@article {2961,
	title = {Persistence of old soil carbon under changing climate: The role of mineral-organic matter interactions},
	journal = {Chemical Geology},
	volume = {587},
	year = {2022},
	month = {Jan-01-2022},
	pages = {120629},
	abstract = {Globally, soils store between 1500 and 2800 Pg of organic carbon (OC). The physical and chemical stability of these terrestrial soil carbon stores under plausible climate change scenarios is unclear. Soil organic carbon (SOC), especially in volcanic soils, is stabilized through mineral matrix interactions. How susceptible are these mineral-organic matter interactions to environmental change? Here we present a study of SOC age along a climate gradient of andisols from Kohala volcano on the Island of Hawai{\textquoteright}i. We measure carbon isotope composition (14C/12C, 13C/12C) in bulk samples and extracted biomarkers for 4{\textendash}8 horizons of 15 soil profiles to understand variability in SOC age and persistence across incremental differences in mean annual precipitation. Bulk OC in the subsoil has radiocarbon fraction modern (Fm) values as low as 0.28 to 0.16 (~10,160 to ~14,630 conventional radiocarbon years). Coexisting plant-derived long chain fatty acids (LCFAs) are older, over 22,500 yrs. (Fm = 0.060), implying that these are among the most stable compounds in the soil, while corresponding shorter-chain (C16) fatty acids are much younger, consistent with an origin from active microbial communities assimilating young OC percolating from surface horizons. There is significant Fe loss at higher mean annual precipitation (MAP) (>2200 mm yr-1) sites associated with episodic soil saturation and microbial Fe reduction. \%OC is higher at these sites, consistent with the expectation that saturated conditions promote SOC storage. However, in these higher MAP sites iron depletion is associated with much younger bulk SOC and LCFAs 14C ages (~2900 14C years) than at equivalent sample depths in sites that retain most Fe (~14,200 14C years). The remaining mineral matrix consists primarily of Si, Al, and Ti as SRO minerals. The data imply that modest increases in precipitation resulting from environmental change at locations near a potential saturation or redox threshold could result in destabilization of Fe-SOC complexes, rendering previously stabilized carbon available for rapid degradation, potentially irreversibly decreasing the size of the old SOC reservoir. The destabilization of an old, persistent Fe-SOC reservoir can decrease SOC storage and ultimately increase the amount of CO2 released to the atmosphere.},
	issn = {00092541},
	doi = {10.1016/j.chemgeo.2021.120629},
	url = {https://linkinghub.elsevier.com/retrieve/pii/S0009254121005726},
	author = {Grant, Katherine E. and Galy, Valier V. and Haghipour, Negar and Eglinton, Timothy I. and Derry, Louis A.}
}
@article {2764,
	title = {Mineral protection regulates long-term global preservation of natural organic carbon},
	journal = {Nature},
	volume = {570},
	year = {2019},
	month = {Jan-06-2019},
	pages = {228 - 231},
	abstract = {The balance between photosynthetic organic carbon production and respiration controls atmospheric composition and climate1,2. The majority of organic carbon is respired back to carbon dioxide in the biosphere, but a small fraction escapes remineralization and is preserved over geological timescales3. By removing reduced carbon from Earth{\textquoteright}s surface, this sequestration process promotes atmospheric oxygen accumulation2 and carbon dioxide removal1. Two major mechanisms have been proposed to explain organic carbon preservation: selective preservation of biochemically unreactive compounds4,5 and protection resulting from interactions with a mineral matrix6,7. Although both mechanisms can operate across a range of environments and timescales, their global relative importance on 1,000-year to 100,000-year timescales remains uncertain4. Here we present a global dataset of the distributions of organic carbon activation energy and corresponding radiocarbon ages in soils, sediments and dissolved organic carbon. We find that activation energy distributions broaden over time in all mineral-containing samples. This result requires increasing bond-strength diversity, consistent with the formation of organo-mineral bonds8 but inconsistent with selective preservation. Radiocarbon ages further reveal that high-energy, mineral-bound organic carbon persists for millennia relative to low-energy, unbound organic carbon. Our results provide globally coherent evidence for the proposed7 importance of mineral protection in promoting organic carbon preservation. We suggest that similar studies of bond-strength diversity in ancient sediments may reveal how and why organic carbon preservation{\textemdash}and thus atmospheric composition and climate{\textemdash}has varied over geological time.},
	issn = {0028-0836},
	doi = {10.1038/s41586-019-1280-6},
	url = {http://www.nature.com/articles/s41586-019-1280-6},
	author = {Hemingway, Jordon D. and Rothman, Daniel H. and Grant, Katherine E. and Rosengard, Sarah Z. and Eglinton, Timothy I. and Derry, Louis A. and Galy, Valier V.}
}
@article {2788,
	title = {Thermal oxidation of carbon in organic matter rich volcanic soils: insights into SOC age differentiation and mineral stabilization},
	journal = {Biogeochemistry},
	volume = {144},
	year = {2019},
	month = {Jan-09-2019},
	pages = {291 - 304},
	abstract = {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 {\textquoteleft}rampedpyrox{\textquoteright} model yielded consistent mean E values of 140 kJ mol-1 below the Oa horizon. The {\textquoteleft}rampedpyrox{\textquoteright} 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 {\textmu}Ef 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.},
	issn = {0168-2563},
	doi = {10.1007/s10533-019-00586-1},
	url = {http://link.springer.com/10.1007/s10533-019-00586-1},
	author = {Grant, Katherine E. and Galy, Valier V. and Chadwick, Oliver A. and Derry, Louis A.}
}
