Non-cellulosic neutral sugar contribution to mineral associated organic matter in top- and subsoil horizons of two acid forest soils

We investigated the polysaccharide composition of bulk and mineral-bound (density fractions >2 g cm⁻³) organic matter in topsoil and subsoil horizons of a Podzol and a Cambisol. Total sugar contents were generally higher in the Cambisol than in the Podzol. For most horizons of both soils, the sugars were enriched in the mineral-bound organic matter fraction. This fraction showed a monosaccharide distribution typical for microbial sugars, whereas in bulk soil horizons higher contributions of plant-derived sugars were observed. A strong relationship with the ¹⁴C activity of the dense fraction suggests that microbial-derived polysaccharides are most likely stabilised preferentially by mineral interactions compared to plant-derived polysaccharides.     

Soil organic matter stabilization in acidic forest soils is preferential and soil type-specific

Minerals with large specific surface areas promote the stabilization of soil organic matter (SOM). We analysed three acidic soils (dystric, skeletic Leptic Cambisol; dystric, laxic Leptic Cambisol; skeletic Leptic Entic Podzol) under Norway spruce (Picea abies) forest with different mineral compositions to determine the effects of soil type on carbon (C) stabilization in soil. The relationship between the amount and chemical composition of soil organic matter (SOM), clay content, oxalate‐extractable Fe and Al (Fe_o; Al_o), and dithionite‐extractable Fe (Fe_d) before and after treatment with 10% hydrofluoric acid (HF) in topsoil and subsoil horizons was analysed. Radiocarbon age, ¹³C CPMAS NMR spectra, lignin phenol content and neutral sugar content in the soils before and after HF‐treatment were determined and compared for bulk soil samples and particle size separates. Changes in the chemical composition of SOM after HF‐treatment were small for the A‐horizons. In contrast, for B‐horizons, HF‐soluble (mineral‐associated) and HF‐resistant (non‐mineral‐associated) SOM showed systematic differences in functional C groups. The non‐mineral associated SOM in the B‐horizons was significantly depleted in microbially‐derived sugars, and the contribution of O/N‐alkyl C to total organic C was less after HF‐treatment. The radiocarbon age of the mineral‐associated SOM was younger than that of the HF‐resistant SOM in subsoil horizons with small amounts of oxalate‐extractable Al and Fe. However, in horizons with large amounts of oxalate‐extractable Al and Fe the HF‐soluble SOM was considerably older than the HF‐resistant SOM. In acid subsoils a specific fraction of the organic C pool (O/N‐alkyl C; microbially‐derived sugars) is preferentially stabilized by association with Fe and Al minerals. Stabilization of SOM with the mineral matrix in soils with large amounts of oxalate‐extractable Al_o and Fe_o results in a particularly stable and relatively old C pool, which is potentially stable for thousands of years.     

Improvement of 13C and 15N CPMAS NMR spectra of bulk soils, particle size fractions and organic material by treatment with 10% hydrofluoric acid

The small organic matter content of mineral soils makes it difficult to obtain ¹³C and ¹⁵N nuclear magnetic resonance (NMR) spectra with acceptable signal-to-noise ratios. Subjecting such samples to hydrofluoric acid removes mineral matter and leads to a relative increase in organic material. The effect of treatment with 10% hydrofluoric acid on bulk chemical composition and resolution of solid-state ¹³C NMR spectra was investigated with six soils, some associated particle size fractions, plant litter and compost. The treatment enhanced the signal-to-noise ratio of the solid-state ¹³C NMR spectra. The improvement in spectrum quality was greatest in the clay fraction of soil contaminated with coal ash. The removal of paramagnetic compounds associated with the ash may be the main reason for the improvement. Based on total C, total N, C/N ratio and intensity distribution of the solid-state ¹³C NMR spectra, no changes in organic matter composition could be detected, except for a possible loss of carbohydrates. After treatment with HF, solid-state ¹⁵N NMR spectra of particle size fractions were obtained and indicated that the observable nitrogen is present mostly as peptides and free amino groups. Extraction with hydrofluoric acid is recommended as a routine treatment prior to solid-state ¹³C and ¹⁵N NMR on soil containing little C or N and soil samples containing paramagnetic compounds from natural or anthropogenic sources.     

