Changes in the Rock-Eval signature of soil organic carbon upon extreme soil warming and chemical oxidation - A comparison

Poeplau, C., Barré, P., Cécillon, L., Baudin, F., & Sigurdsson, B. D. (2019). Changes in the Rock-Eval signature of soil organic carbon upon extreme soil warming and chemical oxidation - A comparison. Geoderma, 337, 181–190.

Soil warming can increase soil organic carbon (SOC) mineralization, triggering a positive climate‑carbon cycle feedback loop. Globally, many soil warming experiments have examined losses of bulk SOC, but few have assessed changes in quality. Accurate knowledge of the latter is required for an in-depth understanding and improved prediction of SOC feedback to climate change. In this study, we used Rock-Eval thermal analysis (RE6) to characterize shifts in SOC thermal stability and bulk chemistry after six years of geothermal warming by 0.6 °C, 1.8 °C, 3.9 °C, 9.9 °C, 16.3 °C, 40 °C, and 80 °C in an Icelandic grassland topsoil (0–10 cm). We also used the strong warming-induced depletion of SOC (up to 92% in the 80 °C soil) in comparisons of chemical oxidation-resistant and biogeochemically resistant SOC, which are generally assumed to be similar in nature. Sodium hypochlorite (NaOCl) and hydrogen peroxide (H2O2) were used for oxidation. Warming-resistant SOC was strongly depleted in hydrocarbons and enriched in oxygen, confirming that SOC oxidation state, and thus energy content, is an important driver for biogeochemical stability. This was supported by findings that thermal stability, i.e., the amount of energy (temperature) necessary to pyrolyze or oxidize SOC, strongly increased with warming intensity. Of the 31 RE6 parameters tested, the most warming-sensitive were hydrogen index (HI, ρ = −0.84), oxygen index (OIRE6, ρ = 0.83), proportion of total pyrolyzed carbon released as hydrocarbons at 200–650 °C (S2/PC, ρ = −0.86), and the temperature at which a certain proportion of CO2 evolved during pyrolysis (ρ > 0.8). Chemical oxidation of unwarmed soil caused average relative SOC losses of 61% (NaOCl) and 91% (H2O2) and shifts in RE6 properties that differed strongly from warming-induced shifts at comparable SOC losses. Chemical oxidation-resistant SOC was more enriched in oxygen, but slightly enriched in hydrocarbons, and less thermostable than comparable naturally depleted SOC at the same time. A certain overlap, especially for NaOCl-treated soils, is likely, while H2O2-oxidized soils showed very distinct RE6 properties. We concluded that i) soil warming leads to strong shifts in SOC bulk chemistry and thermal stability and ii) H2O2 should be avoided in isolation of a slow SOC kinetic pool.