Pyrolytic appraisal of the lignin signature in soil humic acids: assessment of its usefulness as carbon sequestration marker

Abstract

Environmental factors related with the spatial variability of the structure of humic acids (HA) from mountain calcimorphic soils subjected to various use and management were explored by infrared (IR) spectroscopy and analytical pyrolysis (Py-GC/MS). It was found that HAs pyrograms displayed a more or less marked lignin pattern (i.e., characteristic pyrolytic methoxyphenols assemblages), which was also associated to typical infrared pattern for lignin. These two analysis were used as molecular-level surrogates to establish the balance between complementary mechanisms of soil C sequestration; one group of HAs typically produced major yields of methoxyl-lacking aromatics and a poor yield of alkyl compounds, which suggest efficient cleavage of biomacromolecules and active microbial synthesis and condensation processes, whereas other HAs group yielded upon pyrolysis conspicuous series of methoxyphenols, in some cases accompanied by well-defined alkyl series. The former HAs displayed broadband IR spectra, and showed high optical density and polynuclear quinoid chromophors considered of fungal origin. The IR spectra of the second group displayed lignin and amide intense bands, intense 2920 cm-1 alkyl stretching band, a low optical density and high E465/E665 extinction ratio, all indicative of a marked aliphatic character and low macromolecular sizes. This is interpreted as recent diagenetic alteration of young organic matter and suggests the occurrence of C sequestration mechanisms based mainly on the stabilization of plant biomacromolecules together with aliphatic structures. In general, our results indicate that lignin alteration patterns of soil HA are highly responsive for the different soil C sequestration mechanisms, and could be reliable indicators of the changes introduced by soil use and management.