G. Almendros

Fire-induced transformation of soil organic matter from an oak forest: an experimental approach to the effects of fire on humic substances.

This study is about the effects of forest fire on the organic matter from a Dystric Xerochrept under Quercus rotundifolia. In laboratory experiments carried out to simulate the effects of fire on isolated humic fractions, several physicochemical characteristics were described in the heated samples, as well as in humic substances extracted from samples of control and postfire soil areas. The results obtained suggested that humic and fulvic acids underwent moderate structural modifications, but we found important changes in their colloidal properties, particularly regarding solubility. The composition and properties of pyromorphic humus are discussed by using a tentative model for humus transformation on the basis of different fire-induced modifications of the soil organic fractions.

13C- and 15N-NMR spectroscopic examination of the transformation of organic nitrogen in plant biomass during thermal treatment

Abstract Structural changes in lignocellulosic biomass heated under conditions comparable to those encountered in several types of natural or planned burnings have been studied by solid-state 13C- and 15N-CPMAS NMR spectroscopy of 15N-enriched ryegrass (Lolium rigidum) after being subjected to progressive thermal treatment. The solid-state 15N-NMR spectra of biomass subjected to severe heating revealed amide-N in forms which are resistant to the thermal treatment. Progressive burning was found to occur in two well-defined stages: In the early stage the free amino acid and some NH2 groups were removed, but no substantial disruption of the peptide structure was observed. In the final stage of burning the amide-N was converted to heterocyclic structures such as pyrroles, imidazoles and indoles. Some evidence for the presence of pyridines and phenazines was also found at this stage. These findings suggest that a major portion of the N is in forms that may survive most natural fires and that their stability towards further microbial degradation is increased by the heating. The solid-state 13C-NMR spectra revealed that the carbohydrate fraction is converted into condensed dehydrated material producing intense signals in the aromatic region.

13C NMR assessment of decomposition patterns during composting of forest and shrub biomass

A laboratory experiment was designed to investigate the degradation patterns of leaves from 12 forest and shrub species typical of Mediterranean ecosystems by solid-state 13 C NMR. The spectral data have been compared with those for the major organic fractions, and elementary composition in three transformation stages (zero time, intermediated and advanced (168 d)). The plant material in general showed a selective depletion of lipid and water-soluble products and a concentration in acidinsoluble residue (Klason lignin fraction), but the increasing percentage of total alkyl carbons (not observed in pine leaves) suggests that recalcitrant aliphatic material accumulates in the course of the 168 d incubation, when the total weight losses were up to 660 g kg ˇ1 . This contrasts with the fact that the concentration of extractable alkyl C (i.e. the lipid fraction) decreased in all cases. The results for the diAerent plants suggested some general transformation trends simultaneous to specific biodegradation patterns. The non-ameliorant, soil acidifying species (i.e. those a priori considered to favor the accumulation of humus with low biological activity) have high initial concentrations of extractives, alkyl structures and comparatively lower percentages of O-alkyl structures. The decay process in these species is not associated to the increase of the alkyl-to-O-alkyl ratio, which is shown by the ameliorant species. Superimposed on these major trends, the biomass of the diAerent plants underwent divergent paths in the course of composting, leading to, for example, (i) accumulation of recalcitrant, nonhydrolyzable alkyl and aromatic structures (Retama, Genista); (ii) enrichment of resistant O-alkyl structures such are stable fractions of carbohydrate and tannins (Pinus, Calluna); and (iii) accumulation of aliphatic extractives with the lowest stabilization of protein in resistant forms (Arctostaphylos, Ilex). In particular, in the acidifying species, the spectral patterns suggest that the apparent stability of the aromatic domain is compatible with selective preservation of tannins together with aliphatic structures. Such specific tendencies are also illustrated by the diAerence spectra (0 vs 168 d) which suggest that early humification processes are highly heterogeneous and distinct rather than the selective degradation of lipid and water-soluble fractions and carbohydrates, and they may include stabilization of tannins and aliphatic (cutin- and protein-like) macromolecules. 7 2000 Elsevier Science Ltd. All rights reserved.

