Nitrogen mineralization and denitrification in Lake Michigan sediment

Organic nitrogen mineralization mechanisms, fluxes, and fates in Lake Michigan sediments were examined by measuring accumulation rates of inorganic nitrogen in laboratory microcosms. Neither ammonium nor nitrate increased substantially in flow-cell or slurry microcosms of offshore, silty sediments. In experiments with gastight chambers containing intact offshore, silty sediment cores (sampled at 45- and 100-m water depths), the total fluxes of nitrogen (NH4+, N03-, and N2) across the sediment-water interface ranged from 14 to 51 µg-atoms N m-2 h-1. Nitrogen gas accounted for 93-98% of the total inorganic nitrogen flux from the sediment to the water. Inputs of inorganic nitrogen via mineralization processes (mediated by microbial decomposition and invertebrate excretion) appeared to be the major factors controlling the rates of both nitrification and denitrification in these sediments. The overlying water did not serve as a significant net source of nitrate driving the denitrification reaction. These results thus indicate that denitrification is a dominant sink for mineralized nitrogen in these silty Lake Michigan sediments and that this process is closely coupled with the initial mineralization of organic nitrogen in the sediments