Short-term dynamics of slurry-derived plant and microbial sugars in a temperate grassland soil as assessed by compound-specific delta13C analyses

In view of recent discussions about climate change and the anthropogenically enhanced greenhouse effect, the aim of this study was to determine the short-term carbon (C) dynamics in a grassland soil after slurry application. It is known that, depending on cultivation practices, agro-ecosystems can act either as sources or as sinks for atmospheric CO2. C3 and C4 slurries were applied, differing in their stable C isotope signature, to be able to differentiate between native (soil-inherent) and fresh (slurry-applied) C. Samples were taken from 0-2, 2-7.5 and 7.5-15 cm soil depths from 90 days before until 4 weeks after slurry application at various intervals. We carried out compound-specific stable isotope analysis (CSIA) of plant- (arabinose and xylose) and microbial-derived sugars (fucose and rhamnose). Up to 45% of the applied slurry-derived xylose was found in the 0-2 cm soil depth within 24 h after slurry application, with this figure decreasing rapidly and then increasing again towards the end of the experiment. Therefore, during the first phase of slurry incorporation, preferentially the soluble part of slurry entered the first 2 cm of soil while, after about 2 weeks, particulate slurry-derived organic matter was incorporated into the soil. The ratio between plant- and microbial-derived sugars together with delta13C values of individual sugars in the 2-7.5 cm soil depth revealed that the dissipation of sugars from the 0-2 cm soil depth was not only due to leaching, but also was caused by microbial degradation of the fresh C because slurry did not contain significant amounts of rhamnose while the delta13C values of rhamnose became progressively enriched in 13C during the experiment. Stable isotope measurements of bulk soil previously only showed significant differences between C4 and C3 plots at 0-2 cm soil depth. The CSIA of the individual sugars was much more sensitive than bulk isotope measurements, revealing significant differences between C4 and C3 plots even at the 2-7.5 cm soil depth during the first phase of the experiment. Additionally, the dynamics of slurry-derived plant and microbial sugars could be followed specifically.     

Determination of stable carbon isotopes of organic acids and carbonaceous aerosols in the atmosphere

A wet oxidation method for the compound-specific determination of stable carbon isotopes (delta(13)C) of organic acids in the gas and aerosol phase, as well as of water-soluble organic carbon (WSOC), is presented. Sampling of the organic acids was done using a wet effluent diffusion denuder/aerosol collector (WEDD/AC) coupled to an ion chromatography (IC) system. The method allows for compound-specific stable carbon isotope analysis by collecting different fractions of organic acids at the end of the IC system using a fraction collector. delta(13)C analyses of organic acids were conducted by oxidizing the organic acids with sodium persulfate at a temperature of 100 degrees C and determining the delta(13)C value of the resulting carbon dioxide (CO(2)) with an isotope ratio mass spectrometer. In addition, analysis of delta(13)C of the WSOC was performed for particulate carbon collected on aerosol filters. The WSOC was extracted from the filters using ultrapure water (MQ water), and the dissolved organic carbon was oxidized to CO(2) using the oxidation method. The wet oxidation method has an accuracy of 0.5 per thousand with a precision of +/-0.4 per thousand and provides a quantitative result for organic carbon with a detection limit of 150 ng of carbon.     

Use of 13C to monitor soil organic matter transformations caused by a simulated forest fire

Soil organic matter (SOM) transformations caused by heating were analyzed using the stable carbon isotope (13)C as a tracer to follow C mineralization dynamics and C transfers between different organic compartments. A (13)C-labelled soil, obtained by incorporation of (13)C-enriched Lolium perenne phytomass into a pine forest soil, was heated for 10 min at 385 degrees C to reproduce conditions typical of a forest fire and changes in total C content, potential C mineralization activity and C distribution between the different soil organic fractions were determined. Changes caused by heating on the potential soil C mineralization, determined by laboratory aerobic incubation, reveal alterations to the SOM biodegradability; some stabilized SOM showed an increase in biodegradability, whereas less stabilized SOM became more resistant to microorganisms. Chemical fractionations of SOM allowed us to monitor changes in its composition. As a consequence of heating, the less polymerized humic fractions were the most strongly affected, with the total disappearance of fulvic acids. A significant increase in the quantity and degree of polymerization of the humic acids at the expense of other more (13)C-enriched substances was also found. Finally, a large decrease in humin was observed, its solubilizable part disappearing completely, probably as a consequence of the incorporation of the byproducts into the free organic matter fraction.     