Effects of fire on humic and lipid fractions in a Dystric Xerochrept in Spain

Fire-induced changes in humic and lipid soil fractions were assessed by two approaches. Comparisons were made of the nature and amounts of the two fractions in samples of a Dystric Xerochrept under two pine stands, one that had been burned two years earlier and one that had not. Further comparisons were made of humic and lipid fractions in a soil sample before and after heating in the laboratory. Separation and identification of the compounds in these soil fractions were carried out by preparative liquid chromatography followed by GC-MS in the case of lipids; the humic acids were previously degraded by successive treatments with sodium persulphate and potassium permanganate. The greatest differences were in the distribution patterns of soil lipids (alkanes, fatty acids): accumulation of the homologues of lower molecular weight (< C20) was dominant in the post-fire samples. For soil resin acids, these samples had higher values for dehydroabietic and secodehydroabietic acids and lower ones for pimaric acid. The degradative studies on the humic acids showed that the relative yields of aromatic products were comparatively higher in the samples after burning. Alkanoic diacids were found to be the least resistant to heating, whereas greater amounts of alkanes in the less condensed polymer structures of the humic acids were observed in the samples after burning. The relative amounts and distribution patterns of the fatty acids released showed no great differences in the humic acids.

Preservation of aliphatic macromolecules in soil humins

Abstract Two series of alkali-insoluble preparations of soil organic matter (humins) were isolated from three representative types of terrestrial humus by two methods, respectively based on (i) ultrasonic disaggregation followed by flotation in a bromoform-ethanol mixture and (ii) partition in water-methylisobutylketone. The humins amounted to between 7–42% of the total carbon in the soil heavy fraction and were characterized by pyrolysis followed by gas chromatography-mass spectrometry. The pyrolysis patterns of these particulate soil organic fractions differed quantitatively and qualitatively from those typical for terrestrial humic acid or lignocellulosic materials. The humins isolated by both methods yielded major series of alkyl products (56–81% of the total chromatographic area) including very variable amounts of alkanes, fatty acids and olefins. The aromatic pyrolysis products consisted mainly of alkyl-substituted mono and di-phenols, naphthalenes, phenanthrenes, fluorenes and methoxyphenols. The pyrograms of the humins isolated by partition were much more varied and produced a greater yield of alkanes than those isolated by flotation. The differences between humins from the three types of humus were small and tended to coincide in both series of preparations. Both the 13 C NMR spectra and the atomic ratios suggested the presence of stable carbohydrate-like material, which was not evident in the pyrograms or in the total yield of hydrolyzable sugars. The results suggest that humins represent the high-stability aggregate compartments of active terrestrial soils where structural rearrangement of aliphatic biogenic macromolecules may take place, simultaneously to the condensation of recalcitrant macromolecular material tightly bonded on reactive mineral surfaces.

Solid state NMR studies of fire-induced changes in the structure of humic substances

Abstract Several structural changes were recognized by CPMAS 13 C-NMR and resolution-enhanced IR spectroscopy after humic acids (HA) and fulvic acids (FA) were subjected to laboratory heating at 350°C. The NMR results suggested that the HA underwent decarboxylation, selective transformation of aliphatic material, and increase in the number of aromatic carbons when subjected to heating for 120 s, whereas after subsequent heating no predominant aromatic neoformations were observed, and the elimination of alkyl structures occurred preferentially at this stage. The infrared spectroscopy revealed a well-defined lignin pattern in the original HA, that tended to disappear with heating. The results obtained in the laboratory experiments agreed with several of the differences found between the NMR patterns of the humic substances from the unburnt soil area and from that affected by forest fire. It is suggested that the changes observed play important roles in the colloidal properties and the water-repellency observed in the heated humic substances and in the post-fire soil area.

