Organic matter composition in sewage farm soils: Investigations by 13C-NMR and pyrolysis-field ionization mass spectrometry

The composition of soil organic matter (SOM) on sewage farms south of Berlin was investigated by solid-state CP/MAS ¹³C-NMR and pyrolysis-field ionization mass spectrometry (Py-FIMS) of freeze-dried sewage solids and soil samples of differing contamination. These were an untreated soil (USOIL), a former sewage farm used as arable land since 1990 (SF90A), and a recent sewage farm (SF1994). The CP/MAS ¹³C-NMR spectra showed enrichments of the sewage-treated soils with aliphatic C and C in OCH₃-groups and amino acids. In the Py-FI mass spectra the major markers of sewage and SOM in sewage farm soils were (i) N-containing compounds, in particular peptides, (ii) dimethylphthalate (m/z 194), (iii) sterols, and (iv) signals in the mass range m/z 502 to 554 of mono- and diaryl esters which were substituted by long aliphatic chains. The latter signals were intense in the sewage solids, increased in intensity from sample SF90A to SF1994; but they were not present in the USOIL, thus clearly indicating anthropogenic origin. Temperature-resolved Py-FIMS showed that the SOM compounds in the sewage farm soils were generally incorporated into bonds with widely different stabilities which could be relevant for SOM turnover and environmental effects. This is demonstrated for the trapping of dimethylphthalate in a modelled humic substance.     

In-source pyrolysis-field ionization mass spectrometry and Curie-point pyrolysis-gas chromatography/mass spectrometry of amino acids in humic substances and soils

With the aid of in-source pyrolysis-field ionization mass spectrometry (Py-FIMS) and Curie-point pyrolysis-gas chromatography/mass spectrometry (cPy-GC/MS) in the conventional electron impact mode, characteristic signals of 23 amino acid standards were described. Thermal and mass spectrometric fragmentation pathways of these amino acids differed with each method and complemented each other. Pyrolysis products assigned by Py-FIMS extended the range of signals for N-containing compounds in humic substances and soil organic matter and gave marker signals for free amino acids and their subunits in proteinaceous materials. These characteristic signals were correlated with the amino acid content in N-rich humic fractions consisting of seven fulvic acids and eight humic acids. The selected marker signals reflected 25–84% of the variances of the molar distribution of acidic, neutral, neutral aromatic, and basic amino acids in the humic fractions. In addition, a well described agricultural soil (0.08% amino acid N) was spiked with a standard amino acid mixture (0.08 mg amino acid N 100 mg^-1 dry soil) and produced enhancements of the relative abundances of the corresponding amino acid signals. Moreover, for 27 samples of whole agricultural soils of widely different origins, soil types, and vegetations, 15 selected amino acid indicators were correlated significantly with -amino N (r=0.76^***) and total N (r=0.65^***).     

Properties and composition of soil organic matter in forest and arable soils of schleswig-holstein. 2. litter and humic compounds in mineral horizons

Organic matter composition is an important soil constituent with regard to function in soil ecosystems. In the recent paper litter and humic compound contents from about 100 mineral soil investigations are presented. The soil horizons are divided into four groups (Ah, Ap, M. Bh) in order to compare the SOM quality. Ap and Ah horizons showed a similar litter and humic compound distribution. Structural differences in the humic compound fractions were only visible with CPMAS ¹³C-NMR. SOM-containing non-spodic subsoil horizons had a similar SOM quality as the A horizons. In the Bh horizons the humic compounds dominated with about 75% in the SOM. Alkylic and O-alkylic carbon units are the main fractions. The combination of the solid-state ¹³C-NMR spectroscopy of whole soil samples and the wet chemical analysis of litter compounds allowed the estimation of the liner and chemically defined humic compound distribution in soil samples.     

