An oceanic fixed nitrogen sink exceeding 400 Tg N asup−1/sup vs the concept of homeostasis in the fixed-nitrogen inventory

摘要

Measurements of the N₂ produced by denitrification, a betterunderstanding of non-canonical pathways for N₂ production such as theanammox reaction, better appreciation of the multiple environments in whichdenitrification can occur (e.g. brine pockets in ice, within particlesoutside of suboxic water, etc.) suggest that it is unlikely that the oceanicdenitrification rate is less than 400 Tg N a−1. Because this sink termfar exceeds present estimates for nitrogen fixation, the main source foroceanic fixed-N, there is a large apparent deficit (~200 Tg N a−1)in the oceanic fixed-N budget. The size of the deficitappears to conflict with apparent constraints of the atmospheric carbondioxide and sedimentary δ15N records that suggest homeostasisduring the Holocene. In addition, the oceanic nitrate/phosphate ratio tendsto be close to the canonical Redfield biological uptake ratio of 16 (by Nand P atoms) which can be interpreted to indicate the existence of apowerful feed-back mechanism that forces the system towards a balance.The main point of this paper is that one cannot solve this conundrumby reducing the oceanic sink term. To do so would violate an avalanche ofrecent data on oceanic denitrification.A solution to this problem may be as simple as an upwards revision of theoceanic nitrogen fixation rate, and it is noted that most direct estimatesfor this term have concentrated on nitrogen fixation by autotrophs in thephotic zone, even though nitrogen fixing genes are widespread. Anothersimple explanation may be that we are simply no longer in the Holocene andone might expect to see temporary imbalances in the oceanic fixed-N budgetas we transition from the Holocene to the Anthropocene in line with anapparent denitrification maximum during the Glacial-Holocene transition.Other possible full or partial explanations involve plausible changes in theoceanic nitrate/phosphate and N/C ratios, an oceanic phosphorus budget thatmay also be in deficit, and oscillations in the source and sink terms thatare short enough to be averaged out in the atmospheric and geologic records,but which could, perhaps, last long enough to have significant impacts.