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

• Published in: Biogeosciences 2021-03, Vol.18 (5), p.1703-1718
• Main Authors: Chassé, Mathieu, Luftalla, Suzanne, Cécillon, Lauric, Baudin, François, Abiven, Samuel, Chenu, Claire, Barré, Pierre
• Format: Article
• Language: English
• Subjects: Analysis; Analytical methods; Atmospheric carbon dioxide; Benzene; Biogeochemistry; Burning; Carbon; Carbon content; Chemical properties; Chemicophysical properties; Chemistry; Clay; Dynamics; Earth Sciences; Evolution; Experiments; Fractionation; Geochemistry; Granulometry; Greenhouse gases; Heterogeneity; Microorganisms; Mineralization; Mineralogy; Organic carbon; Organic matter; Organic soils; Polycarboxylic acids; Raman spectroscopy; Sciences of the Universe; Soil; Soil chemistry; Soil dynamics; Soil organic matter; Soil properties; Soil stability; Soils; Spectroscopy; Stability analysis; Standard deviation; Stock assessment; Thermal stability; Thermodynamic properties; Total organic carbon
• ISSN: 1726-4189; 1726-4170; 1726-4189
• DOI: 10.5194/bg-18-1703-2021

Evolution of organic carbon content in soils has the potential to be a major driver of atmospheric greenhouse gas concentrations over the next century. Understanding soil carbon dynamics is a challenge due to a wide range of residence times of soil organic matter and limited constraints on the mechanisms influencing its persistence. In particular, large uncertainties exist regarding the persistence of pyrogenic organic carbon in soils. In order to characterize organic matter with varying degrees of persistence and to distinguish pyrogenic organic carbon, we combined Rock-Eval analysis, a thermo-chemical method, with the benzene polycarboxylic acid molecular marker method and Raman spectroscopy to characterize samples from long-term bare-fallow experiments, progressively depleted in the most labile organic carbon over time. Considering the heterogeneity of soil samples, size fractions have been separated to distinguish pools of organic carbon with distinct properties. We observe that organic carbon dynamics is dependent on granulometry. A pool of organic carbon with intermediate residence times, from years to a few decades, representing ca. 65 % of the bulk soil organic carbon stock, is mainly associated with fine fractions ( 20 µm). With time under bare fallow, this organic carbon is progressively transferred towards finer fractions through the breakdown of organic matter. Coarse fractions ( 20 µm) are rich in centennially persistent organic carbon, representing ca. 20 % of the initial organic carbon stock, due to the chemical recalcitrance of organic matter in these fractions, dominated by pyrogenic organic carbon. A second pool of persistent organic carbon, representing ca. 15 % of the initial organic carbon stock, is associated with the clay fraction, indicating mechanisms of protection occurring at the submicron scale ( 2 µm). This persistent organic carbon only represents 30 % of the organic carbon initially present in the clay fraction. Persistent organic carbon exhibits heterogeneous chemical signatures depending on the considered pool but a consistent thermal signature demonstrating the relationship between thermal stability and biogeochemical stability of soil organic carbon. This gives the possibility of assessing the size of the persistent organic carbon pool in soils using thermal parameters. The persistence of pyrogenic organic carbon in the clay fraction is similar to the one of total organic carbon. The different persistence of coarse and