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 (δN and δC) and SOC/SN molar ratios, were determined in subsequent layers of twenty-four sediment cores dated with Pb/Cs and fifty-seven surface sediments. Sedimentation rates in the range 66-736 g/myr (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 δN on incubation (1‰ on average), and gradients of δN, between recent - surface (3.5‰) vs. 100 years old - subsurface (2.1‰) 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 and 3.5 ± 1 g/m.