Identifying recent sources and fate of sedimentary nitrogen in the Baltic Sea based on organic matter elemental composition and nitrogen and carbon stable isotopes ratios

Sedimentary nitrogen (SN) provenience and fate in surface and subsurface sediments collected from the Baltic Sea were assessed. SN and sedimentary organic carbon (SOC) concentrations, stable isotopic signatures (delta N-15(SN) and delta C-13(SOC)) and SOC/SN molar ratios, were determined in subsequent layers of twenty-four sediment cores dated with Pb-210/Cs-137 and fifty-seven surface sediments. Sedimentation rates in the range 66-736 g/m(2)yr(-1) (0.05-0.34 cm/year) were measured. Prolonged incubation of sediments in the laboratory led to a 17-37% decrease of SN in surface sediments, and no SN loss in sediments deposited before 1940. Decrease of delta N-15(SN) on incubation (1 parts per thousand on average), and gradients of delta N-15(SN), between recent-surface (3.5 parts per thousand) vs. 100 years old-subsurface (2.1 parts per thousand) sediments were attributed to varying contributions of labile nitrogen to the SN pool. Annual deposition of SN to sediment surface and burial in subsurface sediments amounted, respectively, to 5 +/- 2 g/m(2) and 3.5 +/- 1 g/m(2).