Long‐term effects of liming on microbial biomass and activity and soil organic matter quality (13C CPMAS NMR) in organic horizons of Norway spruce forests in Southern Germany

Long‐term effects of liming on microbial biomass and activity and soil organic matter (SOM) were investigated in samples from organic horizons (Of/Oh) in spruce forests at Adenau, Höglwald, Idar‐Oberstein, and Schluchsee (Southern Germany) where plots have been manually treated 7 to 13 years ago with dolomitic limestone. At all sites, pH values were markedly increased after liming. The contents of C and N in the organic horizons of the limed plots appeared to be lower with the greatest decrease at Höglwald (Dystric Luvisol) where liming has affected the soil properties for the longest time of all sites. Catalase activity was promoted after liming at Adenau (Cambic Podzol). This was also the case for the Dystric Luvisol where liming resulted also in higher basal respiration. Biomass‐C was higher in samples from the limed plot at Idar‐Oberstein (Dystric Cambisol). The ¹³C CPMAS NMR spectra of organic horizons from the control plots indicate no differences in the gross carbon composition of SOM. Furthermore, spectra from the limed Cambic Podzol, Dystric Cambisol, and Haplic Podzol (Schluchsee) were remarkably similar. However, for the Dystric Luvisol, the lime‐induced promotion of microbial activity resulted in lower O‐alkyl‐C intensity. The observed patterns of microbial biomass and activity were site‐dependent rather than a result of liming. Obviously liming had only small long‐term effects on the humus quality in the organic horizons, as far as detectable by CPMAS NMR spectroscopy. More sensitive techniques like pyrolysis‐GC/MS should be applied to analyze differences in C composition.     

Evaluation of an ultrasonic dispersion procedure to isolate primary organomineral complexes from soils

Soil organic matter can be intimately associated with mineral particles of various sizes. For structural studies, soil organic matter can be isolated in particle size fractions after complete dispersion of the aggregates by ultrasonication. The ultrasonic dispersion energy necessary for complete dispersion was investigated in three A and two B horizons originating from four soils differing in pedogenesis (Gleysol, Phaeozem, Podzol, Alisol), organic C (4.2–34.5 g kg^–1) and clay content (24–294 g kg^–1). Calorimetric calibration of five probe-type ultrasonifiers revealed that the actual energy output from an instrument can depart widely from its nominal output, and that this discrepancy varies from instrument to instrument. Calorimetric calibration is therefore essential for consistency and comparisons between laboratories. Between 450 and 500 J ml^–1 of ultrasonic dispersion energy was enough to disperse completely all samples investigated. The particle size distributions obtained were close to those from standard analysis, except for smaller yields (–20 to –80 g kg^–1) of sand size fractions, which suggests that dispersion by ultrasound is more effective. Based on total C, C:N ratio and distribution of dissolved C, no detachment of soil organic matter from primary organomineral complexes and no redistribution between particle size fractions could be detected in the range 30–590 J ml^–1 of dispersion energy.     

