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An oceanic fixed nitrogen sink exceeding 400 Tg N a<sup>−1</sup> vs the concept of homeostasis in the fixed-nitrogen inventory
L. A. Codispoti
codispot@hpl.umces.edu
University of Maryland Center for Environmental Science, Horn Point Laboratory, P.O. Box 775, Cambridge, MD 21613, USA
Measurements of the N<sub>2</sub> produced by denitrification, a better
understanding of non-canonical pathways for N<sub>2</sub> production such as the
anammox reaction, better appreciation of the multiple environments in which
denitrification can occur (e.g. brine pockets in ice, within particles
outside of suboxic water, etc.) suggest that it is unlikely that the oceanic
denitrification rate is less than 400 Tg N a<sup>−1</sup>. 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<sup>−1</sup>)
in the oceanic fixed-N budget. The size of the deficit
appears to conflict with apparent constraints of the atmospheric carbon
dioxide and sedimentary δ<sup>15</sup>N 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.
<i>The main point of this paper is that one cannot solve this conundrum
by reducing the oceanic sink term</i>. To do so would violate an avalanche of
recent data on oceanic denitrification.
<br><br>
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 nitrogen fixing genes are widespread. 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,
but which could, perhaps, last long enough to have significant impacts.
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