Long-term bare-fallow soil fractions reveal thermo-chemical properties controlling soil organic carbon dynamics

Abstract. Evolution of organic carbon content in soils has the potential to be a major driver atmospheric greenhouse gas concentrations over next century. Understanding soil dynamics is challenge due wide range residence times matter and limited constraints on mechanisms influencing its persistence. In particular, large uncertainties exist regarding persistence pyrogenic soils. order characterize with varying degrees distinguish carbon, we combined Rock-Eval analysis, thermo-chemical method, benzene polycarboxylic acid molecular marker method Raman spectroscopy samples from long-term bare-fallow experiments, progressively depleted most labile time. Considering heterogeneity samples, size fractions have been separated pools distinct properties. We observe that dependent granulometry. A pool intermediate times, years few decades, representing ca. 65 % bulk stock, mainly associated fine (< 20 µm). With time under bare fallow, this transferred towards finer through breakdown matter. Coarse (> µm) are rich centennially persistent initial chemical recalcitrance these fractions, dominated by carbon. second 15 clay fraction, indicating protection occurring at submicron scale 2 This only represents 30 initially present fraction. Persistent exhibits heterogeneous signatures depending considered but consistent thermal signature demonstrating relationship between stability biogeochemical gives possibility assessing using parameters. The fraction similar one total different coarse resides highly condensed nature sand-sized which may result burning temperatures 700 ∘C. Pyrogenic not inert increase level condensation all showing role quality Overall, study helps improve separation, evaluation characterization insight into mechanistic origin dynamics.