Location and chemical composition of stabilized organic carbon in topsoil and subsoil horizons of two acid forest soils

The ¹⁴C age of soil organic matter is known to increase with soil depth. Therefore, the aim of this study was to examine the stabilization of carbon compounds in the entire soil profile using particle size fractionation to distinguish SOM pools with different turnover rates. Samples were taken from a Dystric Cambisol and a Haplic Podzol under forest, which are representative soil types under humid climate conditions. The conceptual approach included the analyses of particle size fractions of all mineral soil horizons for elemental composition and chemical structure of the organic matter by ¹³C cross-polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy. The contribution of phenols and hydroxyalkanoic acids, which represent recalcitrant plant litter compounds, was analyzed after CuO oxidation.In the Dystric Cambisol, the highest carbon concentration as well as the highest percentage of total organic carbon are found in the <6.3 μm fractions of the B and C horizons. In the Haplic Podzol, carbon distribution among the particle size fractions of the Bh and Bvs horizons is influenced by the adsorption of dissolved organic matter. A relationship between the carbon enrichment in fractions <6.3 μm and the ¹⁴C activity of the bulk soil indicates that stabilization of SOM occurs in fine particle size fractions of both soils. ¹³C CPMAS NMR spectroscopy shows that a high concentration of alkyl carbon is present in the fine particle size fractions of the B horizons of the Dystric Cambisol. Decreasing contribution of O-alkyl and aromatic carbon with particle size as well as soil depth indicates that these compounds are not stabilized in the Dystric Cambisol. These results are in accordance with data obtained by wet chemical analyses showing that cutin/suberin-derived hydroxyalkanoic acids are preserved in the fine particle size fractions of the B horizons. The organic matter composition in particle size fractions of the top- and subsoil horizons of the Haplic Podzol shows that this soil is acting like a chromatographic system preserving insoluble alkyl carbon in the fine particle size fractions of the A horizon. Small molecules, most probably organic acids, dominate in the fine particle size fractions of the C horizons, where they are stabilized in clay-sized fractions most likely due to the interaction with the mineral phase. The characterization of lignin-derived phenols indicated, in accordance with the NMR measurements, that these compounds are not stabilized in the mineral soil horizons.     

Organo-mineral associations in sandy acid forest soils: importance of specific surface area, iron oxides and micropores

Organo-mineral associations stabilize soil organic matter, though the mechanisms by which they do so are unclear. We used particle-size fractions < 6.3 μm of two soils to examine the importance of Fe oxides, short-range order Al silicates and the surface areas of minerals and micropores on the formation of organo-mineral associations. In the subsoil Fe oxides were most strongly statistically correlated with the mineral-bound organic carbon. We therefore assume that they are the most important substrates for the formation of organo-mineral associations. There is no indication that this is caused by physical protection of organic matter in their micropores (< 2 nm). In the Haplic Podzol, dithionite–citrate–bicarbonate-soluble short-range order Al silicates may also play a role. Fe oxide particles were calculated to offer specific surface areas of ∼ 200 m² g⁻¹ (goethite) and ∼ 800 m² g⁻¹ (ferrihydrite), corresponding to crystal diameters of only a few nm. We assume that the resulting large amount of oxide-specific reactive surface sites (conditionally charged hydroxyl groups) is responsible for their dominant role as sorbents. With maximum C loadings of 1.3 mg C per m² Fe oxide for the Dystric Cambisol and 1.1 mg C per m² Fe oxide + short-range order Al silicates for the Haplic Podzol the subsoils of both soils seem to have reached saturation with respect to organic matter sorption. In contrast to subsoil horizons, organo-mineral associations from topsoils contain much larger amounts of organic matter. Here a larger C loading on Fe oxides or a greater importance of other sorbents in addition to the oxides must be assumed.     

