Dissolved inorganic nitrogen (DIN), such as NO2– and NH4+, is known to react abiotically with organic matter to form organic N under specific conditions. However, the contribution of abiotic processes to the dynamics of DIN in marine environments has not been sufficiently evaluated. In this study, I demonstrated using 15N-labeled tracers that abiotic immobilization of DIN (particularly NO2–) occurred in coastal marine sediments after samples were autoclaved or treated with HgCl2, and compared it with the biotic immobilization (bacterial assimilation) of DIN in terms of reaction rate, product yield, and the degradability of the organic N produced. Abiotic and biotic immobilization of N from DIN into solid sediment occurred within a period of days to a few weeks. NH4+, NO2–, and NO3– were significantly immobilized by biotic processes in the sediment investigated, although microbial dissimilatory reduction seemed to be the primary sink for NO2– and NO3–. In contrast, only NO2– was significantly immobilized in the sediment by abiotic processes. Abiotic immobilization of NO2– apparently obeyed first-order kinetics when the concentration of NO2– was <200 μM. Decomposition experiments with natural sedimentary bacteria demonstrated that organic N formed biotically from NH4+ and abiotically from NO2– was operationally separated into a readily decomposable fraction and a refractory fraction. The refractory fraction of immobilized N ranged from 22% to 68% and was apparently dependent on the physical nature of the sediment (e.g., specific surface area), rather than whether it had been produced biotically or abiotically. This observation suggested that abiotically immobilized N can be preserved in sediment to a similar extent, and by similar mechanisms, as biotically produced organic N.
2015 by The Geochemical Society of Japan