Characterization of cultivation effects on soil organic matter

Humus properties in various Ap horizons from field plots, that have been cultivated in long-term experiments under different management conditions, were investigated by pyrolysis-field ionization mass spectrometry (Py-FIMS) and ¹³C-NMR spectroscopy. The results of Py-FIMS were evaluated by correlation and principal component analysis from reproducible data sets of bulk soil samples and extracted humic substances, and allowed a distinct discrimination on the basis of humus quality and composition. The chemical subunits suitable for discrimination are the major plant constituents carbohydrates, lignin, and proteinaceous materials as well as their humification products. The contribution of these compound classes to soil organic matter decreased with the intensity of management. CPMAS and solution ¹³C NMR spectra of soils and humic substances demonstrated that with more intense management, both the intensities of the phenolic region (140–160 ppm) and the aromatic region (110–140 ppm) decreased. The combination of both independent methods MS and NMR, together with microbiological and biochemical data, yields the general result that intensive soil management leads to a less active humus.     

Long-term cultivation effects on the quantity and quality of organic matter in selected Canadian prairie soils

Sixteen Orthic Chernozemic surface soil samples, one half from virgin prairie and one half from adjacent cultivated prairie (cultivated for 31 to 94 years), were collected from eight sites throughout Southern Saskatchewan, Canada. Samples were analyzed for total organic C and a number of other chemical and physical properties. The virgin and cultivated soils at site No. 4 were selected for more detailed analysis by CP-MAS ¹³C NMR, Curie-point-pyrolysis-gas chromatography/mass spectrometry (Cp-Py-GC/MS), and by pyrolysis-field ionization mass spectrometry (Py-FIMS). Long-term cultivation resulted in large significant decreases in total SOM (soil organic matter), as represented by total soil organic C. There were significant increases in aromaticity of the SOM as a result of long-term cultivation as indicated by CP-MAS ¹³C NMR spectroscopy. This was mainly attributable to the result of cultivation-enhanced degradation of aliphatic C relative to aromatic C. Organic compounds identified in the Cp-Py-GC/MS spectra of the virgin and cultivated soils at site No. 4 consisted of n-alkanes (ranging from C₁₁ to C₂₂) and alkenes (ranging from C_7:1 to C_21:1), with the virgin soil being richer in alkenes than the cultivated soil. Other components identified were cyclic aromatics, carbocyclics, N-containing aromatics, N-heterocyclics, benzene and substituted benzenes, phenols and substituted phenols and substituted furans. The compounds identified appeared to originate from long-chain aliphatics, lignins, polyphenols, aromatics, polysaccharides, and N-containing compounds in the two soils. While qualitatively similar compounds were identified by Py-FIMS in the two soils, the total ion intensity (TII) of the virgin soil was almost 2.5 times as high as that of the cultivated soil. This suggests that cultivation made the organic matter less volatile, either by favouring the formation of higher molecular weight organic matter or by promoting the formation of non-volatile metal-organic matter complexes. The Py-FIMS spectra showed that the virgin soil contained relatively more lignin dimers, lipids, sterols, and n-C₁₆ to n-C₃₄ fatty acids than the cultivated soil. Thus, conversely, the cultivated soil was richer in carbohydrates, phenols and lignin monomers, alkyl aromatics and N-containing compounds, including peptides, than the virgin soil.     

Colluvisols under cultivation in Schleswig-Holstein. 2. Carbon distribution and soil organic matter composition

Soils under intensive cultivation have altered due to water erosion. This study was conducted to determine whether soil organic matter (SOM) composition of the colluvial source (Ap horizons) differs from the colluvial sink (M horizons). The SOM of a sandy Catena with erodic Cambisols and colluvic soils (Colluvisols) in Schleswig-Holstein, Northwest Germany, was investigated. A wet chemical analysis was combined with CPMAS ¹³C-NMR spectroscopy. In one case a significant correlation between the SOM composition of the Ap horizon of the erodic Cambisol and the M horizon of the Colluvisol was high (r² = 0.904^-), whereas the correlation for the other set was much weaker (r² = 0.640*). Two possible paths of pedogenesis are discussed. About 70% of the SOM of the colluvial source is decomposed during translocation or after deposition. A selective preservation or new formation of humins in the M material is probable. These humins contain, obviously, large amounts of polysaccharides, which were not detected by the wet chemical analysis. Further investigations of colluvic and erodic soils are necessary in order to specify the SOM quality and its possible modification due to soil translocation and accumulation.     

