Abstract
AbstractMeasurements of atmospheric O2 and CO2 concentrations serve as a widely used means to partition global land and ocean carbon sinks. Interpretation of these measurements has assumed that the terrestrial biosphere contributes to changing O2 levels by either expanding or contracting in size, and thus serving as either a carbon sink or source (and conversely as either an oxygen source or sink). Here, we show how changes in atmospheric O2 can also occur if carbon within the terrestrial biosphere becomes more reduced or more oxidized, even with a constant carbon pool. At a global scale, we hypothesize that increasing levels of disturbance within many biomes has favored plant functional types with lower oxidative ratios and that this has caused carbon within the terrestrial biosphere to become increasingly more oxidized over a period of decades. Accounting for this mechanism in the global atmospheric O2 budget may require a small increase in the size of the land carbon sink. In a scenario based on the Carnegie–Ames–Stanford Approach model, a cumulative decrease in the oxidative ratio of net primary production (NPP) (moles of O2 produced per mole of CO2 fixed in NPP) by 0.01 over a period of 100 years would create an O2 disequilibrium of 0.0017 and require an increased land carbon sink of 0.1 Pg C yr−1 to balance global atmospheric O2 and CO2 budgets. At present, however, it is challenging to directly measure the oxidative ratio of terrestrial ecosystem exchange and even more difficult to detect a disequilibrium caused by a changing oxidative ratio of NPP. Information on plant and soil chemical composition complement gas exchange approaches for measuring the oxidative ratio, particularly for understanding how this quantity may respond to various global change processes over annual to decadal timescales.
Highlights
Observed changes in atmospheric O2 and CO2 levels during the 1990s have been used to infer a net land carbon sink (Keeling & Shertz, 1992; Bender et al, 1994; Keeling et al, 1996; Battle et al, 2000; Prentice et al., 2001)
This disequilibrium was equivalent to a 0.113 Pg C yrÀ1 terrestrial carbon source in terms of impacts on global atmospheric O2 levels (obtained via Eqns (4) and (5))
We examined for the first time levels of disequilibrium between Rab and Rba within the terrestrial biosphere that would have an impact on our interpretation of O2 measurements used for land and ocean carbon sink partitioning
Summary
Observed changes in atmospheric O2 and CO2 levels during the 1990s have been used to infer a net land carbon sink (Keeling & Shertz, 1992; Bender et al, 1994; Keeling et al, 1996; Battle et al, 2000; Prentice et al., 2001) The basis for this conclusion is that atmospheric O2 levels decreased more slowly than expected from the oxidation of fossil fuels consumed during the decade, and that a net land carbon sink creates O2 (and could account for the difference between the observations and the fossil fuels) (Keeling, 1988; Keeling et al, 1996). Note that a sink has a negative sign The net ocean carbon flux This quantity is solved for from Eqns (4) and (5). Note that a sink has a negative sign The net flux of O2 from the ocean surface; we used a value of 0.4 ppm O2 yrÀ1 from Plattner et al (2002) in Fig. 1 The oxidative ratio of organic matter.
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