An oceanic fixed nitrogen sink exceeding 400 Tg N a^?1 vs the concept of homeostasis in the fixed-nitrogen inventory

L. A. Codispoti (1)

1 University of Maryland Center for Environmental Science

Measurements of the N₂ produced by denitrification, a better understanding of non-canonical pathways for N₂ production such as the anammox reaction, better appreciation of the multiple environments in which denitrification can occur (e.g. brine pockets in ice), etc. suggest that it is unlikely that the oceanic denitrification rate is less than 400 Tg N a^?1. The total oceanic sink for fixed-N is, therefore, more than 400 Tg N a^?1. Because this sink term far exceeds present estimates for nitrogen fixation, the main source for oceanic fixed-N, there is a large apparent deficit (~200 Tg N a^?1) in the oceanic fixed-N budget. The size of the deficit appears to conflict with apparent constraints of the atmospheric carbon dioxide and sedimentary ?¹⁵N records that suggest homeostasis during the Holocene. In addition, the oceanic nitrate/phosphate ratio tends to be close to the canonical Redfield biological uptake ratio of 16 (by N and P atoms) which can be interpreted to indicate the existence of a powerful feed-back mechanism that forces the system towards a balance. The main point of this paper is that one cannot solve this conundrum by reducing the oceanic sink term without violating an avalanche of recent data.

A solution to this problem may be as simple as an upwards revision of the oceanic nitrogen fixation rate, and it is noted that most direct estimates for this term have concentrated on nitrogen fixation by autotrophs in the photic zone, even though the gene for nitrogen fixation is widespread amongst heterotrophs. Another simple explanation may be that we are simply no longer in the Holocene and one might expect to see temporary imbalances in the oceanic fixed-N budget as we transition from the Holocene to the Anthropocene in line with an apparent denitrification maximum during the Glacial-Holocene transition. Other possible full or partial explanations involve plausible changes in the oceanic nitrate/phosphate and N/C ratios, an oceanic phosphorus budget that may also be in deficit, and oscillations in the source and sink terms that are short enough to be averaged out in the atmospheric and geologic records.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]; Interfaces continentales, environnement; Océan, Atmosphère; Sciences de la Terre

L. A. Codispoti. An oceanic fixed nitrogen sink exceeding 400 Tg N a^?1 vs the concept of homeostasis in the fixed-nitrogen inventory. Biogeosciences Discussions, 2006, 3 (4), pp.1203-1246. ⟨hal-00297838⟩