Relationship between soil organic C degradability and the evolution of the delta13C signature in profiles under permanent grassland

The main objective of this research was to investigate to what extent the potential C dynamics of soil organic matter (SOM) are related to the degree of 13C enrichment with increasing depth in soil profiles under permanent grassland. The evolution of the C content and the 13C natural abundance (delta13C value) of SOM were investigated in three soil profiles (0-40 cm depth) under permanent grassland of varying texture (a loamy sand, a loam and a clay loam soil). The delta13C value of the SOM showed a gradual increase with increasing depth and decreasing C content in the profiles, ranging from 1.9 per thousand (loamy sand soil), 2.9 per thousand (clay loam soil) and 4 per thousand (loam soil) in relation to the delta13C value of SOM at the surface. The relationship between the 13C enrichment and total organic C content at different depths in the profiles (down to 40 cm depth in the loam and clay loam soil, down to 25 cm depth in the loamy sand soil) could be well described by the Rayleigh equation. The enrichment factors epsilon, associated with the Rayleigh approximation of the data, ranged from -1.57 per thousand (clay loam soil) to -1.64 per thousand (loamy sand soil) and -1.91 per thousand (loam soil). The potential C dynamics in four depth intervals from the profiles (0-10, 10-20, 20-30 and 30-40 cm depth) were determined by means of an incubation experiment. The C decomposition rate constants from the four sampling depths in the profiles showed a significant, positive correlation (y = 0.21x + 0.018, R(2) = 0.66, p < 0.005) with the corresponding Deltadelta13C values (change of the delta13C value per depth increment). A better correlation was obtained when only the data from the upper 20 cm in the profiles (y = 0.21x + 0.019, R(2) = 0.78, p < 0.05) were considered. These results suggest that the Deltadelta13C values in the surface layers of profiles under permanent grassland may serve as an indicator of the potential degradability or the stability of the SOM (in terms of C decomposition rate constants).     

Molecular turnover time of soil organic matter in particle‐size fractions of an arable soil

The composition and molecular residence time of soil organic matter (SOM) in four particle‐size fractions (POM >200 µm, POM 63–200 µm, silt and clay) were determined using Curie‐point pyrolysis/gas chromatography coupled on‐line to mass spectrometry. The fractions were isolated from soils, either continuously with a C₃ wheat (soil ¹³C value = ?26.4‰), or transferred to a C₄ maize (soil ¹³C value = ?20.2‰) cropping system 23 years ago. Pyrograms contained up to 45 different pyrolysis peaks; 37 (ca. 85%) were identifiable compounds. Lignins and carbohydrates dominated the POM fractions, proteins were abundant, but lignin was (nearly) absent in the silt and clay fractions. The mean turnover time (MRT) for the pyrolysis products in particulate organic matter (POM) was generally <15 years (fast C pool) and 20–300 years (medium or slow C pools) in silt and clay fractions. Methylcyclopentenone (carbohydrate) in the clay fraction and benzene (mixed source) in the silt fraction exhibited the longest MRTs, 297 and 159 years, respectively. Plant‐derived organic matter was not stored in soils, but was transformed to microbial remains, mainly in the form of carbohydrates and proteins and held in soil by organo‐mineral interactions. Selective preservation of plant‐derived OM (i.e. lignin) based on chemical recalcitrance was not observed in these arable soils. Association/presence of C with silt or clays in soils clearly increased MRT values, but in an as yet unresolved manner (i.e. ‘truly’ stabilized, or potentially still ‘labile’ but just not accessible C). Copyright © 2009 John Wiley & Sons, Ltd.     

