Abstract
Degree of oxidation of organic carbon (Cox) is a fundamental property of the carbon cycle, reflecting the synthesis and decomposition of natural organic matter. Cox is also related to ecosystem oxidative ratio (OR), the molar ratio of O2 to CO2 fluxes associated with net ecosystem exchange (NEE). Here we compare two methods for measuring Cox and OR: (1) %C, %H, %N, and %O elemental analysis, and (2) heat of combustion (ΔHc) measured by means of bomb calorimetry coupled with %C elemental analysis (hereafter referred to as calorimetry). Compared with %C, %N, %H, and %O elemental analysis, calorimetry generates Cox and OR data more rapidly and cheaply. However, calorimetric measurements yield less accurate Cox and OR data. We additionally report Cox and OR data for a pair of biomass standards and a suite of biomass samples. The OR values we measured in these samples were less variable than OR data reported in the literature (generated by simultaneous measurement of ecosystem O2 and CO2 gas mixing ratios). Our biomass OR values had a mean of 1.03 and range of 0.99–1.06. These estimates are lower than the OR value of 1.10 that is often used to partition uptake of fossil fuel CO2 between the ocean and the terrestrial biosphere.
Highlights
We show in this paper that carbon oxidation state (Cox) is mathematically related to ecosystem oxidative ratio (OR), the molar ratio of O2 and CO2 fluxes associated with net ecosystem exchange (NEE)
We calculated theoretical Cox values for the pure standards and compared these values to those generated by elemental analysis (Figure 2a and Table 2a) and by calorimetry (Figure 2b and Table 2b)
Elemental Analysis (EA)-derived Cox values fell along a 1:1 line when plotted versus theoretical Cox values (Figure 2a) with an average error of ±0.045 Cox units and an average error of OR ± 0.011 units
Summary
[2] The oxidation state of organic carbon (Cox) is a fundamental property of the Earth’s carbon cycle. [13] Deriving the relationship between Cox and OR requires balancing the equation for organic matter synthesis (or oxidation) and calculating the ratio of O2/CO2 fluxes. [19] Another situation where the N2-based Cox to OR conversion equation may be appropriate is when accumulation of ecosystem biomass draws upon internal ecosystem reservoirs of N This may occur, for example, if a carbon sink within an ecosystem is driven by climate warming that stimulates decomposition and a transfer of N from soil organic matter (with a low C to N ratio) to woody biomass (with a high C to N ratio) [Field et al, 1992]. [20] In contrast with the case of internal N cycling, terrestrial carbon storage driven by deposition of nitric acid would require use of equation (7) and would yield higher OR values (Table 1).
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