Abstract
Denitrification in continental shelf sediments has been estimated to be a significant sink of oceanic fixed nitrogen (N). The significance and mechanisms of denitrification in organic-poor sands, which comprise 70% of continental shelf sediments, are not well known. Core incubations and isotope tracer techniques were employed to determine processes and rates of denitrification in the coarse-grained, sandy sediments of the Georgia continental shelf. In these sediments, heterotrophic denitrification was the dominant process for fixed N removal. Processes such as coupled nitrification-denitrification, anammox (anaerobic ammonium oxidation), and oxygen-limited autotrophic nitrification-denitrification were not evident over the 24 and 48 h time scale of the incubation experiments. Heterotrophic denitrification processes produce 22.8–34.1 μmole N m-2 d-1 of N2 in these coarse-grained sediments. These denitrification rates are approximately two orders of magnitude lower than rates determined in fine-grained shelf sediments. These lower rates may help reconcile unbalanced marine N budgets which calculate global N losses exceeding N inputs.
Highlights
Recent studies have reported inbalances in global marine fixed N budgets with rates of N loss exceeding rates of N input.[1,2,3] These inbalanced budgets reflect the difficulties in making estimations given the many uncertainties in the pathways and rates of key N supply and removal reactions
These sandy sediment environments are generally characterized by low organic matter and high pore water dissolved oxygen concentrations, properties typically considered unfavorable for heterotrophic denitrification
Because 15NO3Ϫ was the only significant source of 15N–N, it is clear that the production of 29N2 and 30N2 was the result of the reduction of the 15NO3Ϫ tracer. These results indicate that some form of denitrification occurs in Georgia continental shelf sediments
Summary
Recent studies have reported inbalances in global marine fixed N budgets with rates of N loss exceeding rates of N input.[1,2,3] These inbalanced budgets reflect the difficulties in making estimations given the many uncertainties in the pathways and rates of key N supply and removal reactions. Denitrification in continental shelf sediments is one of the largest sinks of oceanic N,2,4,5 accounting for up to 67% of estimates of total global denitrification.[3] Most direct denitrification rate measurements for continental shelves have been made on fine-grained, muddy sediments which cover only 30% of global shelf area.[6] The remaining 70% of continental shelf area is covered by sandy sediments. These sandy sediment environments are generally characterized by low organic matter and high pore water dissolved oxygen concentrations, properties typically considered unfavorable for heterotrophic denitrification. This study of denitrification in the coarse-grained, sandy sediments of the Georgia continental shelf provides new information in an often overlooked but potentially significant sediment type
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