Die Böden von Wallhecken in Schleswig-Holstein. II. Aufbau und Umsetzung der organischen Bodensubstanz

Soils of Mound Hedges in Schleswig-Holstein. II. Composition and Transformation of Soil Organic Matter Two Cumulic Anthrosols of mound hedges and the fossil A-horizons below the deposits, were investigated by means of wet chemistry and ¹³C-NMR spectroscopy in order to characterize soil organic matter (SOM) and to compare their SOM with natural and recent soils. The bulk humus of the Cumulic Anthrosols contains a high amount of easily soluble polysaccharides. Despite their high bioavailability sugars, starch and mobile fulvic acids are not completely consumed by microorganisms and translocated into the subsoil. The initial phase of podzolization is indicated due to this process and the very low pH-level of the soils. After deposition about 30 % of the soil carbon in the A-horizons is mineralized. Litter compounds are predominantly decomposed, whereas humic compounds are selectively preserved or newly synthesized. The significance of the humic compounds in the soil organic matter increases after deposition. This is why the potential cation exchange capacity of the fossil Ahorizons is similar to this of the recent ones.     

The chemistry of soil organic nitrogen: a review

1. From the data presented herein it is possible to deduce the following distribution of total N in humic substances and soils: proteinaceous materials (proteins, peptides, and amino acids) – ca. 40%; amino sugars – 5–6%; heterocyclic N compounds (including purines and pyrimidines) – ca. 35%; NH₃–19%; approximately 1/4 of the NH₃ is fixed NH₄ ⁺. Thus, proteinaceous materials and heterocyclics appear to be major soil N components. 2. Natural ¹⁵N abundance levels in soils and humic materials are so low that direct analysis by ¹⁵N NMR is very difficult or impossible. To overcome this difficulty, the soil or humic material is incubated with ¹⁵N-enriched fertilizer. Even incubation in the laboratory for up to 630 days does not produce the same types of ¹⁵N compounds that are formed in soils and humic materials over hundreds or thousands of years. For example, very few ¹⁵N-labelled heterocyclics are detected by ¹⁵N NMR. Does this mean that heterocyclics are not present? Or are the heterocyclics that are present not labelled under these experimental conditions and therefore not detected by the ¹⁵N NMR spectrometer ? Another possibility is that a large number of N heterocyclics occur in soils, but each type occurs in very low concentrations. Until the sensitivity is improved, ¹⁵N NMR will not provide results that can be compared with data obtained from the same soil and humic material samples by chemical methods and mass spectroscopy. 3. What is most important with respect to agricultural is that all major N forms in soils are available to organisms and are sources of NH₃ or NH₄ ⁺ for plant roots and microbes. Naturally, some of the NH₃ will enter the N cycle. 4. From chemical and pyrolysis-mass spectrometric analyses it appears that N heterocylics are significant components of the SOM, rather than degradation products of other molecules due to pyrolysis. The arguments in favor of N heterocyclics as genuine SOM components are the following: a) Some N-heterocyclics originate from biological precursors of SOM, such as proteinaceous materials, carbohydrates, chlorophyll, nucleic acids, and alkaloids, which enter the soil system as plant residues or remains of animals. b) In aquatic humic substances and dissolved organic matter (DOM) at considerably lower pyrolysis temperatures (200 to 300°C), free and substituted N-heterocyclics such as pyrroles, pyrrolidines, pyridines, pyranes, and pyrazoles, have been identified by analytical pyrolysis (Schulten et al 1997b). c) Their presence in humic substances and soils was also detected without pyrolysis by gel chromatography – GC/MS after reductive acetylation (Schnitzer and Spiteller 1986), by X-ray photoelectron spectroscopy (Patience et al. 1992), and also by spectroscopic, chromatographic, chemical, and isotopic methods (Ikan et al. 1992). 5. While we can see light at the end of the tunnel as far as soil-N is concerned, further research is needed to identify additional N-containing compounds such as N- heterocyclics, to determine whether these are present in the soil or humic materials in the form in which they were identified or whether they originate from more complex structures. If the latter is correct, then we need to isolate these complex N-molecules and attempt to identify them.     

