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Nitrification amplifies the decreasing trends of atmospheric oxygen and implies a larger land carbon uptake


Reichstein,  M.
Research Group Biogeochemical Model-data Integration, Dr. M. Reichstein, Max Planck Institute for Biogeochemistry, Max Planck Society;

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Ciais, P., Manning, A. C., Reichstein, M., Zaehle, S., & Bopp, L. (2007). Nitrification amplifies the decreasing trends of atmospheric oxygen and implies a larger land carbon uptake. Global Biogeochemical Cycles, 21(2), GB2030. doi:10.1029/2006GB002799.

[1] 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 CO2 is accompanied by an opposite flux of O-2. The molar ratio of the CO2 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 CO2. We have calculated that the effective land biotic O-2: CO2 molar ratio ranges between 0.76 and 1.04 rather than 1.10 ( moles of O-2 produced per mole of CO2 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 CO2 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. [References: 38]