Inert Pool Research Articles

Radiocarbon and Stable Carbon Isotopes of Labile and Inert Organic Carbon in the Critical Zone Observatory in Illinois, USA

ABSTRACTWe applied the high temperature pyrolysis-combustion technique to partition the total soil organic carbon (SOC) into labile and inert carbon pools for accelerator mass spectrometry radiocarbon (AMS 14C) dating and stable carbon isotope (δ13C), SOC, and carbonate carbon (CC) content analyses to examine SOC variability at a Critical Zone Observatory site in Illinois, USA. The AMS 14C dates of labile and inert carbon in the top 1.55 m overlap except in the Bt horizon. Below 1.55 m the labile carbon is younger by 8000–14,800 years. The SOC content decreases from 3.61% to 0.12% and CC content increases from 0% to 19.16% at this depth. Results indicate that SOC production exceeds its loss in the weathering zone causing a continuous turnover of both SOC pools. A small amount of modern SOC infiltrates into deeper sediment below 1.55 m, making the labile carbon pool much younger. Their difference of AMS 14C contents, ΔF14C, reveals 3−5% more modern carbon in the labile SOC pools except in the Bt horizon, further quantifying that <3−5% modern carbon with potential pollutants is translocated into the unweathered sediments. The δ13C reveals the sources for SOC cycling dynamics in both carbon pools at this site.

Kazakhstan black soil organic substance agrogenic transformation

Currently despite the extended study of the temporary dynamic of the cultivated soil organic substance on the global scale scientists pay much attention to the establishing the patterns of its quantitative transformation on the regional level depending on the systems of agricultural exploitation for the purpose of clarifying the worldwide estimate of humus reserve in relation to the issue of the greenhouse effect. In this regard we have examined the modern dynamic (19902015) of the regular and southern black soil organic substance in the conditions of long-term stationary field experiments in the primary grain-sowing region of the Northern Kazakhstan. Based on the data from alternative time periods on the proportion of Соргand N in the upper (mainly 025 cm) layer of virgin and agrogenic black soil of Akmolinsk and Kostanay regions of Kazakhstan we have discovered the distinctive characteristics of changes in the content of soil organic substance. It has been demonstrated that the content of humus in the regular black soil is significantly higher than that in the southern carbonated black soil. Evaluation of transforming and inert pool of organic carbon in virgin and agrogenic variants of southern black soil has been made. It has been established that currently in the result of influence from various methods of southern black soil treatment the rate of humus percentage decrease is slowing down in comparison with that of the previous period of its agricultural exploitation, however, in practice, those methods, including the subsurface tilling, do not halt the de-humusization process. Content of humus in soil of the variants with continuous spring wheat is approaching that with continuous complete fallow. There are sufficient grounds to suggest that the reason for de-humusization of the southern black soil could reside in biochemical (mineralization), as well as physical (deflation) loss of humus. Particular danger comes from the wind erosion resulting in percentage of humus dropping even below its minimal value, i.e. due to the decrease of nearly unrecoverable inert part. In agrogenic regular black soil, during the closing period of its agricultural exploitation, there has been no humus loss detected, which is most probably related to the existence of protective anti-erosion forest belts in the studied fields area.

Parametrization consequences of constraining soil organic matter models by total carbon and radiocarbon using long-term field data

Abstract. Soil organic carbon (SOC) dynamics result from different interacting processes and controls on spatial scales from sub-aggregate to pedon to the whole ecosystem. These complex dynamics are translated into models as abundant degrees of freedom. This high number of not directly measurable variables and, on the other hand, very limited data at disposal result in equifinality and parameter uncertainty. Carbon radioisotope measurements are a proxy for SOC age both at annual to decadal (bomb peak based) and centennial to millennial timescales (radio decay based), and thus can be used in addition to total organic C for constraining SOC models. By considering this additional information, uncertainties in model structure and parameters may be reduced. To test this hypothesis we studied SOC dynamics and their defining kinetic parameters in the Zürich Organic Fertilization Experiment (ZOFE) experiment, a > 60-year-old controlled cropland experiment in Switzerland, by utilizing SOC and SO14C time series. To represent different processes we applied five model structures, all stemming from a simple mother model (Introductory Carbon Balance Model – ICBM): (I) two decomposing pools, (II) an inert pool added, (III) three decomposing pools, (IV) two decomposing pools with a substrate control feedback on decomposition, (V) as IV but with also an inert pool. These structures were extended to explicitly represent total SOC and 14C pools. The use of different model structures allowed us to explore model structural uncertainty and the impact of 14C on kinetic parameters. We considered parameter uncertainty by calibrating in a formal Bayesian framework. By varying the relative importance of total SOC and SO14C data in the calibration, we could quantify the effect of the information from these two data streams on estimated model parameters. The weighing of the two data streams was crucial for determining model outcomes, and we suggest including it in future modeling efforts whenever SO14C data are available. The measurements and all model structures indicated a dramatic decline in SOC in the ZOFE experiment after an initial land use change in 1949 from grass- to cropland, followed by a constant but smaller decline. According to all structures, the three treatments (control, mineral fertilizer, farmyard manure) we considered were still far from equilibrium. The estimates of mean residence time (MRT) of the C pools defined by our models were sensitive to the consideration of the SO14C data stream. Model structure had a smaller effect on estimated MRT, which ranged between 5.9 ± 0.1 and 4.2 ± 0.1 years and 78.9 ± 0.1 and 98.9 ± 0.1 years for young and old pools, respectively, for structures without substrate interactions. The simplest model structure performed the best according to information criteria, validating the idea that we still lack data for mechanistic SOC models. Although we could not exclude any of the considered processes possibly involved in SOC decomposition, it was not possible to discriminate their relative importance.

