New developments in the marine nitrogen cycle.

Nitrogen, as a building block in the structures of nucleic and amino acids, porphyrins, and amino sugars, is a fundamental player in many biogeochemical cycles. 1 It also shares with many elements a role in reduction xidation reactions in the marine environment. 2,3 Additionally, nitrogen is strongly impacted by anthropogenic activities. 4-6 Most nitrogen in marine environments is present in five forms: N2, a quite stable molecule that requires specialized enzymatic systems to break and use; nitrate, the most oxidized form of nitrogen and the dominant biologically utilizable form of N within oxic environments; ammonium, the most reduced natural form of N and the dominant biologically available form found in anoxic environments; particulate nitrogen, predominant within sediments and primarily in the form of organic N, and dissolved organic N (DON), a complex mixture of compounds with a wide range of compositions. 7-9 Nitrate, nitrite, ammonium, and organic nitrogen are typically grouped together as “fixed N” in discussions of nitrogen availability, although each form has a different level of reactivity. A complex web of reactions links these different compounds in ways that are still being determined. In the simplest sense, these reactions, together with major flux terms, describe a marine nitrogen cycle reduced to six terms: N 2 fixation, riverine inputs, atmospheric fallout, sediment organic matter burial, and water column and sedimentary denitrification (conversion of fixed N to N2). Our understanding of the relative and absolute importance of each process has changed dramatically over the past 40 years. Early marine nitrogen studies focused on the role of N as a primary productivity-limiting element. The advent of the “Redfield Ratio (RR)”13,14 provided a simple metric to determine whether nitrogen or phosphorus would limit overall levels of primary productivity in a particular ecosystem. Simply put, the RR hypothesis posits that all marine organic matter consists of material with roughly 16 N for every one P. One can thus use this assumption to both predict the usage ratios and remineralization ratios of inorganic N and P within the water column. Global studies of dissolved nutrient patterns show strong correlations between the abundances of PO 4 and NO3 that would be expected if “Redfieldian” organic matter was being remineralized. 15 One can thus use this assumption to predict the usage ratios and remineralization ratios of N and P. Deviations in the N/P stoichiometry of dissolved nutrient concentrations, 16 defined by the tracer N* (N*) [NO3] 16[PO4] + 2.9),17 therefore reflect non-Redfield biological nutrient inputs, such as N 2 fixation, which causes N* to increase, and losses such as denitrification, which reduces N*. In suboxic water columns, such as occur in the eastern tropical Pacific (ETP) Ocean and the northern Arabian Sea (AS), N* values indicated the loss of NO3 as (unmeasured) N 2 via the process of denitrification. An earlier version of the N* relationship was used to estimate the difference between observed and calculated fixed N levels. 16 This result, together with residence time estimates, was then used to estimate the fixed nitrogen loss from the ETP and subsequently expanded to cover other suboxic and anoxic regions. 18-21