C. Cerli

Molecular Trickery in Soil Organic Matter: Hidden Lignin

Binding to minerals is one mechanism crucial toward the accumulation and stabilization of organic matter (OM) in soils. Of the various biochemicals produced by plants, lignin-derived phenols are among the most surface-reactive compounds. However, it is not known to what extent mineral-bound lignin-derived phenols can be analytically assessed by alkaline CuO oxidation. We tested the potential irreversible binding of lignin from three litters (blue oak, foothill pine, annual grasses) to five minerals (ferrihydrite, goethite, kaolinite, illite, montmorillonite) using the CuO-oxidation technique, along with bulk organic carbon (OC) sorption. Up to 56% of sorbed lignin could not be extracted from the minerals with the CuO-oxidation technique. The composition of the irreversibly bound lignin component differed markedly between minerals and from that of the parent litter leachates, indicating different bonding strengths related to individual monomers and conformations. The difference in extractability of individual phenols suggests that abiotic processes, such as sorption/desorption, should be taken into account when using CuO oxidation data for assessing lignin turnover in mineral matrixes. However, given the apparent relationship between aromaticity as indicated by carbon-specific UV absorbance (SUVA) and bulk OC sorption, it is likely that irreversible sorption is a concern for any technique that addresses the broad class of aromatic/phenolic compounds in soils and sediments.

Robust analysis of underivatized free amino acids in soil by hydrophilic interaction liquid chromatography coupled with electrospray tandem mass spectrometry

Amino acids are an important and highly dynamic fraction of organic N in soils and their determination in soil without derivatization is challenging due to the difficulties in separation and detection of trace amounts of these polar analytes. In the present work, we developed an analytical method to quantify 20 free amino acids in aqueous soil extracts without derivatization. The method employed hydrophilic interaction liquid chromatography-tandem mass spectrometry (HILIC-MS/MS) technique combined with a cation exchange solid phase extraction (SPE). Four stable isotope labelled amino acids were used as internal standards to improve the method performance. Good separation of 20 underivatized amino acids was achieved within 12min. The limit of detection (LODs) and limit of quantification (LOQs) were in the range of 13-384ngg(-1) and 43-1267ngg(-1) (dry soil basis), respectively. The results showed that overall recoveries with high precision were obtained for the extracted free amino acids from ten different soils. The overall recoveries of 18 amino acids were similar for the ten soils used, which differed substantially in organic C content and in other properties as soil texture and pH. For most of the amino acids, the average recoveries from soil extracts were between 74% and 117%, with the exception of Met (31%), Pro (52%) and Arg (68%). Variability was within acceptable limits (relative standard deviations were between 4% and 13%), with the exception of Met (relative standard deviation=90%) and Arg (relative standard deviation=53%). Thus the proposed method with high throughout and high analyte specificity shows great promise for consistent analysis of free amino acids extracted from soils and offers new horizons for the analysis of amino acids in terrestrial and aquatic ecosystem.

A novel high-temperature combustion based system for stable isotope analysis of dissolved organic carbon in aqueous samples. : II optimization and assessment of analytical performance

RATIONALE Dissolved organic carbon (DOC) plays an important role in carbon cycling, making precise and routine measurement of δ(13)C values and DOC concentration highly desirable. A new promising system has been developed for this purpose. However, broad-scale application of this new technique requires an in-depth assessment of analytical performance, and this is described here. METHODS A high-temperature combustion Total Organic Carbon analyzer was interfaced with continuous flow isotope ratio mass spectrometry (TOC/IRMS) for the simultaneous analysis of the bulk DOC concentration and δ(13)C signature. The analytical performance (precision, memory effects, linearity, volume/concentration effects, accuracy) was thoroughly evaluated, including realistic and challenging conditions such as low DOC concentrations and natural DOC. RESULTS High precision (standard deviation, SD predominantly ≤ 0.15 ‰) and accuracy (R(2) = 0.9997) were achieved for the δ(13)C analysis of a broad diversity of DOC solutions. Simultaneously, good results were obtained for the measurement of DOC concentration. Assessment of natural abundance and slightly (13)C-enriched DOC, a wide range of concentrations (~0.2-150 mgC/L) and injection volumes (0.05-3 mL), demonstrated minor/negligible memory effects, good linearity and flexible usage. Finally, TOC/IRMS was successfully applied to determine low DOC concentrations (<2 mgC/L) and DOC from diverse terrestrial, freshwater and marine environments (SD ≤ 0.23 ‰). CONCLUSIONS TOC/IRMS enables fast and reliable measurement of DOC concentrations and δ(13)C values in aqueous samples, without pre-concentration and freeze-drying. Further investigations should focus on complex, saline matrices and very low DOC concentrations, to achieve a potential lower limit of 0.2 mgC/L. Thus, TOC/IRMS will give DOC research in terrestrial and aquatic environments a huge impulse with high-resolution, routine δ(13)C analysis.