Retention and release of dissolved organic matter in Podzol B horizons

The main objectives were to study the effects of pH on the retention and release of organic matter in acid soil, and to determine the main differences in results obtained from batch experiments and experiments in columns. We took soil material from the B horizons of a Podzol at Skånes Värsjö (southern Sweden). In batch experiments, soil was equilibrated with solutions varying in pH and concentration of dissolved organic C. In Bh samples, the release of dissolved C gradually increased with increase in pH. In the Bs1 material there was a minimum at pH 4.1, and in the Bs2 soil the minimum occurred at pH 4.6. The ability to retain added dissolved C increased in the order Bh < Bs1 < Bs2. The column experiment was run for 160 days under unsaturated flow conditions. Columns were packed with Bh, Bh + Bs1 or Bh + Bs1 + Bs2 samples to calculate mass balances for each horizon. Solutions either without any dissolved organic C or ones containing 49 mg C dm^?3 with pH of 4.0 or 3.6 were used to leach columns. The pH of input solutions only little affected the concentration of dissolved C in the effluent. Relative proportions of hydrophobic substances decreased with increasing column length and decreasing pH. For input solutions containing dissolved C, near steady state was achieved for both the Bs1 and Bs2 horizons with approximately 25% dissolved organic matter retention. Thus, no maximum sorption capacity for dissolved C could be defined for these horizons. This behaviour could not have been predicted by batch data, showing that column experiments provide useful additional information on interactions between organic compounds and solid soil material.     

稳定性氮同位素技术研究太湖梅梁湾区域营养盐差异

稳定同位素技术通常用于指示外源输入对环境的影响,采用该技术分析了太湖北部不同湖区悬浮颗粒有机物和浮游动物的碳氮稳定同位素比值的差异。结果表明,不同粒径的悬浮颗粒有机物和浮游动物稳定同位素比值存在显著的季节变化,夏季普遍高于冬季,其中δ13C高3.19‰~7.21‰,δ15N高4.20‰~9.36‰。各有机物δ13C不能有效区分各湖区的环境差异,而δ15N在各湖区存在显著差异。δ15N聚类分析显示,河口区由于受外源输入的影响,其δ15N显著低于其他湖区,单独聚为一类,而交汇处和湖心区影响较小,同河口区存在显著差异。整体来讲,总颗粒有机物和浮游动物δ15N同水体TP浓度呈显著正相关,同总溶解性无机氮浓度、铵根离子浓度呈显著负相关。可见,太湖北部悬浮颗粒有机物和浮游动物δ15N受环境营养盐差异的影响强烈,悬浮颗粒有机物δ15N可以作为指示湖区区域差异的有效指标。     

Examining the Microbial Degradation of Naphthenic Acids Using Stable Isotope Analysis of Carbon and Nitrogen

Naphthenic acids (NAs) are a complex group of naturally occurring oil sands constituents that constitute a significant portion of the dissolved organic carbon (DOC) pool available for microbial degradation in the process-related waste water associated with oil sands mine sites. One approach to understanding the biological fate of oil sands process-derived carbon and nitrogen in aquatic reclamation of the mine sites involves the use of stable isotope analyses. However, for stable isotope analyses to be useful in such field-based assessments, there is a need to determine how microbial degradation of a complex mixture of NAs might change the stable isotope values (δ ¹³C, δ ¹⁵N). In batch cultures and semi-continuous laboratory microcosms, utilization of a commercial mixture of NAs by oil sands-derived microbial cultures resulted in microbial biomass that was similar or slightly ¹³C enriched (1.4‰ to 3.0‰) relative to the DOC source, depending on the length of incubation. Utilization of a NA-containing extract of oil sands processed water resulted in greater ¹³C enrichment of microbial biomass (8.5‰) relative to the DOC source. Overall, the δ ¹³C of the DOC comprised of complex mixtures of NAs showed minimal change (?0.5‰ to ?0.1‰) during the incubation period whereas the δ ¹³C of the dissolved inorganic carbon (DIC) was more variable (?5.0‰ to +5.4‰). In tests where the concentration of available nitrogen was increased, the final biomass values were ¹⁵N enriched (3.8‰ to 8.4‰) relative to the initial biomass. The isotope trends established in this study should enhance our ability to interpret field-based data from sites with hydrocarbon contamination, particularly in terms of carbon source utilization and ¹⁵N enrichment.     

