Abiotic versus biotic immobilization of inorganic nitrogen in sediment as a potential pathway of nitrogen sequestration from coastal marine ecosystems

Abstract

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.