The use of δ2H and δ18O isotopic analyses combined with chemometrics as a traceability tool for the geographical origin of bell peppers

Two approaches were investigated to discriminate between bell peppers of different geographic origins. Firstly, δ(18)O fruit water and corresponding source water were analyzed and correlated to the regional GNIP (Global Network of Isotopes in Precipitation) values. The water and GNIP data showed good correlation with the pepper data, with constant isotope fractionation of about -4. Secondly, compound-specific stable hydrogen isotope data was used for classification. Using n-alkane fingerprinting data, both linear discriminant analysis (LDA) and a likelihood-based classification, using the kernel-density smoothed data, were developed to discriminate between peppers from different origins. Both methods were evaluated using the δ(2)H values and n-alkanes relative composition as variables. Misclassification rates were calculated using a Monte-Carlo 5-fold cross-validation procedure. Comparable overall classification performance was achieved, however, the two methods showed sensitivity to different samples. The combined values of δ(2)H IRMS, and complimentary information regarding the relative abundance of four main alkanes in bell pepper fruit water, has proven effective for geographic origin discrimination. Evaluation of the rarity of observing particular ranges for these characteristics could be used to make quantitative assertions regarding geographic origin of bell peppers and, therefore, have a role in verifying compliance with labeling of geographical origin.

Molecular Features of Humic Acids and Fulvic Acids from Contrasting Environments

Insight in the molecular structure of humic acid (HA) and fulvic acid (FA) can contribute to identify relationships between their molecular properties, and further our quantitative abilities to model important organic matter functions such as metal complexation and association with mineral surfaces. Pyrolysis gas chromatography/mass spectrometry (Py-GC-MS) is used to compare the molecular composition of HA and FA. A systematic comparison was obtained by using samples from different environmental sources, including solid and aqueous samples from both natural and waste sources. The chemical signature of the pyrolysates was highly variable and no significant difference between HA and FA was found for major chemical groups, that is, carbohydrates, phenols, benzenes, and lignin phenols, together accounting for 62-96% of all quantified pyrolysis products. However, factor analysis showed that within each sample, FAs consistently differed from corresponding HAs in a larger contribution from mono- and polyaromatic hydrocarbons and heterocyclic hydrocarbons, together accounting for 3.9-44.5% of the quantified pyrolysis products. This consistent difference between FAs and corresponding HAs, suggests that their binding properties may, in addition to the carboxyl and phenolic groups, be influenced by the molecular architecture. Py-GC-MS may thus contribute to identify relationships between HA and FA binding- and molecular-properties.

Centennial-scale lake-level lowstand at Lake Uddelermeer (The Netherlands) indicates changes in moisture source region prior to the 2.8-kyr event

The Uddelermeer is a unique lake for The Netherlands, containing a sediment record that continuously registered environmental and climatic change from the late Pleistocene on to the present. A 15.6-m-long sediment record was retrieved from the deepest part of the sedimentary basin and an age–depth model was developed using radiocarbon dating, 210Pb dating, and Bayesian modeling. Lake-level change was reconstructed using a novel combination of high-resolution palaeoecological proxies (e.g. pollen, non-pollen palynomorphs, chironomids), quantitative determinations of lake-level change (ground-penetrating radar), and estimates of changes in precipitation (lipid biomarker stable isotopes). We conclude that lake levels were at least as high as present-day water levels from the late glacial to 3150 cal. yr BP, with the exception of at least one lake-level lowstand during the Preboreal period. Lake levels were ca. 2.5 m lower than at present between 3150 and 2800 cal. yr BP, which might have been the result of a change in moisture source region prior to the so-called 2.8-kyr event. Increasing precipitation amounts around 2800 cal. yr BP resulted in a lake-level rise of about 3.5–4 m to levels that were 1–1.5 m higher than at present, in line with increased precipitation levels as inferred for the 2.8-kyr event from nearby raised bog areas as well as with reconstructions of higher lake levels in the French Alps, all of which have been previously attributed to a phase of decreased solar activity. Lake levels decreased to their present level only during recent times, although the exact timing of the drop in lake levels is unclear.