Compost Changed Soil Organic Matter Molecular Composition: A Py-GC/MS and Py-FIMS Study

Changes in the molecular composition of soil organic matter (SOM) resulting from compost application are not sufficiently known at the molecular scale even though this is a major issue for soil fertility and soil carbon sequestration. Therefore, the present study investigated effects of long-term compost application in comparison to mineral fertilizer on the molecular composition of SOM in a 34-year-old experiment. Soil samples were taken after 19 and 34 years of constant management and analyzed by Curie point Pyrolysis-Gas Chromatography/Mass Spectrometry (Cp Py-GC/MS) and Pyrolysis-Field Ionization Mass Spectrometry (Py-FIMS). In general, compost application increased the organic carbon (C) content. The Cp PyGC/MS revealed larger relative intensities of alkylphenols/lignin monomers at the expense of carbohydrates in the compost treatments. Py-FIMS indicated higher proportions of labile n-fatty acids, lipids and sterols in the compost than in the mineral fertilizer treatment. Permanent cropping of grass between years 19 and 34 revealed similar signal patterns, which is also maintained after conversion of soil from permanent grass to arable use. Thermograms of volatilization indicated enrichments of stable (compounds volatilized in between 370°C and 570°C) phenols/lignin monomers, lipids and alkylaromatics between years 19 and 34 in compost fertilized soils. This was a result of enhanced losses of compounds that are considered easily metabolized by microorganisms (e.g. carbohydrates) after compost addition as derived from Py-GC/MS and Py-FIMS. In summary, long-term application of mature compost was shown to have a positive, long lasting effect on the organic carbon sequestration in agricultural soils.     

Litter decomposition and humification in acidic forest soils studied by chemical degradation,IR and NMR spectroscopy and pyrolysis field ionization mass spectrometry

Two forest soils (Typic Dystrochrept, Entic Haplorthod) with mor and moder were investigated by chemical degradation, IR and CPMAS ¹³C NMR spectroscopy and pyrolysis (Py) field ionization (FI) mass spectrometry (MS). Chemical analyses show that during litter decomposition, humification, and podzolisation, cellulose and lignin structures decrease considerably, whereas no distinct changes were found for the hemicellulose and protein fractions. These results are consistent with current hypotheses on the conversion of plant residues to stable humic substances, but the sum of chemically identified organic soil components of the litter layers only accounts for 40–50% of total organic carbon. The amounts of different carbon types were estimated by the integration of CPMAS ¹³C NMR spectra. For the L layers this calculation assigns 56–58% as O-alkyl-C, 20–22% as alkyl-C, 14–16% as aryl-C, and 6–8% as carboxyl-C. With increasing soil depth O-alkyl-C (with polysaccharides as main source) decrease to 31–42%, aliphatic C increases to 36–43%, and aryl- and carboxyl-C show no distinct changes. The hypothesis of an increasing aromaticity during humification in soils therefore is questionable. Data from Py-FIMS confirm and extend the results' of chemical methods as well as IR and ¹³C NMR spectroscopy. In particular, the Fi mass spectra of the generated pyrolysates show that the increase in polymethylene carbon during the biodegradation and humification of beech and spruce litter is partly due to an increase of saturated fatty acids. This means, Py-FIMS is able to describe the structure of wet-chemically unaccounted, individual humus constituents and thus improves the knowledge about the genesis of humic substances.     

Decomposition and the contribution of glomalin-related soil protein (GRSP) in heavy metal sequestration: Field experiment

Glomalin is a metal-sorbing glycoprotein excreted by arbuscular mycorrhizal fungi (AMF). One method of estimating glomalin in soils is as glomalin-related soil protein (GRSP). In this study the role of GRSP in sequestering Pb and Cd was investigated in an in situ field experiment. The effect of metal sequestration on the subsequent decomposition of GRSP was also investigated. GRSP was determined using the Bradford method as total glomalin-related soil protein (T-GRSP) and as easily extractable glomalin-related soil protein (EE-GRSP). After 140 days, GRSP bound Pb accounted for 0.21–1.78% of the total Pb, and GRSP bound Cd accounted for 0.38–0.98% of the total Cd content in the soil. However when compared on a soil organic matter (SOM) basis, only 4% of the Pb or Cd was bound to the GRSP fraction of the SOM compared with 40–54% of the Pb or Cd bound to the humin and fulvic acids in the SOM fraction. In soils contaminated with the highest levels of Pb and Cd, the T-GRSP (EE-GRSP) decomposition after 140 days was reduced by 8.0 (6.6)% and 7.0 (7.5)%, respectively, when compared with the controls. In the high Pb or Cd treatment groups we found that the fraction of metal bound to GRSP increased even though the total GRSP content declined over time. The mass ratio between Pb and GRSP-carbon changed from 2.3 to 271.4 mg (100 g)⁻¹ in all Pb levels soil, while with the high-Cd treatment group the mass ratio between Cd and GRSP-carbon (0.36 mg (100 g)⁻¹) was higher than the mass ratio seen with Cd-bound humic acid fractions. Our in situ field study shows that while GRSP does bind Pb and Cd, in the soils we investigated, the levels are insignificant compared to soil organic matter such as humic and fulvic acids.     

