Pilar Tinoco

Impact of the vegetation on the lignin pyrolytic signature of soil humic acids from Mediterranean soils

Abstract Humic acids (HAs) from 16 soils in Continental Mediterranean areas under potential vegetation consisting of sclerophyllic, mesophyllic or conifer forests in Madrid (Spain) were studied by Curie–Point pyrolysis. Statistical analyses based on the absolute and the relative abundances of the whole compound assemblages released by pyrolysis suggested that a considerable portion of the variance (inertia) associated with the vegetation was accounted for the lignin-derived compounds. In the studied samples, a set of 12 index-methoxyphenols was considered to have a discriminating potential comparable to that of the classical index phenols released from sedimentary organic matter by wet CuO alkaline oxidation. The ecological features most accurately reflected by the methoxyphenolic patterns consisted of: (i) the expected lack of syringol derivatives in HAs formed in soils under Gymnosperm plants, (ii) the diagnostic patterns of guaiacols in the HAs from soils under different species of Angiosperms ( Quercus spp., Fraxinus , Castanea , grasses); (iii) the responsivity of the methoxyphenols to the extent of some types of soil perturbations: the highest environmental impact on the HA structure was found in soil after severe burning, not in the soil affected by medium-intensity fire.

Nitrogen speciation and pyrolytic patterns of 15N-labelled soil and compost fractions

A mixture of urban waste, wheat straw and K 15NO3 was subjected to 80-day laboratory composting in order to analyse the qualitative and quantitative speciation patterns of the newly formed N compounds in the different compost fractions, i.e. water-soluble fraction (WS), colloidal fractions — humic acid-like (HA) and fulvic acid-like (FA), and alkali-insoluble organic fractions. In a further experiment, the compost was added to a mineral soil (3.1 g compost per 100 g soil), which was subjected to additional incubation for 80 days. The above organic fractions in addition to two particulate ones were isolated from the soil. The stable isotope ratios (15N/14N) of the resulting compost (incorporating 21.8% N as 15N) and soil fractions were analysed to monitor the distribution of the N in the different soil organo-mineral compartments. Finally, a preliminary identification of the major groups of nitrogen-bearing molecules in soil and compost fractions was carried out by Curie-point analytical pyrolysis. It was found that, after the incubation experiments, most of the newly formed N-compounds tend to concentrate in the alkali-insoluble residue (30% 15N enrichment) but, about 28% of the N remains in colloidal fractions (HA+FA). Upon pyrolysis, the FA yielded typical anhydrosugar and furan compounds suggesting an origin from carbohydrate material. The 15N in this soil fraction amount to 0.7% total soil nitrogen (TSN) (up to 9% of the N from the nitrate added). The HA showed a similar composition, but comparatively released higher amounts of carbohydrate-derived products, typical methoxyphenols and some nitrogen-containing compounds, suggesting a lignoprotein content (9% N derived from the nitrate added=1.7% of the TSN). The WS showed a very complex composition yielding after pyrolysis a series of methoxyphenols and carbohydrate-derived products and substantial yields of fatty acids. The slight differences between the pyrolytic patterns of the different soil and compost fractions suggest a predominantly physical incorporation of the compost to the soil, as well as a non-selective biodegradation of the different C and N forms. The isotopic ratios indicate that the tendency for accumulation of stable N forms followed the order soluble

13C-NMR of forest soil lipids

Molecular characterization of soil lipids often provides valuable biogeochemical information about the impact of vegetation, microorganisms, and abiotic factors on the soil C sequestration proccess. The total lipid extracted with petroleum ether from nine soils developed under three types of Mediterranean forest (stone pine (Pinus pinea L.), evergreen oak (Quercus rotundifolia L.), and Spanish juniper (Juniperus thurifera L)) has been analyzed by high-resolution 13C nuclear magnetic resonance (13C-NMR) under quantitative acquisition conditions. Tentative assignments of the spectral peaks are presented, and the spectra of soil lipids are compared with those from the lipids extracted directly from leaves of the corresponding trees. This comparison evidenced that soil lipids behaved as biomarker soil fractions when analyzed by 13C-NMR as a whole. Analysis by gas chromatography-mass spectrometry (GC/MS) reveals that the volatile fraction of the lipid extract (46%, on average, as estimated by internal reference) consisted mainly of free alkanes, alkanoic acids (<C30), and diterpene resin acids. We observed some differences between the chemical structures suggested by 13C-NMR and GC/MS. This was interpreted as a portion of soil and plant lipids consisting of extractable material that cannot be detected by standard GC methods. The complex signal pattern in the 0 to 30 ppm chemical shift range showed typical signals for carbons in acyl polymethylene chains, which overlapped with a pattern suggesting isoprenoid-like branching in long-chain structures (major signals at ca. 22, 26, and 32 ppm). In addition, periodic unsaturations suggested by signals at ca. 124 and 135 ppm are also compatible with polyprenoid-type backbones. The alkyl region coincided with those of mono- to triacyl glycerol fatty esters. It seems evident that 13C-NMR allows us to characterize structures present in nonvolatile complex material.

Soil Perturbation in Mediterranean Ecosystems Reflected by Differences in Free-Lipid Biomarker Assemblages

Environmental information provided by free lipids in soil samples collected from control and disturbed plots (Madrid, Spain) was assessed by comparing molecular assemblages of terpenoids and distribution patterns of alkanes and fatty acids (FAs) analyzed by gas chromatography-mass spectrometry (GC-MS). Wildfires in pine forests led to increased proportions of retene, dehydroabietin, and simonellite. Friedo-oleananes were characteristic in soils under angiosperms, and norambreinolide-type diterpenes were characteristic in soils encroached by Cistus bushes. Steroids were major compounds in pastured sites. Enhanced Shannons lipid biodiversity index in disturbed soils compared with in control soils suggested patterns of recent lipids overlapping a preserved original lipid signature. The extent of the environmental impacts was illustrated as Euclidean distances between paired control and disturbed sites calculated using the compounds in alkyl homologous series as descriptors. As expected, reforestation, bush encroachment, wildfires, and cultivation were reflected by changes in the molecular record of lipids in soils.