Technical note: An inverse method to relate organic carbon reactivity to isotope composition from serial oxidation

Abstract

Abstract. Serial oxidation coupled with stable carbon and radiocarbon analysis of sequentially evolved CO 2 is a promising method to characterize the relationship between organic carbon (OC) chemical composition, source, and residence time in the environment. However, observed decay profiles depend on experimental conditions and oxidation pathway. It is therefore necessary to properly assess serial oxidation kinetics before utilizing decay profiles as a measure of OC reactivity. We present a regularized inverse method to estimate the distribution of OC activation energy (E), a proxy for bond strength, using serial oxidation. Here, we apply this method to ramped temperature pyrolysis or oxidation (RPO) analysis but note that this approach is broadly applicable to any serial oxidation technique. RPO analysis directly compares thermal reactivity to isotope composition by determining the E range for OC decaying within each temperature interval over which CO 2 is collected. By analyzing a decarbonated test sample at multiple masses and oven ramp rates, we show that OC decay during RPO analysis follows a superposition of parallel first-order kinetics and that resulting E distributions are independent of experimental conditions. We therefore propose the E distribution as a novel proxy to describe OC thermal reactivity and suggest that E vs. isotope relationships can provide new insight into the compositional controls on OC source and residence time.

Ramped thermal analysis for isolating biologically meaningful soil organic matter fractions with distinct residence times

“…The 1973 soil sample was initially run using a fast ramp (20 • C min −1 ) where only CO 2 concentration was recorded. Using an inversion method (Hemingway et al, 2017a), five distinct thermal fractions were identified from this thermogram with temperature ranges of 100-325, 325-400, 400-445, 445-515 and > 515 • C (Fig. A1).…”

Evolution of biomolecular loadings along a major river system

“…A concern is the efficacy of OM removal at 350°C due to the formation of charred residues (Masiello, 2004;Wakeham et al, 2009) and thermally recalcitrant OM components (i.e. petrogenic carbon; Hemingway et al, 2017). Chars may exhibit variable and potentially high SA (>50 m 2 /g; e.g.…”

Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils

“…Rapid OC petro oxidation can occur (i) abiotically without chemical alteration, (ii) abiotically with chemical alteration, (iii) biotically without chemical alteration, or (iv) biotically with chemical alteration and 14 C-depleted biomass production (20)(21)(22). To assess alteration and track multiple OC sources within a single sample, we used Ramped PyrOx (RPO) serial combustion (23). This technique heats each sample at a constant ramp rate to separate OC on the basis of thermal lability and determines Fm values for specific temperature intervals (termed RPO fractions) (14).…”

“…Because calculated f mid depends on our choice of mid-E range (here, 150 to 185 kJ mol −1 ), it is possible that mixing distributions highlighting the differences between OC endmembers: LiWu POC exported during typhoon events (average, n = 27; black), organic-rich A-and E-horizon topsoil (green), and C-horizon saprolite (orange). Each p(0,E) distribution integrates to unity ( y-axis values not shown) (14,23). (B) E versus Fm relationships for all soils (green circles and orange triangles) and LiWu POC (white diamonds) in which RPO-fraction 14 C activity was measured.…”

“…Constraints on end-member E and Fm ranges are described in the text (blue, vascular plant OC bio ; gray, OC petro ). Black arrows represent theoretical trends for end-member mixing (vertical) and chemical alteration (horizontal) (23) and indicate that alteration is necessary to explain the presence of mid-E OC. In both panels, dashed lines separate OC into low-E (<150 kJ mol −1 ), mid-E (150 ≤ E < 185 kJ mol −1 ), and high-E (≥185 kJ mol −1 ) regions.…”