Microbial contribution to SOM quantity and quality in density fractions of temperate arable soils

The formation of soil organic matter (SOM) very much depends on microbial activity. Even more, latest studies identified microbial necromass itself being a significant source of SOM and found microbial products to initiate and enhance the formation of long-term stabilized SOM. The objectives of this study were to investigate the microbial contribution to SOM in pools of different stability and its impact on SOM quality. Hence, four arable soils of widely differing properties were density-fractionated into free and occluded particulate organic matter (fPOM, oPOM < 1.6 g cm⁻³ and oPOM < 2.0 g cm⁻³) and mineral associated organic matter (MOM > 2.0 g cm⁻³) by using sodium polytungstate. These fractions were characterized by in-source pyrolysis-field ionization mass spectrometry (Py-FIMS). Main SOM compound classes of the fractions were determined and further SOM properties were derived (polydispersity, thermostability). The contribution of microbial derived input to arable soil OM was estimated from the hexose to pentose ratio of the carbohydrates and the ratio of C₄–C₂₆ to C₂₆–C₃₆ fatty acids. Additionally, selected samples were investigated by scanning electron microscopy (SEM) for visualizing structures as indicators for the origin of OM. Results showed that, although the samples differed significantly regarding soil properties, SOM composition was comparable and almost 50% of identifiable SOM compounds of all soils types and all density fractions were assigned to phenols, lignin monomers and alkylaromatics. Most distinguishing were the high contents of carbohydrates for the MOM and of lipids for the POM fractions. Qualitative features such as polydispersity or thermostability were not in general assignable to specific compounds, density fractions or different mean residence times. Only the microbial derived part of the soil carbohydrates could be shown to be correlated with high SOM thermostability (r² = 0.63**, n = 39). Microbial derived carbohydrates and fatty acids were both enriched in the MOM, showing that the relative contribution of microbial versus plant-derived input to arable SOM increased with density and therefore especially increased MOM thermostability. Nevertheless, the general microbial contribution to arable SOM is suggested to be high for all density fractions; a mean proportion of about 1:1 was estimated for carbohydrates. Despite biomolecules released from living microorganisms, SEM revealed that microbial mass (biomass and necromass) is a considerable source for stable SOM which is also increasing with density.     

Structure of “unknown” soil nitrogen investigated by analytical pyrolysis

Curie-point pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and in-source pyrolysis-field ionization mass spectrometry (Py-FIMS) were applied for the first time to the structural characterization of organic nitrogen in hydrolyzates and hydrolysis residues resulting from the classical 6M HCl hydrolysis of mineral soils. Two well-described soils of widely different origin (i.e., a Gleysol Ah and a Podzol Bh) were investigated. Py-GC/MS was performed using a nitrogen-selective detector to detect and identify N-containing pyrolysis in the hydrolyzate (e.g., pyrazole and/or imidazole, N,N-dimethylmethanamine, benzenacetonitrile, propane- and propenenitriles) and the hydrolysis residue (e.g., pyrroles, pyridines, indoles, N-derivatives of benzene, benzothiazol, and long-chain aliphatic nitriles). Moreover, temperature-resolved Py-FIMS allowed us to record the thermal evolution of the N-containing compounds during pyrolysis. These were characterized by a particularly high thermostability compared to their thermal release from whole soils. The combination of pyrolysis with mass spectrometric methods permitted analyses of the identities and thermal stabilities of complex nitrogen compounds in hydrolysis residues of whole soils, which cannot be done by wet-chemical methods. Pyrolysis-methylation GC/MS with tetramethyl-ammonium hydroxide (TMAH) as methylating agent enabled the identification of N,N-dimethylbenzenamine and so confirmed the identification of benzeneamine by Py-GC/MS in nonmethylated hydrolysis residues. The described N-derivatives of benzene and long-chain nitriles are usually not detectable by pyrolysis-mass spectrometry of plants and microorganisms. These compounds are characteristic of soils, terrestrial humic substances and hydrolysis residues and seem to be specific, stable transformation products of soil nitrogen. Received: 15 April 1996     