Competition and surface conditioning alter the adsorption of phenolic and amino acids on soil minerals

Summary Adsorptive interactions of organic molecules with soil minerals often impair their bioavailability. However, little is known about the adsorption behaviour of phenolic and nitrogenous compounds on different minerals and their mutual interaction with respect to competition and surface conditioning (i.e. surface modification induced by preceding adsorption of the other class of compounds). Therefore, batch adsorption experiments were done to study the interaction between phenolic acids (PAs; salicylic acid, Sal; syringic acid, Syr; ferulic acid, Fer; vanillic acid, Van) and amino acids (AAs; lysine, Lys; glutamic acid, Glu; leucine, Leu; phenylalanine, Phe) during adsorption on goethite and Ca2+-montmorillonite at pH 6 by applying adsorbate concentrations of 0.01, 0.05 and 0.1 mm. Larger adsorption of PAs was observed on goethite than montmorillonite, whereas the phyllosilicate was a better adsorbent for AAs than the oxide. Among all tested PAs, Sal was preferentially adsorbed on both minerals. For the AAs, Glu was preferentially adsorbed on goethite and Lys on montmorillonite. The AAs were more competitive than PAs and partially suppressed the adsorption of PAs on both minerals. The adsorption of PAs or AAs on both minerals was enhanced by surface conditioning with the other group, with larger effects for goethite than montmorillonite. For goethite, surface conditioning by PAs enhanced the adsorption of AAs more (by 97–161%) than did AAs for PAs (9–48%). The results support the hypothesis that pre-adsorption of one class of organic compound can enhance the retention of another class. This suggests that adsorbed organic matter on soil mineral phases might be subject to a self-strengthening effect. Highlights Phenolic acids (PAs) were preferentially retained on goethite. Amino acids (AAs) were preferentially retained on montmorillonite. AAs were more competitive than PAs for adsorption sites on both minerals. Pre-adsorption of one class of compound can enhance the retention of another class.

Organic matter coatings of soil minerals affect adsorptive interactions with phenolic and amino acids: Adsorption of phenolic and amino acids on OM-coated minerals

Phenolic (PAs) and amino acids (AAs) contribute potentially to the stabilization of organic matter (OM) in soil by forming mineral–organic associations (MOAs). However, little is known about how natural OM coating affects the adsorption of these compounds and their mutual interaction upon adsorption. Therefore, batch adsorption experiments were carried out to study how coatings of minerals with dissolved organic matter (DOM) obtained from forest leaf litter (L‐DOM) and from the O horizon (O‐DOM) affect the adsorption of phenolic acids (salicylic, syringic, ferulic and vanillic acid) and amino acids (lysine, glutamic acid, leucine and phenylalanine) at pH 6. Moreover, the adsorption of PAs on OM‐coated minerals was tested after preconditioning by AAs and vice versa. Adsorption of DOM depended on the mineral (goethite ≫ kaolinite > montmorillonite) and sources of DOM (O‐DOM > L‐DOM for goethite). Coatings of all minerals with both DOM sources reduced the adsorption of PAs and the acidic glutamic acid but enhanced that of the basic lysine. The adsorption of PAs decreased with increasing OM coating of the minerals. Strong bonds between AAs and OM‐coated minerals generally enhanced adsorption of PAs with preconditioning by AAs. This indirectly indicates a multilayer model of MOA. Adsorption of AAs, however, was rarely influenced by preconditioning with PAs. Competition between AAs and PAs generally suppressed the adsorption of AAs on coated minerals, whereas the PAs were hardly affected. These results emphasize that the amount and composition of OM coating must be considered when studying the adsorption of PAs and AAs in soil.