Chemistry of soil organic matter as related to C : N in Norway spruce forest (Picea abies(L.) Karst.) floors and mineral soils

Due to high nitrogen deposition in central Europe, the C : N ratio of litter and the forest floor has narrowed in the past. This may cause changes in the chemical composition of the soil organic matter. Here we investigate the composition of organic matter in Oh and A horizons of 15 Norway spruce soils with a wide range of C : N ratios. Samples are analyzed with solid‐state ¹³C nuclear magnetic resonance (NMR) spectroscopy, along with chemolytic analyses of lignin, polysaccharides, and amino acid‐N. The data are investigated for functional relationships between C, N contents and C : N ratios by structural analysis. With increasing N content, the concentration of lignin decreases in the Oh horizons, but increases in the A horizons. A negative effect of N on lignin degradation is observed in the mineral soil, but not in the humus layer. In the A horizons non‐phenolic aromatic C compounds accumulate, especially at low N values. At high N levels, N is preferentially incorporated into the amino acid fraction and only to a smaller extent into the non‐hydrolyzable N fraction. High total N concentrations are associated with a higher relative contribution of organic matter of microbial origin.     

Black carbon contribution to soil organic matter composition in tropical sloping land under slash and burn agriculture

Tropical soils are generally depleted in organic carbon (OC) due to environmental conditions favouring decomposition and mineralisation of soil organic matter (SOM). In Northern Laos, sloping soils are subjected to slash and burn agriculture, which leads to production of black carbon (BC), a stable SOM fraction. BC may directly influence the quantity and quality of SOM sequestered in tropical soils. The aim of this study was to quantify BC content and evaluate its impact on the chemical and stable isotope composition of SOM along a catena composed of Dystrochrepts at the bottom of the slope, Alfisols (midslope) and Inceptisols at the top of the slope for different burning frequencies. Six soil profiles, situated on a slope ranging from a river bank to the summit of a hill, were sampled. The stable isotope compositions (¹³C and ¹⁵N) of samples from both organo-mineral A and mineral B and C horizons were determined. The chemical composition of SOM analysed by ¹³C CPMAS NMR spectroscopy and the contribution of BC determined by dichromate oxidation were compared to OC and iron oxide content as well as land management including the burning cycle.The highest C contents were recorded at midslope positions. At any position on the slope, δ¹³C and δ¹⁵N ratios showed an enrichment in ¹³C and ¹⁵N with increasing soil depth. The OC content of soil horizons was related to their aryl C content, which is the component most likely driven by BC inputs. The BC contributions analysed by dichromate oxidation ranged from 3% to 7% of total OC. A positive correlation was obtained between aryl C and the BC content of SOM. Comparison of BC content and stable isotope composition of SOM showed that BC influenced the δ¹³C and the δ¹⁵N stable isotope ratios of these soils. BC was not associated with the mineral phase. The highest BC contents were measured under intensive slash and burn practice in the vicinity of the boundary of Alfisols at the top of the slope, where erosion was severe. Therefore, BC, a SOM component strongly influencing OC sequestration of these soils, is susceptible to translocation down the slope.     

13C/12C Ratios of soil organic matter as indicators of vegetation changes in the congo

The ¹³C/¹²C ratios were determined for the organic matter of all horizons of a podzol profile and of the A₁ horizons of some ferrallitic soils, in some grass shoots and in a fossil root fragment from the B_2h horizon of the podzol. The isotope ratio in the organic matter of the A₁ horizon of the podzol matches those in grass shoots from the present savanna vegetation. The ratios in the lower horizons match those of organic matter in the A₁ horizons of soils under forest and that of the fossil root fragment in the B_2h horizon. The ratios thus demonstrate that the humus enrichment of the B_2h horizon of the podzol occurred while it was under forest vegetation and that the present grass vegetation did not take part in the podzolization process. The differences also indicate that savanna replaced forest vegetation after the profile had been formed.     