Influence of recent vegetation on labile and recalcitrant carbon soil pools in central Queensland, Australia: evidence from thermal analysis-quadrupole mass spectrometry-isotope ratio mass spectrometry

The effect of a recent vegetation change (<100 years) from C(4) grassland to C(3) woodland in central Queensland, Australia, on soil organic matter (SOM) composition and SOM dynamics has been investigated using a novel coupled thermogravimetry-differential scanning calorimetry-quadrupole.mass spectrometry-isotope ratio mass spectrometry (TG-DSC-QMS-IRMS) system. TG-DSC-QMS-IRMS distinguishes the C isotope composition of discrete SOM pools, showing changes in labile, recalcitrant and refractory carbon in the bulk soil and particle size fractions which track the vegetation changes. Analysis of evolved gases (by QMS) from thermal decomposition, rather than observed weight loss, proved essential in determining the temperature at which SOM decomposes, because smectite and kaolinite clays contribute to observed weight losses. The delta(13)C analyses of the CO(2) evolved at different temperatures for bulk soil and particle size-separates showed that most of the labile SOM under the more recent woody vegetation was C(3)-derived carbon whereas the delta(13)C values in the recalcitrant SOM showed greater C(4) contributions. This indicated a shift from grass (C(4))- to tree (C(3))-derived carbon in the woodland, which was also supported by the two-phase (13)C enrichment with depth, i.e. C(3) vegetation dominated the top soil (0-10 cm), but the C(4) contribution increased with depth (more gradual). This is perturbed by the inclusion of charcoal from forest fires ((14)C age incursions) and by the deep incorporation of C(3) carbon due to root penetration.     

Assessing the use of delta(13)C natural abundance in separation of root and microbial respiration in a Danish beech (Fagus sylvatica L.) forest

Our understanding of forest biosphere-atmosphere interactions is fundamental for predicting forest ecosystem responses to climatic changes. Currently, however, our knowledge is incomplete partly due to inability to separate the major components of soil CO(2) effluxes, viz. root respiration, microbial decomposition of soil organic matter and microbial decomposition of litter material. In this study we examined whether the delta(13)C characteristics of solid organic matter and respired CO(2) from different soil-C components and root respiration in a Danish beech forest were useful to provide information on the root respiration contribution to total CO(2) effluxes. The delta(13)C isotopic analyses of CO(2) were performed using a FinniganMAT Delta(PLUS) isotope-ratio mass spectrometer coupled in continuous flow mode to a trace gas preparation-concentration unit (PreCon). Gas samples in 2-mL crimp seal vials were analysed in a fully automatic mode with an experimental standard error +/-0.11 per thousand. We observed that the CO(2) derived from root-free mineral soil horizons (A, B(W)) was more enriched in (13)C (delta(13)C range -21.6 to -21.2 per thousand ) compared with CO(2) derived from root-free humus layers (delta(13)C range -23.6 to -23.4 per thousand ). The CO(2) evolved from root respiration in isolated young beech plants revealed a value intermediate between those for the soil humus and mineral horizons, delta(13)C(root) = -22.2 per thousand, but was associated with great variability (SE +/- 1.0 per thousand ) due to plant-specific differences. delta(13)C of CO(2) from in situ below-ground respiration averaged -22.8 per thousand, intermediate between the values for the humus layer and root respiration, but variability was great (SE +/- 0.4 per thousand ) due to pronounced spatial patterns. Overall, we were unable to statistically separate the CO(2) of root respiration vs. soil organic matter decomposition based solely on delta(13)C signatures, yet the trend in the data suggests that root respiration contributed approximately 43% to total respiration. The vertical gradient in delta(13)C, however, might be a useful tool in partitioning respiration in different soil layers. The experiment also showed an unexpected (13)C-enrichment of CO(2) (>3.5 per thousand ) compared with the total-C signatures in the individual soil-C components. This may suggest that analyses of bulk samples are not representative for the C-pools actively undergoing decomposition.     

Temperature-dependent shift from labile to recalcitrant carbon sources of arctic heterotrophs

Soils of high latitudes store approximately one-third of the global soil carbon pool. Decomposition of soil organic matter (SOM) is expected to increase in response to global warming, which is most pronounced in northern latitudes. It is, however, unclear if microorganisms are able to utilize more stable, recalcitrant C pools, when labile soil carbon pools will be depleted due to increasing temperatures. Here we report on an incubation experiment with intact soil cores of a frost-boil tundra ecosystem at three different temperatures (2 degrees C, 12 degrees C and 24 degrees C). In order to assess which fractions of the SOM are available for decomposition at various temperatures, we analyzed the isotopic signature of respired CO2 and of different SOM fractions. The delta13C values of CO2 respired were negatively correlated with temperature, indicating the utilization of SOM fractions that were depleted in 13C at higher temperatures. Chemical fractionation of SOM showed that the water-soluble fraction (presumably the most easily available substrates for microbial respiration) was most enriched in 13C, while the acid-insoluble pool (recalcitrant substrates) was most depleted in 13C. Our results therefore suggest that, at higher temperatures, recalcitrant compounds are preferentially respired by arctic microbes. When the isotopic signatures of respired CO2 of soils which had been incubated at 24 degrees C were measured at 12 degrees C, the delta13C values shifted to values found in soils incubated at 12 degrees C, indicating the reversible use of more easily available substrates. Analysis of phospholipid fatty acid profiles showed significant differences in microbial community structure at various incubation temperatures indicating that microorganisms with preference for more recalcitrant compounds establish as temperatures increase. In summary our results demonstrate that a large portion of tundra SOM is potentially mineralizable.     

