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

Abstract

Lignin markers in humic acids (HA, the alkali-soluble, acid-insoluble soil organic matter fraction) molecular features are explored to assess the extent to which plant biomacromolecules are progressively transformed by humification processes leading to stable C-forms in soils. Humic acids extracted from a collection of mountain calcimorphic soils from Sierra María-Los Vélez Natural Park (Southeastern Spain) under different use and management practices were studied in detail by visible and infrared (FT-IR) spectroscopies and analytical pyrolysis (Py-GC/MS). The HAs display a more or less marked lignin pattern defined by characteristic methoxyphenol assemblages released after pyrolysis that are associated to a typical infrared pattern including absorption frequencies bands at 1510, 1460, 1420, 1270, 1230 and 1030cm−1. This variability in the HA spectroscopic and pyrolytic patterns was used as a source of molecular-level surrogates to establish the balance between complementary mechanisms of soil C sequestration i.e., a selective preservation of lignin associated to raw organic matter and other plant-inherited macromolecules, or alternative mechanisms involving microbial breakdown or plant precursors and its condensation with microbial metabolites.We found that HAs in which the lignin signature was comparatively less marked also show high optical density values suggesting unsubstituted, condensed aromatic units and a chaotic organic structure, pointing to the presence of highly resilient carbon forms. Upon analytical pyrolysis, one group of HAs produced major yields of methoxyl-lacking aromatics (alkylbenzenes and alkylphenols), and poor yields of alkyl compounds, which suggest efficient cleavage of biomacromolecules and the occurrence of active microbial synthesis and condensation processes. In fact, these HAs also displayed broadband IR spectra, and visible spectra showing high optical density and polynuclear quinoid chromophors considered of fungal origin. Other group of HAs yielded upon pyrolysis conspicuous series of methoxyphenols and well-defined alkyl series (alkanes, alkenes and fatty acids). The IR spectra also displayed clear lignin and amide bands, as well as intense 2920cm−1 band and a low optical density, indicative of a marked aliphatic character. This latter is interpreted as the result of recent diagenetic alteration processes of young organic matter and suggests that C sequestration mechanisms in these soils are mainly based on the stabilization of HAs from plant biomacromolecules and aliphatic structures.These differential lignin alteration patterns indicate that HAs are responsive to soil C sequestration mechanisms, which in the studied soils seem to relay upon microtopographical features rather than to changes in soil use and management.