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

In Press.-- 9 páginas.-- 4 figuras.-- 3 tablas.-- 40 referenciasLignin 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 1030 cm−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 2920 cm−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.the Spanish Ministry of Science and Innovation by CTM2005-04739/CGL2008-04926/CARBORAR CGL2011-27493/GEOFIRE CGL2012-38655-C04-01 research projects. Dr. Ana Piedra Buena has been contracted by the CCMA-CSIC via I3P Program supported by the European Social Fund. Dr. Isabel Miralles received postdoctoral fellowships Juan de la Cierva 2008-39669 and a Marie Curie Intra-European Fellowship (FP7-PEOPLE-2013-IEF, Proposal No. 623393).Peer reviewe

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

Poster presentado en el 20th International Symposium on Analytical and Applied Pyrolysis (PYRO2014) 19th-23rd May, 2014Environmental 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.Peer Reviewe