Dissolved inorganic nitrogen (DIN), such as NO
2– and NH
4+, 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 NO
2–) occurred in coastal marine sediments after samples were autoclaved or treated with HgCl
2, 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. NH
4+, NO
2–, and NO
3– were significantly immobilized by biotic processes in the sediment investigated, although microbial dissimilatory reduction seemed to be the primary sink for NO
2– and NO
3–. In contrast, only NO
2– was significantly immobilized in the sediment by abiotic processes. Abiotic immobilization of NO
2– apparently obeyed first-order kinetics when the concentration of NO
2– was <200 μM. Decomposition experiments with natural sedimentary bacteria demonstrated that organic N formed biotically from NH
4+ and abiotically from NO
2– 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.
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