Changes in the chemical composition of soil organic matter after application of compost

Is the composition of soil organic matter changed by adding compost? To find out we incubated biowaste composts with agricultural soils and a humus‐free mineral substrate at 5°C and 14°C for 18 months and examined the products. Organic matter composition was characterized by CuO oxidation of lignin, hydrolysis of cellulosic and non‐cellulosic polysaccharides (CPS and NCPS) and ¹³C cross‐polarization magic angle spinning nuclear magnetic resonance (CPMAS ¹³C‐NMR) spectroscopy. The lignin contents in the compost‐amended soils increased because the composts contained more lignin, which altered little even after prolonged decomposition of the composts in soil. A pronounced decrease in lignin occurred in the soils amended with mature compost only. Polysaccharide C accounted for 14–20% of the organic carbon at the beginning of the experiment for both the compost‐amended soils and the controls. During the incubation, the relative contents of total polysaccharides decreased for 9–20% (controls) and for 20–49% (compost‐amended soils). They contributed preferentially to the decomposition as compared with the bulk soil organic matter, that decreased between < 2% and 20%. In the compost‐amended agricultural soils, cellulosic polysaccharides were decomposed in preference to non‐cellulosic ones. The NMR spectra of the compost‐amended soils had more intense signals of O–alkyl and aromatic C than did those of the controls. Incubation for 18 months resulted mainly in a decline of O–alkyl C for all soils. The composition of the soil organic matter after compost amendment changed mainly by increases in the lignin and aromatic C of the composts, and compost‐derived polysaccharides were mineralized preferentially. The results suggest that decomposition of the added composts in soil is as an ongoing humification process of the composts themselves. The different soil materials affected the changes in soil organic matter composition to only a minor degree.     

Application of diffuse reflectance FT-IR spectroscopy and partial least-squares regression to predict NMR properties of soil organic matter

Carbon 13 nuclear magnetic resonance spectroscopy (¹³C NMR) is a powerful technique for studying the structure and turnover of soil organic matter, but is time consuming and expensive. It is therefore worth seeking swifter and cheaper methods. Diffuse reflectance FT‐IR spectroscopy (DRIFT), along with partial least squares (PLS) algorithms, provides statistical models to quantify soil properties, such as contents of C, N and clay. I have applied DRIFT?PLS to quantify soil organic C species, as measured by solid state ¹³C NMR spectroscopy, for several bulk soils and physical soil fractions. Calibration and prediction models for organic C and for particular NMR regions, namely alkyl C, O?alkyl C and carboxyl C, attained R² values of between 0.94 and 0.98 (calibration) and 0.70–0.93 (cross‐validation). The prediction of unknown soil samples, after pre‐selection by statistical indices, confirmed the applicability of DRIFT?PLS. The prediction of aromatic C failed, probably because of superimposition of aromatic bands by signals from minerals. Results from fractions of particulate organic matter suggest that the chemical homogeneity of the material hampers the quantification of its constituting C species by DRIFT?PLS. For alkyl C, prediction of carbon species by DRIFT?PLS was better than direct peak‐area quantification in the IR spectra, but advantageous in parts only compared with a linear model correlating C species with soil C contents. In conclusion, DRIFT?PLS calibrated with NMR data provides quantitative information on the composition of soil organic matter and can therefore complement structural studies by its application to large numbers of samples. However, it cannot replace the information provided by more specific methods. The actual potential of DRIFT?PLS lies in its capacity to predict unknown samples, which is helpful for classification and identification of environmental outliers or benchmarks.     

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.     

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.     

应用~(13)C核磁共振技术研究土壤有机质化学结构进展

土壤有机质化学结构对准确评价土壤有机质的稳定性及其在土壤中的功能具有重要意义。土壤有机质化学结构的研究方法中,固态~(13)C核磁共振波谱技术(Solid-state ~(13)C-NMR spectroscopy)具有独特优势,对土壤有机质化学结构的解析更贴近真实状态,近年来已取得诸多新进展和新突破。综述了近年来应用~(13)C-NMR测定土壤全土、团聚体和密度组分、腐殖质组分的有机碳化学结构特征,分析了影响化学结构变化的因素。不同气候条件、植被类型、土地利用管理方式、土壤类型、土壤有机碳含量的全土中有机碳化学结构比较相似,均表现为烷氧碳比例最高,其次为烷基碳和芳香碳,羧基羰基碳比例最低。土壤有机碳主要来源于外源植物残体,植物残体化学结构的相似性可能是导致土壤有机碳化学结构相似的主要原因,环境条件、土壤自身属性和微生物活性的差异使土壤有机碳化学结构产生微小差异。土壤颗粒及化学组分间的有机碳分子结构差异较大,大颗粒有机碳中烷氧碳比例最高,小粒径及与矿物颗粒结合的有机碳中烷基碳和羧基羰基碳比例更高,粉黏粒和腐殖酸组分的有机碳化学结构在土壤类型间差异较大。今后的研究重点应更多地关注土壤有机质来源的定量化分析、土壤微生物对土壤有机碳组分和结构稳定性的贡献及调控机制、土壤有机碳稳定性的生物物理化学保护机制、空间大尺度环境因子/土壤生态过程与微观尺度的有机碳化学分子结构的耦合作用机制、跨学科的多种土壤有机碳化学分子结构测定辨识技术等方面的研究。     

