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

Abstract

Atmospheric O2 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 O2. The molar ratio of the CO2 and O2 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 O2. 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 O2 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 O2 and slightly increase atmospheric CO2. We have calculated that the effective land biotic O2:CO2 molar ratio ranges between 0.76 and 1.04 rather than 1.10 (moles of O2 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 O2 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 O2 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.