Automated analysis of 15N and 14C in biological samples

Abstract

An automated method for the simultaneous analysis of total N, total C, ¹⁵N and ¹⁴C in small plant and soil samples is described. A commercial C‐N analyser ‐ continuous flow isotope ratio mass spectrometer (ANCA‐MS) has been extended to also measure CO₂ and collect ¹⁴CO₂ produced by sample combustion. Samples containing 20 ‐ 200 μg N and up to 5 mg C can be analysed directly with no sample preparation other than drying and fine grinding. The precision of total elemental analysis is comparable to that by conventional methods. The average standard deviation of ¹⁵N analyses of plant material at natural abundance was ±1 ‰. This is accurate enough for all ¹⁵N studies except those using natural abundance and possibly long term studies of soil organic matter. Recovery of ¹⁴C in test samples was 100%. The instrument can be operated by graduate students under supervision and operating costs are primarily for sample cups, combustion catalyst and quartz tubes.     

Stabilization of organic matter by soil minerals — investigations of density and particle‐size fractions from two acid forest soils

We tested the hypothesis whether organic matter in subsoils is a large contributor to organic carbon (OC) in terrestrial ecosystems and if survival of organic matter in subsoils is the result of an association with the soil mineral matrix. We approached this by analyzing two forest soil profiles, a Haplic Podzol and a Dystric Cambisol, for the depth distribution of OC, its distribution among density and particle‐size fractions, and the extractability of OC after destruction of the mineral phase by treatment with hydrofluoric acid (HF). The results were related to indicators of the soil mineralogy and the specific surface area. Finally, scanning electron microscopy combined with energy dispersive X‐ray spectroscopy (SEM‐EDX) was used to visualize the location of OC at mineral surfaces and associations with elements of mineral phases. The subsoils (B and C horizons) contained 40—50% of the soil OC including the organic forest floor layers. With increasing depth of soil profiles (1) the radiocarbon ages increased, and (2) increasing portions of OC were either HF‐soluble, or located in the density fraction d >1.6 g cm^—3, or in the clay fraction. The proportions of OC in the density fraction d >1.6 g cm^—3 were closely correlated to the contents of oxalate and dithionite‐citrate‐bicarbonate‐extractable Fe (r² = 0.93 and 0.88, P <0.001). SEM‐EDX analyses suggested associations of OC with aluminum whereas silicon‐enriched regions were poor in OC. The specific surface area and the microporosity of the soil mineral matrix after destruction of organic matter were less closely correlated to OC than the extractable iron fractions. This is possibly due to variable surface loadings, depending on different OC inputs with depth. Our results imply that subsoils are important for the storage of OC in terrestrial ecosystems because of intimate association of organic matter with secondary hydrous aluminum and iron phases leading to stabilization against biological degradation.     

Biological influences on the morphology and micromorphology of selected Podzols (Spodosols) and Cambisols (Inceptisols) from the eastern United States and north-east Scotland

Biological activities greatly influence the formation of many soils, especially forest soils under cool humid climates. The objective of this study was to investigate the effects of vegetation and soil biota on the formation of selected soils. Field morphology, micromorphology, and carbon and organic matter analysis were determined on six Podzols (Spodosols) and two Cambisols (Inceptisols) from the eastern United States and north-east Scotland. Humification of plant material by soil fauna and fungi occurs in all organic horizons. Thick organic coatings are observed on soil peds and rock fragments from the E1 to the Bs horizon in a Haplic Podzol from Clingmans Dome Mt., TN. Thin sections reveal large accumulations of root material in different stages of decomposition in the spodic horizons of a Haplic Podzol from Whiteface Mt., NY. Organic carbon ranges from 5.4 to 8.5% in the spodic B horizons of the Whiteface Mt. Podzol. Earthworms and enchytraeids have a great effect on the structure of the surface and subsurface horizons in the Dystric Cambisols from Huntly and Clashindarroch Forests, Scotland and a Cambic Podzol from the Corrie Burn Basin, Scotland. Podzols from Speymouth Forest, Scotland (Gleyic Podzol), Clingmans Dome Mt., and Whiteface Mt. have thick organic horizons. The Podzols from the Flatwoods in Georgia, the Pine Barrens in New Jersey, the Corrie Burn Basin, and the Cambisol from Huntly Forest have only A horizons at the surface. The Clashindarroch Forest soil has a very thin organic horizon. Warm and humid climates and sandy parent material are responsible for thick E horizons and lack of thick organic horizons in the Flatwoods (Carbic Podzol) and Pine Barrens (Ferric Podzol) soils. Earthworms and enchytraeids thrive in the Corrie Burn Basin and Huntly Forest soils due to the vegetation and the highly weathered basic parent material. The site at Clashindarroch once carried oak, and then birch forest, both of which produce a mild litter and also encourage earthworm and enchytraeids. This fauna is responsible for much mixing of the topsoil. The present conifer vegetation will eventually produce a deep litter and cause podzolization.     