Long-term release of carbon from grassland soil amended with different slurry particle size fractions: a laboratory incubation study

Application of animal manure to agricultural soils enhances both native soil carbon (C) and overall (native soil C and added C) respiration. CO(2) effluxes were measured in a laboratory incubation study for 1465 days after the application of different slurry fractions (>2000, 425-2000, 250-425, 150-250, 45-150 and <45 μm) to a grassland soil. The slurry-derived C present in the soil was traced using the natural abundance δ(13)C method. We used two kinetic (single and two pool) models to fit the experimental data and to test the model validity with respect to long-term data sets. Mean residence times (MRTs) of the particle size based slurry-C fractions were estimated using these models and a linear (13)C natural abundance based approach. The results showed that slurry-C degradation in soil over time varied between the different particle size based slurry treatments. The two kinetic soil-C models were successful to predict medium- to long-term carbon release from soil amended with animal slurry. The estimated MRTs did vary between the linear (3.8-5.6 years) and non-linear based (0.8-3.8 years) (model) approaches. Slurry-derived C could still be (isotopically) detected in the soil 4 years after slurry application using the natural abundance δ(13)C method. This suggests that it may take a decadal timescale or longer before the entire amount of C introduced through whole slurry amendments to grassland soils is fully dissipated.     

土壤有机质对菲的吸附-解吸平衡的影响

以自然土壤和过氧化氢分级土壤为实验模拟样品,测定了菲在这些样品上的吸附一解吸等温线,用线性和Freundlich模型拟合了这些等温线．^13C NMR谱表明,随着土壤有机质腐殖化程度的加深,有机质将含有较多的长链烷烃化合物,含氧、氮化合物有所减少,芳香环的数量变化不大．吸附实验结果表明,土壤有机质含量与菲的吸附容量存在一定的线性相关关系．有机质腐殖质化程度较深的样品比原土壤具有更大的吸附容量,其吸附等温线表现出更为明显的非线性,而且具有更明显的解吸滞后现象．说明土壤中一些结构紧密和含极性官能团较少的有机质是引起菲的非线性吸附过程和解吸滞后现象的主要原因。     

Three sources of CO2 efflux from soil partitioned by 13C natural abundance in an incubation study

This study describes a novel approach to separate three soil carbon (C) sources by one tracer method (here 13C natural abundance). The approach is based on the combination of C3 and C4 sources in different treatments, identical decomposition of C3 and C4 substances in soil, and subsequent calculation of their contribution to the total CO2 efflux. We used the temporal dynamics of the CO2 efflux from a C3 grassland soil amended with added C3 or C4 slurry and/or C3 or C4 sugar to estimate contributions of three separate C sources: native soil organic matter (SOM), slurry and sugar, to CO2 efflux. Soil with slurry and/or sugar was incubated under controlled conditions, and concentration and delta13C values of evolved CO2 were measured over a 2-week period. The main assumption needed for separation of three C sources in CO2 efflux, i.e. identical decomposition of applied C3 and C4 sugars in soil, was investigated and proven. The relative contribution to the CO2 efflux increased, but its duration decreased with an increased microbial availability of the C source, i.e. sugar > slurry > SOM. The microorganisms used the C sources according to their availability. The contribution of sugar to the CO2 efflux was finished after 2-4 days. Separation of three CO2 sources and comparison of CO2 from different treatments tracing the changes of SOM and slurry decomposition induced by addition of sugar were investigated. During the sugar decomposition (the first 2-4 days), the SOM decomposition strongly decreased. At the same time the contribution of slurry-C to CO2 increased. The shortcomings and limitations as well as possible future applications of the suggested method including FACE (Free Air Carbon dioxide Enrichments) and continuous labelling experiments are discussed.     

