Biogeochemical Controls on Fixed Nitrogen Loss Processes in the Marine Environment

Abstract

Fixed nitrogen availability can regulate atmospheric carbon dioxide concentrations and climate as a whole. Its loss via two anaerobic microbial processes - anammox and denitrification - only occurs in two types of marine environments where oxygen is sufficiently depleted: (1) benthic sediments, concentrated on coastal shelves, and (2) pelagic oxygen deficient zones (ODZs). The same factors were found to control fixed nitrogen loss in all three ecosystems investigated - Chesapeake Bay sediments and the ODZs of the Eastern Tropical North and South Pacific (ETNP and ETSP). Very low oxygen concentrations are required for fixed nitrogen loss by either anammox or denitrification, but organic matter (OM) quantity determines the magnitudes of these rates and OM quality controls the partitioning between the two pathways.

In mesocosms containing Chesapeake Bay sediments, two magnitudes of OM stimulated anammox and denitrification proportionally. Higher amounts of OM also induced a shift in the denitrifying bacterial community. But, because the same quality OM (i.e., C/N ratio) was applied to both treatments, the proportions of nitrogen loss attributed to anammox and denitrification did not change.

Complementary water column experiments using isotope tracers in the ETNP showed the proportion of nitrogen loss attributed to anammox increased with OM nitrogen content and identically matched theoretical expectations. Moreover, the depth distribution of in situ nitrogen loss rates showed OM mass fluxes regulated the total rates of nitrogen loss. Dissolved oxygen concentrations also affected the production of N2: nitrogen loss was reduced to negligible rates with an oxygen amendment as low as 3 &mu;mol L<super>-1</super>. OM and oxygen also controlled nitrous oxide production and consumption rates in the ETNP. Measured rates indicated extremely rapid turnover times (as low as 1 day) and a simple one-dimensional biogeochemistry model identified denitrification as a major N2O production pathway.

In the ETSP ODZ, OM drove nitrate reduction rates in the same way it controlled fixed nitrogen loss in the ETNP. Anammox rates positively correlated with the measured nitrate deficit, suggesting regulation by the supply of nitrite and ammonium from nitrate reduction. Meanwhile, rates of anaerobic nitrite oxidization were significant at the ODZ boundaries, with iodate shown to be a plausible oxidant. A one-dimensional model revealed that the measured rates of nitrate reduction provided anammox with sufficient nitrite but not ammonium. The model also reasonably reproduced the observed nutrient distributions from all directly measured rates in a time frame of 260 days, suggesting the residence time for nitrite in the ODZ to be of this scale.