基于固态13C核磁共振波谱研究植物残体分解和转化机制的进展

植物残体在土壤中的分解和转化影响了其养分归还和有机质形成过程。由于缺乏高分辨率的分析方法,对不同气候、植被和土壤类型条件下植物残体在分解过程中化学结构组成的演变特征和机制仍不清楚。核磁共振波谱技术在解析自然有机物化学组成方面具有独特的优势,本文综述了基于固态~(13)C核磁共振波谱(solid-state ~(13)C-NMR spectroscopy)技术评价植物残体的基质质量、解析植物残体的分解速率及其官能团组成的变化特征、揭示土壤腐殖质特性等方面的主要进展。未来针对植物残体分解和有机质形成机制的研究,应该结合稳定性同位素质谱和扫描电镜分析方法,综合分析植物残体中的有机化合物组成和物理结构;从多时空尺度揭示不同类型植物残体中有机碳官能团的降解路径;结合高通量测序和基因芯片分析方法,深入研究土壤微生物群落与植物残体化学结构的协同演变机制,提出不同气候–土壤–植被类型区促进土壤有机质形成的调控措施。     

Similarities in chemical composition of soil organic matter across a millennia-old paddy soil chronosequence as revealed by advanced solid-state NMR spectroscopy

This study examined the chemical composition of soil organic matter (SOM) along a 2,000-year paddy soil chronosequence in eastern China by use of advanced solid-state nuclear magnetic resonance (NMR) spectroscopy as well as Fourier transform infrared spectroscopy (FTIR), aiming to identify changes in the chemical composition of SOM over a millennium timescale. The results showed that soil organic carbon concentration in the surface soil reached a steady state after 100 years of rice (Oryza sativa L.)–wheat (Triticum sp.) cropping on coastal tidal flats. The ¹³C NMR spectra and fractions of structural groups or components of the whole SOM samples differed little along the chronosequence, suggesting a similar chemical composition in SOM samples regardless of the duration of rice cultivation. The FTIR spectral pattern and relative intensities of some resolved functional groups or components of whole SOM were also similar along the soil chronosequence. The similarities in chemical composition of SOM can be attributed to the rice–wheat cropping system, in which SOM has undergone ongoing turnover under periodical fresh plant material input and wet–dry cropping alternation, leading to a similar chemical composition of bulk SOM.     

Application of <Superscript>13</Superscript>C NMR Spectroscopy to the Study of Soil Organic Matter: A Review of Publications

Possibilities of NMR spectroscopy with ¹³C nuclei application to the study of soil organic matter and its various fractions is considered. This is a non-destructive method, which is particularly valuable in the analysis of various fractions of soil organic matter. It is regarded as a direct method, and, unlike most of indirect methods, it allows one to obtain reliable estimates of the ratio between virtually all groups of carbon atoms in different organic molecules, including those in humus specimens. Owing to impulse technique and high sensitivity, ¹³C-NMR spectra may be obtained immediately from soil samples without any extraction operations. The modern technique of obtaining spectra, their mathematical processing (Fourier transform), and data interpretation are considered. The results of applying ¹³C-NMR to the study of humus substances, water-soluble fractions of soil organic matter, and soil litters from different natural zones are discussed.     