Evolution of the delta13C signature related to total carbon contents and carbon decomposition rate constants in a soil profile under grassland

Evolution of the total carbon (C) content and the (13)C enrichment (delta(13)C signature) of soil organic matter (SOM) with increasing depth in a soil profile under permanent grassland (C(3) vegetation) were investigated. The relationship between the total C content and the delta(13)C signature at different depths in the upper 30 cm of the soil profile could be well fitted by the Rayleigh equation (y = -29.8 - 2.3x, R(2) = 0.95, p < 0.001), describing the enrichment in (13)C as resulting from isotopic fractionation associated with C mineralization (isotope enrichment factor epsilon = -2.3 per thousand). Potential C dynamics of SOM in four depth intervals of the profile (0-10, 10-20, 20-30 and 30-40 cm depth) were investigated through an incubation study. The C decomposition rate constants decreased with increasing sampling depth from 0.0479 yr(-1) (0-10 cm sampling depth) to 0.0256 yr(-1) (30-40 cm sampling depth) and were highly correlated (y = 0.02 + 0.13x, R(2) = 0.93, p < 0.05) with the corresponding deltadelta(13)C values (average change of the delta(13)C signature per depth increment). These results suggest that changes of the delta(13)C signature of SOM in undisturbed soil profiles under continuous C(3) vegetation may serve as an indicator of the variation of SOM quality with increasing depth.     

Applications of stable isotope ratio mass spectrometry in cattle dung carbon cycling studies

Understanding the fate of dung carbon (C) in soils is challenging due to the ubiquitous presence of the plant‐derived organic matter (OM), the source material from which both dung‐derived OM and soil organic matter (SOM) predominantly originate. A better understanding of the fate of specific components of this substantial source of OM, and thereby its contribution to C cycling in terrestrial ecosystems, can only be achieved through the use of labelled dung treatments. In this short review, we consider analytical approaches using bulk and compound‐specific stable carbon isotope analysis that have been utilised to explore the fate of dung‐derived C in soils. Bulk stable carbon isotope analyses are now used routinely to explore OM matter cycling in soils, and have shown that up to 20% of applied dung C may be incorporated into the surface soil horizons several weeks after application, with up to 8% remaining in the soil profile after one year. However, whole soil δ¹³C values represent the average of a wide range of organic components with varying δ¹³C values and mean residence times in soils. Several stable ¹³C isotope ratio mass spectrometric methods have been developed to qualify and quantify different fractions of OM in soils and other complex matrices. In particular, thermogravimetry‐differential scanning calorimetry‐isotope ratio mass spectrometry (TG‐DSC‐IRMS) and gas chromatography‐combustion‐IRMS (GC‐C‐IRMS) analyses have been applied to determine the incorporation and turnover of polymeric plant cell wall materials from C₄ dung into C₃ grassland soils using natural abundance ¹³C isotope labelling. Both approaches showed that fluxes of C derived from polysaccharides, i.e. as cellulose or monosaccharide components, were more similar to the behaviour of bulk dung C in soil than lignin. However, lignin and its 4‐hydroxypropanoid monomers were unexpectedly dynamic in soil. These findings provide further evidence for emerging themes in biogeochemical investigations of soil OM dynamics that challenge perceived concepts of recalcitrance of C pools in soils, which may have profound implications for the assessment of the potential of agricultural soils to influence terrestrial C sinks. Copyright © 2010 John Wiley & Sons, Ltd.     

Advanced CPMAS-13C NMR techniques for molecular characterization of size-separated fractions from a soil humic acid

A humic acid extracted from a volcanic soil was subjected to preparative high-performance size-exclusion chromatography (HPSEC) to reduce its molecular complexity and eleven different size fractions were obtained. Cross-polarization magic-angle spinning ¹³C NMR (CPMAS ¹³C NMR) analysis performed with variable contact-time (VCT) pulse sequences showed that the largest molecular-size fractions contained aromatic, alkyl, and carbohydrate-like components. The carbohydrate-like content and the alkyl chain length seemed to decrease with decreasing molecular size. Progressive reduction of aromatic carbon atoms was also observed with decreasing molecular size of the separated fractions. Mathematical treatment of the results from VCT experiments enabled cross polarization (T _CH) and proton spin–lattice relaxation () times to be related to structural differences among the size fractions. The conformational distribution indicated that the eleven size fractions could be allocated to two main groups. The first group, with larger nominal molecular sizes, was characterized by molecular domains with slower local molecular motion. The second group of size fractions, with smaller nominal molecular sizes, was characterized by a larger number of molecular domains with faster local molecular motion. The T _CH and values suggested that either condensed or strongly associated aromatic systems were predominant in the size fractions with the largest apparent molecular dimensions.     

