot separate analysis reduced C/N ratios to 6 to 8. It is suggested that the addition of sulphurous acid to eliminate inorganic carbon volatilises nitrogenous organic matter.