Soil organic matter transformations induced by Hieracium pilosella L. in tussock grassland of New Zealand

 To study the effect of Hieracium pilosella L. invasion on the transformations of soil organic matter of New Zealand tussock grassland soils (Ustochrepts), plant material and soils underneath Hieracium, the surrounding halo, and the adjacent herbfield (depleted tussock grassland) were examined for their chemical composition. An attempt was made to reveal possible changes in chemical composition of the soil organic matter induced by H. pilosella invasion. Small differences were detected by solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy in the composition of the plant and soil materials from these zones. Most of the differences in soil organic matter occurred due to differences in the amount and quality of plant-residue inputs. Comparable amounts of phenolic C were detected in the solid-state ¹³C NMR spectra of H. pilosella and herbfield vegetation, while alkaline CuO oxidation yielded considerable lower lignin oxidation products for H. pilosella. A slightly higher proportion of these compounds in H. pilosella soil revealed an accumulation and a low degradation rate of lignin compounds under H. pilosella. The HCl hydrolysis and solid-state ¹⁵N NMR spectroscopy showed similar chemical compositions of the N fractions of the three different soils. The absence of ¹⁵N NMR signal intensity assignable to aniline derivatives or aromatic heterocyclic N indicates that the condensation of phenolic compounds with N groups plays a minor role in N sequestration in these soils. Received: 6 September 1999     

C and N NMR spectroscopy as a tool in soil organic matter studies

Nuclear magnetic resonance (NMR) is a valuable tool for the characterization of soil organic matter and humification processes in soils. This review highlights soil organic matter studies based mainly on solid-state ¹³C and ¹⁵N NMR spectroscopy and some emerging applications, that may provide significant progress in our knowledge on soil organic matter. A major advantage of Nmr spectroscopy is that it can be used as a non-invasive method for solid soil samples or soil fractions. Although resolution is limited, one can obtain an overview on the organic matter structures present in the soil sample. Application of ¹³C and ¹⁵N NMR to soils has, for a long time, been confined to the study of bulk soils or humic extracts for structural characterization. The transformations of soil organic C and N are now being investigated after addition of ¹³C- and ¹⁵N-labelled parent materials to the soil and following their evolution in different C and N pools. With labelling techniques it is also possible to study the interaction of organic pollutants with soil organic matter. Contamination of a soil with man-made additives, such as soot or brown coal dust, can also be detected in soils or individual soil fractions.     

Composition of organic matter in a subtropical Acrisol as influenced by land use, cropping and N fertilization, assessed by CPMAS 13C NMR spectroscopy

We know much about the influence of management on stocks of organic matter in subtropical soils, yet little about the influence on the chemical composition. We therefore studied by CPMAS ¹³C NMR spectroscopy the composition of the above-ground plant tissue, of the organic matter of the whole soil and of silt- and clay-size fractions of the topsoil and subsoil of a subtropical Acrisol under grass and arable crops. Soil samples were collected from three no-till cropping systems (bare soil; oats−maize; pigeon pea + maize), each receiving 0 and 180 kg N ha⁻¹ year⁻¹, in a long-term field experiment. Soil under the original native grass was also sampled. The kind of arable crops and grass affected the composition of the particulate organic matter. There were no differences in the composition of the organic matter in silt- and clay-size fractions, or of the whole soil, among the arable systems. Changes were observed between land use: the soil of the grassland had larger alkyl and smaller aromatic C contents than did the arable soil. The small size fractions contain microbial products, and we think that the compositional difference in silt- and clay-size fractions between grassland and the arable land was induced by changes in the soil's microbial community and therefore in the quality of its biochemical products. The application of N did not affect the composition of the above-ground plant tissue nor of the particulate organic matter and silt-size fractions, but it did increase the alkyl C content in the clay-size fraction. In the subsoil, the silt-size fraction of all treatments contained large contents of aromatic C. Microscopic investigation confirmed that this derived from particles of charred material. The composition of organic matter in this soil is affected by land use, but not by variations in the arable crops grown.     

Organic N forms of a subtropical Acrisol under no-till cropping systems as assessed by acid hydrolysis and solid-state NMR spectroscopy