Application of Pyrolysis Cryogenic Trap Gas Chromatography/Atomic Emission Detection (Py-CryT-GC/AED) for the Study of Nitrogen Forms and Dynamics in a Grassland Soil

Soil organic nitrogen (N) was characterized for its chemical species and chemical transformations in a grassland soil profile by using a combination of cryogenic pyrolysis gas chromatography atomic emission detection method with soil physical size fractionation. The soils taken from 0–12, 12–25, and 25–38 cm depth layers were separated into five fractions, <2, 2–38, 38–53, 53–105, and 105–250 µm and each of which was analyzed for total organic C and N, and different N forms. Our results showed that (1) total organic carbon has a positive correlation with the total organic nitrogen (TON) with correlation coefficient increased with soil depth; (2) deep and small particle-size fraction soils yielded more volatile pyrolysate N than the surface and large particle-size fractions and the amount of volatile pyrolysate N has a linear positive correlation with TON and correlation coefficient increased with soil depth; (3) the major components of the volatile pyrolysate N include ammonia, acetonitrile, hydrogen cyanide, pyridine, and pyrrole; (4) of the total volatile pyrolysate N, ammonia accounted for more than 40%, and the sum of acetonitrile and hydrogen cyanide accounted for approximately 30–50%; and (5) the amounts of acetonitrile, hydrogen cyanide, and pyridine had increased positive correlations with TON with increasing soil depth, but the correlation between the amount of pyrrole and depth decreased in the opposite direction. Our research result sheds some light into soil organic nitrogen forms and its transformations in the processes of soil organic C aging and stabilization.     

Changes in microbial community structure and function within particle size fractions of a paddy soil under different long-term fertilization treatments from the Tai Lake region, China

Greenhouse gas (GHG) production and emission from paddy soils impacts global climate change. Soil particle size fractions (PSFs) of different sizes act as soil microhabitats for different kinds of microbial biota with varying conditions of redox reactions and soil organic matter (SOC) substrates. It is crucial to understand the distribution of soil microbial community structure within PSFs and linkage to the GHG production from paddy soils of China. The change of bacterial and methangenic archaeal community and activity relating to CH₄ and CO₂ production with PSFs under different fertilizer applications was studied in this paper. The fertilization trial was initiated in a paddy soil from the Tai Lake region, Jiangsu, China with four treatments of non-fertilized (NF), fertilized with inorganic fertilizers only (CF), inorganic with pig manure (CFM) and inorganic with straw return (CFS), respectively since 1987, and the PSFs (<2 μm, 2–20 μm, 20–200 μm, and 200–2000 μm) were separated by a low energy sonication dispersion procedure from undisturbed samples. Analysis of bacterial community within different size particles was conducted by PCR-DGGE. The results indicated significant variation of bacterial community structure within different PSFs. The methane was predominantly produced in the coarser fractions, while more species and higher diversity of bacteria survived in the size of <2 μm fractions, in which the bacterial community structure was more significantly affected by fertilizer application practices than in the other coarser fractions. Higher bacterial species richness and more diversities in the smallest size fractions was due to the vicinity between microbes, access to carbon resource outside the microaggregates, and smaller pore size as protective agent suitable habitats for microbes rather than high SOC. Whereas, higher CO₂, CH₄ production and methanogenic archaeal community in coarser fractions may be contributed to storage of labile organic carbon in these fractions. It indicated that availability of SOC in PSFs is mainly factor affected survival of methanogenic archaeal community structure, whereas, bacterium community habitation more affected by physical protection of their location in PSFs. Their activity greatly depended on liability of SOC access to PSFs. Fertilizer application caused more change of bacteria community in clay fraction and greatly increased bacterium and methanogen activity in coarser fractions but only a slight effect on methanogenic archaeal community in the particle size fractions.     