Reconstruction of Late Glacial and Holocene landscape-climatic changes in the central Selenga Middle Mountains based on the isotopic composition of organic matter

The results of investigation into the composition of stable carbon and nitrogen isotopes of organic matter in the soils developed within soil–sedimentary sequences in the central part of the Selenga Middle Mountains in the Late Glacial and Holocene are presented. In the past 15000 years, the organic matter of the investigated soils has only been formed from the biomass of C3 plants (without the participation of C4 plants). This is confirmed by the of δ¹³С values from–27.00 to–23.35‰. A combined analysis of the parameters of the organic matter (С_org, N_total, C/N, δ¹³С, and δ¹⁵N) of soils formed in different periods makes it possible to assume that the isotopic composition of carbon and nitrogen reflects changes in the climate humidity during the Late Glacial and Holocene periods. The specified intervals of soil formation correspond to the climate humidification and stabilization of the surface owing to the development of dense vegetation. Aridization periods were characterized by the accumulation of sediments that buried soil horizons. The most pronounced stages of climate aridization occurred at the transition from the Late Glacial to the Holocene, from the Boreal to the Atlantic, and from the Atlantic to the Subboreal periods. The optimum soil-forming conditions existed in the periods of 11700–11000, 8800–6900, and 4700–1000 years ago, which is confirmed by the published data on the landscape-climatic changes in the adjacent areas in the past 15000 years.     

A reassessment of podzol formation processes

Translocated (oxalate-soluble) Al and Fe are present predominantly in inorganic forms in the B₂ horizons of the five pcdzol profiles examined: A1 as imogolite and proto-imogolite allophanes, and Fe as a separate oxide phase. Below the top few cm of the B₂ horizon, over 75 per cent of the extractable (acid-plus alkali-soluble) organic matter is present as Al-fulvates, largely sorbed on allophanic material. The Bh horizons of the Iron Humus Podzol and Iron Podzol intergrades are distinguished by very high levels of organically bound Fe (soluble in EDTA solution), five to ten times more than in immediately adjacent A₂ or B₂ horizons, and also by larger humic acid contents than in comparable B₂, levels in typical Iron Podzols. Inorganic forms of translocated Al and Fe are probably absent from two of the three Bh horizons examined, and also from the Bhg horizon overlying the thin iron pan in the Peaty Podzol. The organic matter in this Bhg horizon is saturated with Al rather than Fe. Chemical and physical processes which could lead to evolution of a profile along the genetic sequence, Iron Podzol, Iron Humus Podzol, Peaty Podzol, are postulated. During the formation of an Iron Podzol, positively charged inorganic sols carry aluminium, silicon and iron from the A₂ and deposit them in the B₂ horizon; subsequently, with the development of an H layer, colloidal humus migrates through the A₂ and precipitates on the positive colloids at the top of the B₂ horizon to form a Bh horizon, in which remobilized ferric species are trapped by the organic matter. In higher rainfall areas, occasional waterlogging above the oxide-impregnated B₂ leads to a thin iron pan, separating permanently oxidizing conditions below from seasonally waterlogged and reducing conditions above.     