Molecular alterations of organic fractions from urban waste in the course of composting and their further transformation in amended soil

Abstract The evolution of various analytical characteristics (including the concentrations of water-soluble fractions, free and esterified lipids, and humic-like substances) of solid urban waste in the course of a 7-week composting process have been monitored in two independent piles composted during different seasons (Spring/Autumn). Whereas the concentration of water-soluble and mineral fractions tended to increase during composting, the opposite was observed for total organic matter and free lipid. Unmatured compost showed comparatively high amounts of esterified lipid. The absence of monotonic trends shown by this fraction, and by total humic-like substances indicate two successive stages of compost microbial reworking. Gas chromatography-mass spectrometry (GC-MS) analysis of the lipid fractions progressively removed by step-wise chemolysis allows the appraisal of different lipid species, which differ in their resistance to biodegradation. Extended composting does not contribute to the accumulation of progressively aromatised humic-type materials similar to those present in soil. Both pyrolysis (Py)-GC-MS and spectroscopic techniques (Fourier transform infrared spectroscopy, FT-IR, and 13 C nuclear magnetic resonance spectroscopy, NMR) confirm that compost-derived humic acid-like substances are not structurally comparable to soil humic acids. Finally, the fate of the compost organic fractions has been investigated in two soils amended with different doses of the final compost. In general, the most conspicuous compost-induced effects were reflected by the differences in the qualitative and quantitative composition of the soil lipid. The results indicate that compost application cannot be considered to contribute to the mid-term accumulation of stable forms of organic matter in soil.

13C and 15N NMR analysis of some fungal melanins in comparison with soil organic matter

A variety of fungal melanins with natural 15N abundance are characterized by solid-state 13C and 15N NMR spectroscopy and are compared to solid-state 13C and 15N NMR spectra of organic matter from representative soils. In all solid-state 15N NMR spectra the peptide/amide region (−220 to −285 ppm) dominates with more than 70% of the total intensity. The region between −285 and −375 ppm, assigned to amino and ammonium groups, always contains more than half of the remaining intensity. The area in the region from −30 to −220 ppm, where aromatic heterocycles would show signals, makes up less than 10% of the total intensity. These findings call into question common structural models for melanins. The solid-state 13C NMR spectra, on the other hand, reveal large differences when the melanins are compared to each other, and to composts and soils. The concentration of the aromatic carbon varies from 5 to 40% in the melanin series. The ratio CaroNtot and CaliNtot were calculated, and confirm that nitrogen in these samples is bound in Ca-groups rather than in aromatic heterocyclic structures.

Solid-state spectroscopic analysis of lignins from several Austral hardwoods

In order to gain information about lignin molecular characteristics with a direct bearing on the remarkable susceptibility of some Austral hardwoods to biological delignification, milled-wood lignins were isolated and analyzed by spectroscopic techniques in the solid state. Cross polarization and magic-angle spinning 13C nuclear magnetic resonance (13C CPMAS NMR) and Fourier-transform infrared (FTIR) spectra of the lignin preparations were obtained. The most diagnostic peaks were assigned and quantified as percentages of the total spectral area, and the differences observed discussed in terms of lignin composition. The spectral patterns obtained revealed that the woods from Gevuina avellana, Eucryphia cordifolia and Nothofagus dombeyii have lignin with high syringyl/guaiacyl ratio, as evidenced by relative areas of 13C NMR signals at 153 and 148 ppm, and FTIR bands at 1,335 and 1,275 cm-1. The presence of syringyl-rich lignins, characterized by lower redox potential and condensation degree than guaiacyl-rich lignins, could be a structural factor contributing to the ease of extensive delignification of these woods by white-rot fungi.

Transformation of wheat straw in the course of solid-state fermentation by four ligninolytic basidiomycetes

Biological upgrading of wheat straw by the white-rot fungi Phanerochaete chrysosporium , Pleurotus eryngii, Phlebia radiata, and Ceriporiopsis subvermisporawas monitored during 60-day solid-state fermentation. Analysis of straw included determination of weight loss and lignin content, color analysis, and infrared spectroscopy, whereas the studies on the water-soluble fractions were carried out by infrared spectroscopy, elementary analyses and quantification of the total phenols and reducing sugars. The most selective degradation of lignin was produced by P. eryngii and especially by C. subvermispora, the former species releasing the greatest amount of colored water-soluble products, whereas an increase in straw brightness was caused by C. subvermispora.In general, the composition of the water-soluble fraction correlated with the extent of straw transformation. The initial fermentation stage (0 ‐15 days) was characterized by the accumulation of water-soluble products from lignin degradation and fungal metabolism, the concentration of which tended to stabilize in the second stage (16 ‐ 60 days). The degree of delignification at the second stage tended to coincide with the decrease of the water-soluble nitrogen.