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Nitrification amplifies the decreasing trends of atmospheric oxygen and implies a larger land carbon uptake
Author(s) -
Ciais P.,
Manning A. C.,
Reichstein M.,
Zaehle S.,
Bopp L.
Publication year - 2007
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/2006gb002799
Subject(s) - reactive nitrogen , nitrogen , environmental chemistry , carbon cycle , carbon fibers , environmental science , nitrogen cycle , nitrification , nitrate , oxygen , chemistry , ecology , ecosystem , materials science , biology , organic chemistry , composite number , composite material
Atmospheric O 2 trend measurements are used to partition global oceanic and land biotic carbon sinks on a multiannual basis. The underlying principle is that a terrestrial uptake or release of CO 2 is accompanied by an opposite flux of O 2 . The molar ratio of the CO 2 and O 2 terrestrial fluxes should be 1, if no other elements are considered. However, reactive nitrogen produced by human activities (e.g., fertilizers, N deposition) is also being incorporated into plant tissues. The various reaction pathways of the terrestrial nitrogen cycle cause fluxes of atmospheric O 2 . Thus the cycles of nitrogen, carbon, and oxygen must be linked together. We report here on previously unconsidered anthropogenic nitrogen‐related mechanisms which impact atmospheric O 2 trends and thus the derived global carbon sinks. In particular, we speculate that anthropogenic‐driven changes are driving the global nitrogen cycle to a more oxidized state, primarily through nitrification, nitrate fertilizer industrial production, and combustion of fossil fuels and anthropogenic biomass burning. The sum of these nitrogen‐related processes acts to additionally decrease atmospheric O 2 and slightly increase atmospheric CO 2 . We have calculated that the effective land biotic O 2 :CO 2 molar ratio ranges between 0.76 and 1.04 rather than 1.10 (moles of O 2 produced per mole of CO 2 consumed) over the period 1993–2003, depending on which of four contrasting nitrogen oxidation and reduction pathway scenarios is used. Using the scenario in which we have most confidence, this implies a 0.23 PgC yr −1 correction to the global land biotic and oceanic carbon sinks of most recently reported estimates over 1993–2003, with the land biotic sink becoming larger and the oceanic sink smaller. We have attributed large uncertainties of 100% to all nitrogen‐related O 2 and CO 2 fluxes and this corresponds up to ±0.09 PgC yr −1 increase in global carbon sink uncertainties. Thus accounting for anthropogenic nitrogen‐related terrestrial fluxes of O 2 results in a 45% larger land biotic sink of 0.74 ± 0.78 PgC yr −1 and a slightly smaller oceanic sink of 2.01 ± 0.66 PgC yr −1 for the decade 1993–2003.