Zur Definition von Humusformen ackerbaulich genutzter Böden. II. Quantität und Qualität der organischen Bodensubstanz

The definition of humusforms from soils under cultivation. II. Quantity and quality of soil organic matter In the new edition of the German textbook “Practical Studies in Soil Science” the authors presented a proposal of mapping humusforms in arable soils in order to characterize soil and site ecology (Schlichting et al., 1995). This proposal was developed from the definitions “Ochric”, “Mollic” and “Umbric” of the Soil Taxonomy and the FAO classification. The characterization of humusforms in 45 arable surface soils was carried out according to this proposal while soil organic matter (SOM) composition was investigated by means of wet chemistry and CPMAS ¹³C-NMR spectroscopy. “Mollic” in contrast to “Umbric” humusforms could be characterized by a higher carbonyl/carboxyl carbon content probably deriving from proteins, polysaccharides and humic substances. In addition the mollic epipedon contains 10% more litter compounds, whereas in the umbric epipedon humic acids are of major importance. The humin fraction in the mollic epipedon is thought to be raised by the formation of Ca-humates. Our data suggest, that with regard to microbial decomposition a surplus of available organic matter is present in the mollic horizons. The ochric-like epipedon has a much lower humus content compared to “Mollic” and “Umbric” horizons and exhibits the highest amounts of soluble organic matter as well as aromatic and carboxylic C-compounds in the humic fraction. Our data suggest, that SOM quantity and quality of the mollic, umbric and ochric epipedons differ substantially. These findings suggest that the proposal of Schlichting et al. (1995), which was extended by Blume & Beyer (1996), should be regarded as a useful basis to discuss the development of humusforms in soils under cultivation and facilitate soil survey in order to improve site characterization.     

Properties and composition of soil organic matter in forest and arable soils of Schleswig-Holstein: 1. Comparison of morphology and results of wet chemistry,CPMAS-13C-NMR spectroscopy and pyrolysis-field ionization mass spectrometry

Properties and composition of 25 soil samples (0.8–51% C_org) were determined by morphology, wet chemistry, CPMAS-¹³C-NMR-spectroscopy and pyrolysis-field ionization mass spectrometry (Py-FIMS). The recovery rate of organic carbon was 102% (±15%). The correlation between the litter compound/humic compound ratio and humification grade, estimated with morphological observations, was strong (r² = 0.502^***). A typical classification of horizons (L, O, H, Ah+M) with regard to organic compounds (wet chemistry and ¹³C-NMR) was not always significant. The pyrolysis-mass spectra confirmed and extended on the basis of molecular chemical structures the results of wet chemistry, especially with polysaccharides, nitrogen compounds, lignin, lipids and alkylaromatics (r² = 0.818–0.937^***). A correlation between the NMR-aliphatics, the humic compounds isolated by wet chemistry, and the long-chain aliphatic carbon units in the pyrolysis-mass spectra was established.     

Investigating the effects of wettability and pore size distribution on aggregate stability: the role of soil organic matter and the humic fraction

Soil organic matter (SOM) is an important factor influencing aggregate stability. Interactions between SOM and soil structure are widely studied, although the subtle relationship between SOM content, pore size distribution and aggregate stability is not fully known. Here we investigate such a relationship by means of a long‐term experiment established in 1962 in northeastern Italy, which considers different fertilizer practices (organic, mineral and mixed) applied to a continuous maize crop rotation. We measured wet stability of 1–2 mm aggregates subjected to different pretreatments. Both soil physical properties (such as pore size distribution and hydrophobicity) and chemical properties (soil organic and humic carbon content) affecting aggregate stability were considered. The chemical structure of humic substances was characterized by thermal and spectroscopic analyses (TG‐DTA, DRIFT and ¹H HR MAS NMR). The Pore‐Cor network model was then applied to evaluate the contribution of hydrophobicity and porosity to aggregate wetting. Our study suggests that SOM and its humic fraction can affect aggregate wetting and consequently slaking by modifying the pore size distribution with a shift from micropores (5–30 µm) and mesopores (30–75 µm) to ultramicropores (0.1–5 µm); hydrophobicity was also increased as a result of different humic composition. Spectroscopic analysis showed that hydrophobic compounds were mostly associated with complex humic molecules. Models of fast wetting dynamics, however, suggest that the contribution that hydrophobicity makes to aggregate stability, especially to soils with large carbon inputs, may not be the most significant factor.     