This study was conducted to investigate the influence of land-use systems (grassland and cropland) and of long-term no-till cropping systems [bare soil, oat/maize (O/M), pigeon pea+maize (P+M)] on the composition of organic N forms in a subtropical Acrisol. Soil samples collected from the 0- to 2.5-cm layer in the study area (Eldorado do Sul RS, Brazil) were submitted to acid hydrolysis and cross-polarization magic angle spinning (CPMAS) ¹⁵N and ¹³C nuclear magnetic resonance (NMR) spectroscopies. The legume-based cropping system P+M contained the highest contents of non-hydrolysable C and N, hydrolysable C and N, amino acid N and hydrolysed unknown N. The relative proportion of non-hydrolysable N was higher in bare soil (30.0%) and decreased incrementally in other treatments based on the total C and N contents. The amino acid N corresponded to an average of 37.2% of total N, and was not affected by land use and no-till cropping systems. The non-hydrolysable residue contained lower O-alkyl and higher aromatic C concentrations, as revealed by CPMAS ¹³C NMR spectroscopy, and higher C:N ratio than the bulk soil. No differences in the bulk soil organic matter composition could be detected among treatments, according to CPMAS ¹³C and ¹⁵N NMR spectra. In the non-hydrolysable fraction, grassland showed a lower concentration of aromatic and a higher concentration of alkyl C than other treatments. From CPMAS ¹⁵N NMR spectra, it could be concluded that amide N from peptide structures are the main organic N constituent. Amide structures are possibly protected through encapsulation into hydrophobic sites of organic matter and through organomineral interaction.     

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.     

Carbon and nitrogen isotope composition of bulk soils, particle-size fractions and organic material after treatment with hydrofluoric acid

Soils and sediments contain only small amounts of organic matter, and large concentrations of paramagnetic metals can give poor solid‐state nuclear magnetic resonance (NMR) spectra of organic matter. Pretreatment of samples with hydrofluoric acid (HF) dissolves significant proportions of the mineral matrix and extracts paramagnetic elements. We investigated the effects of 10% HF treatment on the stable isotope content of carbon (C) and nitrogen (N) of organic matter from soils, composts and shales. Additionally we inferred molecular and isotopic characteristics of lost materials from calculations of isotope mass balances. Treatment with HF enriched C and N in mineral samples substantially (factors 2.5–42.4), except for Podzol B horizons (1.1–1.7) and organic material (1.0–1.3). After treatment most of the C (59.7–91.7%) and N (53.7–86.6%) was recovered, although changing C/N ratios often indicated a preferential loss of N‐rich material. Isotope ratios of C and N in the remaining material became more negative when net alterations exceeded 0.3‰. The isotope ratios of the lost material contained more ¹³C (1–2‰) and ¹⁵N (1–4‰) than the initial organic matter. Acid hydrolysis typically removes proteins, amino acids and polysaccharides, all of which are enriched in ¹³C, and in the case of proteins and amino acids, enriched in ¹⁵N as well. We conclude that HF treatment released fresh, soluble, probably microbial, biomass in addition to carbohydrates. Net changes of the bulk chemical composition of organic matter were small for most soils, size fractions and plant material, but not for samples containing little organic matter, or those rich in easily soluble organic matter associated with iron oxides, such as Podzol B horizons.     

The phenanthrene‐sorptive interface of an arable topsoil and its particle size fractions

Sorption of organic chemicals in soil is affected by the properties and availability of surfaces. These surfaces are composed of diverse mineral, organic and biological components, forming a soil's ‘biogeochemical interface’. Phenanthrene was used to probe the hydrophobic sorptive capacity of the interface of an arable soil. Batch sorption experiments were carried out with the bulk soil as well as the fine (0.2–6.3 µm) and coarse (6.3–63 µm) particle size fractions of two arable topsoil samples with different organic matter (OM) contents from a Eutric Cambisol. The specific surface area (SSA) of the bulk soil and particle size fractions was determined by BET‐N₂ and EGME sorption. OM composition was characterized by solid‐state ¹³C NMR spectroscopy. No clear relationship was found between phenanthrene sorption and SSA. We conclude that phenanthrene probes a specific fraction of the soil interface that is not well represented by the traditional methods of SSA detection such as BET‐N₂ and EGME sorption. The sorption behaviour of phenanthrene may therefore provide a useful additional tool to characterize the specific affinity of the soil biogeochemical interface for hydrophobic molecules. Sorption capacity for phenanthrene increased after particle‐size fractionation, indicating that the reduced availability of the interface caused by the aggregated structure is important for the sorptive capacity of a soil. This should be considered when projecting data obtained from extensively treated and fractionated samples to the actual interaction with biogeochemical interfaces as they are present in soil.     