Xe NMR spectroscopy of adsorbed xenon as an approach for the characterisation of soil meso- and microporosity

¹²⁹Xe nuclear magnetic resonance (NMR) spectroscopy of adsorbed xenon was applied for the characterisation of soil meso- and microporosity. Model systems (Ca-montmorillonite, quartz sand) and three soil types (Luvisol Alh, Bt and Cv horizons; Gleysol Go horizon; Podzol Bvs horizon) were studied. For Ca-montmorillonite, the average intercrystallite pore size has been evaluated. In natural soils, ¹²⁹Xe resonance parameters were shown to be affected by different factors: pore heterogeneity, influence of organic functional groups, possible presence of paramagnetic compounds, occurrence of xenon exchange between inter- and intraparticle void spaces. The effect of those factors on the pattern of ¹²⁹Xe NMR spectra was tested. In the three soils studied, no micropores within the mineral phase available for xenon adsorption were found. The most probable reason is that such pores are occupied by small molecules of the soil organic matter (SOM). Variable extent of accessibility of mesopores within the mineral phase of the various soils was revealed. It was highest in the Podzol. Here, xenon exchange between different adsorption zones (i.e., pores of differing size, e.g., internal and external void spaces) was slow on an NMR time scale that allowed to detect separate signals, each characterising xenon behaviour in the respective adsorption zone. The pore system of the soil organic matter was shown to be not accessible for xenon, as it is accepted for N₂ and other nonpolar adsorbates. Based on analysis of the spectra, a model for the possible mutual location of organic matter and iron compounds in natural soils was suggested. According to this model, a certain part of organic matter species can form the layers above iron species, thus masking them and preventing ¹²⁹Xe NMR spectra from significant low-field shifts and signal broadening.     

Cold and hot water–extractable organic matter as indicators of litter decomposition in forest soils

Mild extractions were used as indicators of easily decomposable organic matter (OM). However, the chemical composition of extracted OM often remained unclear. Therefore, the composition of cold and hot water–extractable OM was investigated in the O horizons (Oi, Oe, Oa) of a 170 y old beech stand (Fagus sylvatica) in the Ore Mtns., SE Germany. To simulate litter decomposition, the O horizon samples were incubated for 1 week under defined conditions. Cold‐ and hot‐water extracts were analyzed and chemically characterized by pyrolysis–field ionization mass spectrometry (Py‐FIMS). The C and N concentrations were always lower in the cold‐(C: 2.69 to 3.95 g kg^–1; N: 0.14 to 0.29 g kg^–1) than in the hot‐water extracts (C: 13.77 to 15.51 g kg^–1; N: 0.34 to 0.83 g kg^–1). The C : N ratios of both extracts increased with increasing depth. Incubation increased the concentrations of C and N in all water extracts, while C : N ratios of extracts decreased. The molecular‐chemical composition of cold and hot water–extracted OM revealed distinct differences. Generally, cold water–extracted OM was thermally more stable than hot water–extracted OM. The mass spectra of the hot water–extracted organic matter revealed more intensive signals of carbohydrates, phenols, and lignin monomers. Additionally, the n‐C₂₈ fatty acid and the n‐C₃₈–to–n‐C₅₂ alkyl monoesters clearly distinguished the hot‐ from the cold‐water extract. A principle‐component analysis visualized (1) alterations in the molecular‐chemical composition of cold‐ and hot‐water extracts due to previous incubation of the solid O horizon samples and (2) a decomposition from the Oi to the Oh horizon. This provides evidence that the macromorphological litter decomposition was reflected by the chemical composition of water extracts, and that Py‐FIMS is well‐suited to explain at the molecular level why OM decomposability is correlated with water‐extracted C.