NaOH‐extractable organic matter of andic soils from Galicia (NW Spain) under different land use regimes: a pyrolysis GC/MS study

The objective of this study was to determine to what extent the attenuation or loss of andic soil properties caused by land use change – from forest (FOR, average C content 118.2 ± 23.7 g kg^?1) to agricultural land (AGR, average C content 55.7 ± 16.7 g kg^?1) use – is reflected in soil organic matter (SOM) at the molecular level. For this, NaOH‐extractable SOM of A horizons from 17 soils developed on amphibolitic parent material in NW Spain was studied by pyrolysis gas chromatography spectrometry (Py‐GC/MS). We also included two buried andic A horizons (PAL, 2200 cal yr BP in age) on the same parent material, as a reference for the molecular composition of SOM from soils without recent litter additions. Organic matter of PAL soils had a composition largely different from that of superficial soils (FOR and AGR), with an important relative contribution of microbial polysaccharides and N‐compounds, and an absence of compounds that characterize fresh plant litter (e.g. lignins). In the superficial soils, the relative contribution of lignin‐derived compounds was greater in AGR than in FOR soils. Differences were also observed in the relative contribution of aliphatic compounds, FOR soils being enriched in this type of components compared with AGR soils. The results indicated that land use change from FOR to AGR, which was accompanied by a decrease in total SOM, resulted in an enrichment in primary SOM. The smaller relative abundance of primary SOM derivatives in andic FOR soils indicates that these compounds were quickly degraded in Andisols.     

Cultivation-Induced Effects on the Organic Matter in Degraded Southern African Soils

We studied quantitative and qualitative changes in soil organic matter (SOM) due to different land uses (reference woodland versus cultivated) on six soils from Tanzania (Mkindo and Mafiga), Zimbabwe (Domboshawa and Chickwaka), and South Africa (Hertzog and Guquka). Structural characteristics of the humic acids (HAs) were measured by Curie-point pyrolysis–gas chromatography/mass spectrometry (P_y–GC/MS) and solid-state ¹³C nuclear magnetic resonance (CPMAS ¹³C NMR) spectroscopy. Significant changes in concentration and composition of SOM were observed between land uses. Losses of organic carbon after cultivation ranged from 35% to 50%. Virgin soils showed large proportions of colloidal humus fractions: humic acids (HAs) and fulvic acids (FAs) but negligible amounts of not-yet decomposed organic residues. The change in land use produced a contrasting effect on the composition of the HAs: a noteworthy “alkyl enhancement” in Mkindo soil and “alkyl depletion” in Chikwaka and to a lesser extent in Domwoshawa. The remaining soils displayed only minor alterations.     

The impact of a low humus level in arable soils on microbial properties, soil organic matter quality and crop yield

 In arable soils in Schleswig-Holstein (Northwest Germany) nearly 30% of the total organic C (TOC) stored in former times in the soil has been mineralized in the last 20 years. Microbial biomass, enzyme activities and the soil organic matter (SOM) composition were investigated in order to elucidate if a low TOC level affects microbial parameters, SOM quality and crop yield. Microbial biomass C (C_mic) and enzyme activities decreased in soils with a low TOC level compared to soils with a typical TOC level. The decrease in the C_mic/TOC ratio suggested low-level, steady-state microbial activity. The SOM quality changed with respect to an enrichment of initial litter compounds in the top soil layers with a low TOC level. Recent management of the soils had not maintained a desirable level of humic compounds. However, we found no significant decrease in crop yield. We suggest that microbial biomass and dehydrogenase and alkaline phosphatase activities are not necessarily indicators of soil fertility in soils with a high fertilization level without forage production and manure application. Received: 12 December 1997