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 of plant tissue submerged in an extremely acidic mining lake sediment: phenolic CuO-oxidation products and solid-state C NMR spectroscopy

In extremely acidic mining sediments of the Lusatian mining district, the alkalinisation process relies on organic C, which can serve as electron donor for microbially induced sulfate reduction. Plant material of the pioneer plant Juncus bulbosus is an important organic matter source in lake sediments. Therefore, decomposition of the plant tissue was assessed during the exposure of litterbags for 30 months in the 0-5 cm layer of waterlogged mining sediments, which have a pH between 2.5 and 3. The ash free dry weight (AFDW) and elemental content of the plant tissue were recorded several times during the exposure. Changes in chemical structure were analyzed by solid-state ¹³C cross polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy and the lignin component characterized by wet-chemical CuO oxidation. The AFDW accounted for about 34% of initial biomass after field exposure for 30 months. Mass loss of biomass occurred in two phases with decomposition rates varying between 30 and 430 mg AFDW d⁻¹. The mass loss increased considerably after 5-7 months when litterbags were invaded by fresh J. bulbosus plants. With respect to higher mass loss, ¹³C CPMAS NMR spectroscopy, showed slight changes of the bulk chemical composition after 11 months, indicating that microorganisms present in the sediments or in the rhizosphere degrade plant material as a whole, rather than selectively. During the second phase from about 11 months until the end of the exposure period, contribution of O-alkyl C most probably assignable to easily degradable polysaccharides decreased. In contrast, the contribution of alkyl, aromatic and carboxyl C increased. CuO oxidation showed that the lignin component of J. bulbosus is degraded oxidatively during field exposure. Our results indicate that the exposed plant material is decomposed in the sediment due to changes in sediment conditions that followed plant invasion of the litterbags. It is suggested that the rhizosphere of J. bulbosus by its influence on the redox potential, pH and the microbial component plays a crucial role in organic matter degradation in acidic mining sediments.     

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.     

Transformation of organic matter from maize residues into labile and humic fractions of three European soils as revealed by 13C distribution and CPMAS-NMR spectra

The dynamics of incorporation of fresh organic residues into the various fractions of soil organic matter have yet to be clarified in terms of chemical structures and mechanisms involved. We studied by ¹³C‐dilution analysis and CPMAS‐¹³C‐NMR spectroscopy the distribution of organic carbon from mixed or mulched maize residues into specific defined fractions such as carbohydrates and humic fractions isolated by selective extractants in a year‐long incubation of three European soils. The contents of carbohydrates in soil particle size fractions and relative δ¹³C values showed no retention of carbohydrates from maize but rather decomposition of those from native organic matter in the soil. By contrast, CPMAS‐¹³C‐NMR spectra of humic (HA) and fulvic acids (FA) extracted by alkaline solution generally indicated the transfer of maize C (mostly carbohydrates and peptides) into humic materials, whereas spectra of organic matter extracted with an acetone solution (HE) indicated solubilization of an aliphatic‐rich, hydrophobic fraction that seemed not to contain any C from maize. The abundance of ¹³C showed that all humic fractions behaved as a sink for C from maize residues but the FA fraction was related to the turnover of fresh organic matter more than the HA. Removal of hydrophobic components from incubated soils by acetone solution allowed a subsequent extraction of HA and, especially, FA still containing much C from maize. The combination of isotopic measurements and NMR spectra indicated that while hydrophilic compounds from maize were retained in HA and FA, hydrophobic components in the HE fraction had chemical features similar to those of humin. Our results show that the organic compounds released in soils by mineralization of fresh plant residues are stored mainly in the hydrophilic fraction of humic substances which are, in turn, stabilized against microbial degradation by the most hydrophobic humic matter. Our findings suggest that native soil humic substances contribute to the accumulation of new organic matter in soils.     

Liming upland grassland: the effects on earthworm communities and the chemical characteristics of carbon in casts

Different earthworm species have different tolerances of acid soil conditions, and the application of lime to upland grassland to improve the grazing quality may therefore alter the size and diversity of the earthworm community. Altering soil properties may also affect the chemical characteristics of organic C in earthworm casts. We surveyed the earthworm community of an upland grassland in southern Scotland at the outset of annual lime applications, and after 3 years, and used ¹³C nuclear magnetic resonance (NMR) spectroscopy to assess the distribution of C between different functional groups in the organic matter. In addition, soil was incubated for 8 weeks with several earthworm species in the presence or absence of lime, and the earthworm casts were subsequently analysed by ¹³C NMR spectroscopy. Liming did not significantly affect earthworm abundance or species diversity, but it did affect the chemical composition of the casts. Casts from earthworms incubated in unlimed soil had greater ratios of alkyl‐C to O?alkyl‐C, indicative of more decomposed, recalcitrant C, and spectra from litter‐feeding species had the greatest intensities of O?alkyl‐C signals. In limed soil, the largest O?alkyl‐C signal intensities were not restricted to litter‐feeding species, indicating an increase in the quality of organic matter ingested by geophagous species.