Thermal oxidation of carbon in organic matter rich volcanic soils: insights into SOC age differentiation and mineral stabilization
Radiocarbon ages and thermal stability measurements can be used to estimate the stability of soil organic carbon (OC). Soil OC is a complex reservoir that contains a range of compounds with different sources, reactivities, and residence times. This heterogeneity can shift bulk radiocarbon values and impact assessment of OC stability and turnover in soils. Four soil horizons (Oa, Bhs, Bs, Bg) were sampled from highly weathered 350 ka Pololu basaltic volcanics on the Island of Hawaii and analyzed by Ramped PyrOX (RPO) in both the pyrolysis (PY) and oxidation (OX) modes to separate a complex mixture of OC into thermally defined fractions. Fractions were characterized for carbon stable isotope and radiocarbon composition. PY and OX modes yielded similar results. Bulk radiocarbon measurements were modern in the Oa horizon (Fm = 1.013) and got progressively older with depth: the Bg horizon had an Fm value of 0.73. Activation energy distributions (p(E)) calculated using the ‘rampedpyrox’ model yielded consistent mean E values of 140 kJ mol−1 below the Oa horizon. The ‘rampedpyrox’ model outputs showed a mostly bimodal distribution in the p(E) below the Oa, with a primary peak at 135 kJ mol−1 and a secondary peak at 148 kJ mol−1, while the Oa was dominated by a single, higher E peak at 157 kJ mol−1. We suggest that mineral-carbon interaction, either through mineral surface-OC or metal-OC interactions, is the stabilization mechanism contributing to the observed mean E of 140 kJ mol−1 below the Oa horizon. In the Oa horizon, within individual RPO analyses, radiocarbon ages in the individual thermal fractions were indistinguishable (p > 0.1). The flat age distributions indicate there is no relationship between age and thermal stability (E) in the upper horizon (> 25 cm). Deeper in the soil profile higher µEf values were associated with older radiocarbon ages, with slopes progressively steepening with depth. In the deepest (Bg) horizon, there was the largest, yet modest change in Fm of 0.06 (626 radiocarbon years), indicating that older OC is slightly more thermally stable.
KeywordsSoil organic carbon (SOC) Ramped pyrolysis/oxidation (RPO) Radiocarbon
We would like to thank the staff at NOSAMS for their generous laboratory help and guidance, especially A. Mchnicol, M. Lardie Gaylord, and A. Gagnon. K. Grant would like to thank J. Hemingway for his assistance with the RPO instrument and help with the ‘rampedpyrox’ code, and Gregg McElwee for assistance with ICP instrumentation at Cornell. Marc Kramer helped with field sampling and interpretation of soil organic C properties in the soil horizon.
This work was partially supported by the Cornell University Atkinson Center Small Grant program and by NSF EAR 1660923 (Derry, PI). KEG was supported by the Cross-Scale Biogeochemistry and Climate – National Science Foundation Integrative Graduate Education and Research Traineeship grant#DGE-1069193 and the Cornell Graduate Fellowship.
- Barre P, Plante AF, Cecillon L, Lutfalla S, Baudin F, Bernard S, Christensen BT, Eglin T, Fernandez JM, Houot S, Katterer T, Le Guillou C, Macdonald A, van Oort F, Chenu C (2016) The energetic and chemical signatures of persistent soil organic matter. Biogeochemistry 130(1–2):1–12CrossRefGoogle Scholar
- Giambelluca TW, Nullet MA, Schroeder TA (1986) Rainfall atlas of Hawai’i. Department of Land and Natural Resources, Hawai’iGoogle Scholar
- Hemingway JD (2016) rampedpyrox: open-source tools for thermoanalytical data analysis. http://pypi.python.org/pypi/rampedpyrox. Accessed 5 Aug 2018
- Kleber M, Mikutta R, Torn, Jahn R (2005) Poorly crystalline mineral phases protect organic matter in acid subsoil horizons. Eur J Soil Sci 56(6):717–725Google Scholar
- Lopez-Capel E, Krull ES, Bol R, Manning DAC (2008) Influence of recent vegetation on labile and recalcitrant carbon soil pools in central Queensland, Australia: evidence from thermal analysis-quadrupole mass spectrometry-isotope ratio mass spectrometry. Rapid Commun Mass Spectrom 22(11):1751–1758CrossRefGoogle Scholar
- Marín-Spiotta E, Gruley KE, Crawford J, Atkinson EE, Miesel JR, Greene S, Cardona-Correa C, Spencer RGM (2014) Paradigm shifts in soil organic matter research affect interpretations of aquatic carbon cycling: transcending disciplinary and ecosystem boundaries. Biogeochemistry 117(2–3):279–297CrossRefGoogle Scholar
- Mikutta R, Schaumann GE, Gildemeister D, Bonneville S, Bonneville S, Kramer MG, Chorover J, Chadwick OA, Guggenberger G (2009) Biogeochemistry of mineral-organic associations across a long-term mineralogical soil gradient (0.3–4100 kyr), Hawaiian Islands. Geochim et Cosmochim Acta 73(7):2034–2060CrossRefGoogle Scholar
- Zhang XW, Bianchi TS, Cui XQ, Rosenheim BE, Ping CL, Hanna AJM, Kanevskiy M, Schreiner KM, Allison MA (2017) Permafrost organic carbon mobilization from the watershed to the Colville River delta: evidence from C-14 ramped pyrolysis and lignin biomarkers. Geophys Res Lett 44(22):11491–11500CrossRefGoogle Scholar