Characterizing soil organic carbon spatial and seasonal variability using Rock-Eval and CO₂/O₂ fluxes measurements

Abstract. Soil organic matter (SOM) stores most of the terrestrial carbon, and changes in this storage can have a significant effect on the global carbon cycle. Various approaches have been used to understand the SOM transformations and stability. Here we tested the combination of two complementary approaches: 1) We estimated the long-term SOM stability, and the effect of decomposition on the stability and composition of the remaining fraction, by Rock-Eval pyrolysis. 2) We measured the respiratory CO₂ and O₂ fluxes, and their ratio (the Apparent Respiratory Quotient – ARQ) in soil incubations at different temperatures, to learn about the short-term processes. To study in detail the spatial and temporal variability, we examine soil samples from two sample sets: a regional set, and a local set that was used to study the seasonality and environmental variability within a site. The Rock-Eval analysis showed an effect of the slope aspect on SOM. The south-facing slope organic matter was more mature and stable. For the particulate organic matter fraction, we found an increase in O₂ uptake rate with an increase in Hydrogen Index (HI), indicating that the respiration rates are higher when the reduced and easily degradable fraction is larger. The soil incubation experiments showed an increase in the ARQ values with temperature. This can be explained by higher respiration rates at high temperatures and the formation of anoxic microsites where electron acceptors alternative to O₂ are used. This suggestion was supported by incubation of soil clods that at 23°C resulted in ARQ values >1, implying anerobic conditions, while the addition of O₂ to the headspace lowered the ARQ. Based on additional experiments, we further suggest that incubations at low temperatures can reflect the history of the soil and indicate past anaerobic conditions that resulted in soil rich in reduced chemical species, resulting in a lower ARQ.  This line of reasoning can explain the lower ARQ measured for soil sampled in winter, since high moisture content limits oxygen diffusion and creates anaerobic microsites. Combining the measurements of ARQ with Rock Eval pyrolysis can provide a more complete understanding of the state of the organic matter in the soil.