Effects of soil pH and organic matter on distribution of thorium fractions in soil contaminated by rare-earth industries

The labilities of thorium fractions including mobility and bioavailability vary significantly with soil properties. The effects of soil pH and soil organic matter on the distribution and transfer of thorium fractions defined by a sequential extraction procedure were investigated. Decrease of soil pH could enhance the phytoavailability and the potential availability of thorium in soil. Increase of organic matter reduced the phytoavailability of thorium, but enhanced the potential availability of it. The reasons why soil pH and soil organic matter affect thorium fractions were discussed, and the behavior of the effects of soil properties on thorium fractions was elucidated. Fourier-transform infrared (FTIR) spectra were employed to reveal the positive relationship between the amounts adsorbed in humic material and/or amorphous oxides and the content of soil organic matter.     

Rapid extraction of water soluble organic compounds from airborne particulate matter.

Water soluble organic compounds (WSOC) in airborne particulate matter (PM) have received considerable attention in recent years due to their abundance and their importance in atmospheric processes. The analysis of WSOC is necessary for quantifying the relative contribution of individual organic compounds to the total WSOC mass. In the present work, we evaluated the performance of a microwave-assisted extraction (MAE) method for the determination of WSOC in PM and compared the data with those of a conventional ultrasonic extraction (USE). The experimental results showed that the MAE method requires a shorter extraction time (5 min) compared to USE. The isolated water-soluble organic fraction of PM was subsequently analyzed using ion chromatography (IC) for low molecular weight organic acids. The rapid MAE method was used in conjunction with IC for the analysis of organic acids in PM samples, collected from different sources.     

Movement of newly assimilated 13C carbon in the grass Lolium perenne and its incorporation into rhizosphere microbial DNA

One of the key processes that drives rhizosphere microbial activity is the exudation of soluble organic carbon (C) by plant roots. We describe an experiment designed to determine the impact of defoliation on the partitioning and movement of C in grass (Lolium perenne L.), soil and grass‐sterile sand microcosms, using a ¹³CO₂ pulse‐labelling method. The pulse‐derived ¹³C in the shoots declined over time, but that of the roots remained stable throughout the experiment. There were peaks in the atom% ¹³C of rhizosphere CO₂ in the first few hours after labelling probably due to root respiration, and again at around 100 h. The second peak was only seen in the soil microcosms and not in those with sterilised sand as the growth medium, indicating possible microbial activity. Incorporation of the ¹³C label into the microbial biomass increased at 100 h when incorporation into replicating cells, as indicated by the amounts of the label in the microbial DNA, started to increase. These results indicate that the rhizosphere environment is conducive to bacterial growth and replication. The results also show that defoliation had no impact on the pattern of movement of ¹³C from plant roots into the microbial population in the rhizosphere. Copyright © 2010 John Wiley & Sons, Ltd.     

TMAH-preparative thermochemolysis for the characterization of organic matter in densimetric fractions of a Mediterranean forest soil

Physical protection is one of the most important ways for stabilization of organic carbon in soils, and in order to properly manage soils as a sink for carbon, it is necessary to know how much organic carbon a given soil could protect and to have information on the molecular composition of this protected organic matter in soil. To this end, we studied individual horizons taken from a soil profile under Quercus rotundifolia stands over calcareous parent material. Horizons were subjected to a sequential extraction using solutions of sodium polytungstate (NaPT) of increasing density (1.6, 1.8 and 2.0) to differentiate five fractions: a free light, extractable without sonication, three occluded (extractable by sonication) and a dense (retained in the dense residue, after sonication). The obtained fractions were analyzed by preparative thermochemolysis followed by gas chromatography–mass spectrometry (GC/MS) in order to get some insight on the molecular composition. The total ion chromatograms obtained for the pyrolysates of both of the densimetric fractions show various series of fatty acids (as their methyl esters), n-alkanols (as their methyl ethers), methylated α,ω-diacids, methylated ω-hydroxyacids, various lignous subunits and permethylated deoxy aldonic acids derived from carbohydrates. The comparison of the distributions of the thermochemolysis products shows that organic carbon in the dense fractions of the deepest horizons were more influenced by a microbial reworking than the others dense fractions from the upper horizons. It is also the case for the occluded fraction 1 of the H horizon even the vegetal part of the organic carbon in that occluded fraction appears to have a non-woody origin. On the other hand, the dense fraction of the H horizon is strongly marked by vegetal origin.