A two-step thermochemolysis for soil organic matter analysis. Application to lipid-free organic fraction and humic substances from an ombrotrophic peatland

• An original two-step thermochemolysis method based on reactivity differences is proposed.
• Two chemical agents HMDS (silylation) and TMAH (transmethylation) were selected.
• Additional information on complex natural organic matters were provided.
• Mostly sugars and monomers from biopolymers such as lignin, cutins and suberins were identified in the first and second step respectively.

A two-step off-line thermochemolysis was developed to characterize successively free and bound compounds in humic fractions from a wooded Sphagnum-dominated mire peat (Frasne, Jura Mountains, France). Two reactants with different strengths were used: hexamethyldisilazane (HMDS) followed by tetramethylammonium hydroxide (TMAH). Analysis of the thermolysates showed obvious differences between the two steps regarding the nature and/or the distribution of the numerous identified products. The first step with HMDS revealed the occurrence of thermolabile compounds and/or molecular units occurring initially in (bio)polymers susceptible to be cracked by HMDS, mostly carbohydrates and aromatics. The second step with TMAH released products resulting from the cracking of units with a relatively high thermal and/or chemical stability (monomers from biopolymers such as lignin and cutins, suberins or waxes). Two-step thermochemolysis thus appears as an interesting tool to help deciphering the composition, origin and nature of soil and sediment organic matter. Moreover, the comparison of the three humic substances fractions (fulvic acids, humic acids and humin) shows that a part of the analyzed compounds (lipids, aromatic and carbohydrates) were lost during this chemical fractionation.