Release of Pb in soils washed with various extractants

The suitability of an agent for washing heavy metal contaminated soils depends not only on the efficiency of heavy metal removal but also on the persistence of the washing effect, especially for agricultural soil. Redistribution of residual heavy metals in washed soil from inert pools to labile pools can reverse the initial washing effect. In order to study the redistribution of residual Pb in Pb-contaminated soils washed with citric acid (CA), ethylenediaminetetraacetic acid (EDTA), FeCl3, and HCl solutions, washed soils were incubated under both flood and 70% field water capacity (FWC) conditions for 180days. The Pb availability in CA-, EDTA-, FeCl3-, and HCl-washed soils varied with incubation time. Diethylenetriaminepentaacetic acid (DTPA)-Pb generally increased with incubation time, except in CA-washed soil under flood incubation condition. Pb associated with the exchangeable/acid extractable fraction (F1) increased, whereas Pb in the easily reducible fraction (F2) decreased over time, for all washed soils under flood incubation condition; the changes in the amount of Pb associated with each fraction were much smaller under 70% FWC incubation. The transformation of Pb from F2 to F1 during flood incubation was largely caused by the mobilization of Fe and Mn oxides. The mobility factors (MF) of Pb for samples under flood incubation increased with increasing incubation time while those for samples under 70% FWC incubation changed only slightly, which implies increasing environmental risks of Pb from washed soils that are flooded. Therefore, the redistribution and potential environmental risks posed by heavy metals in washed soils, especially in paddy rice soils, should be taken into account when the suitability of a washing agent is evaluated. In the present study, CA was found to be unsuitable for washing Pb-contaminated agricultural soil, whereas the FeCl3 solution was the most stable washing agent tested, as it had high removal efficiency and low release of Pb after washing.

Redefining the inert organic carbon pool

Radiocarbon measurements reveal that soil carbon is often hundreds to thousands of years old; significantly older than the annual flux of carbon through the soil would suggest. Models deal with this discrepancy by conceptualizing soil carbon as having fast and slow cycling pools. The Rothamsted Soil Carbon Model contains an inert pool for this reason. Here we use a unique record of time-series radiocarbon measurements from long-term trials to demonstrate that the inert pool is hardly inert, and that its mean age varies from 2000 to as little as 90 years depending on carbon flow through the soil. This finding suggests that the concept of truly inert organic matter requires redefinition to account for the enhanced probability that microorganisms will overcome barriers to previously inaccessible organic matter as their activity increases.

Memory CD8 T cells that are bystander-activated during an infection control early pathogen replication in an innate-like manner (P4371)

Abstract Recent studies illustrate that during an infection the antigen non-specific memory CD8 T cell compartment is not simply an inert pool of cells, but can participate in the immune response. This bystander activation of memory CD8 T cells occurs in the absence of cognate antigen recognition, but the overall role of T cell receptor (TCR) mediated signals has not been defined. Furthermore, it is unknown if and how these cells contribute to the clearance of an infection. We measured the strength of TCR signals that bystander activated CD8 T cells (BA-CTLs) receive in vivo and found evidence of TCR signaling, albeit limited. Given this marginal contribution of the TCR, we sought to determine how BA-CTLs efficiently kill target cells. We demonstrate that BA-CTLs recognize and directly eliminate target cells in an innate-like, granzyme-dependent manner. We next selectively inhibited BA-CTL mediated killing in vivo during the course of an infection, which led to a significant defect in pathogen clearance. Our data suggest that BA-CTLs play a previously unappreciated, essential role in the early immune response to pathogens before the onset of a de-novo generated, primary CD8 T cell response.

Bystander-Activated Memory CD8 T Cells Control Early Pathogen Load in an Innate-like, NKG2D-Dependent Manner

During an infection the antigen-nonspecific memory CD8 T cell compartment is not simply an inert pool of cells, but becomes activated and cytotoxic. It is unknown how these cells contribute to the clearance of an infection. We measured the strength of T cell receptor (TCR) signals that bystander-activated, cytotoxic CD8 T cells (BA-CTLs) receive in vivo and found evidence of limited TCR signaling. Given this marginal contribution of the TCR, we asked how BA-CTLs identify infected target cells. We show that target cells express NKG2D ligands following bacterial infection and demonstrate that BA-CTLs directly eliminate these target cells in an innate-like, NKG2D-dependent manner. Selective inhibition of BA-CTL-mediated killing led to a significant defect in pathogen clearance. Together, these data suggest an innate role for memory CD8 T cells in the early immune response before the onset of a de novo generated, antigen-specific CD8 T cell response.

How useful are MIR predictions of total, particulate, humus, and resistant organic carbon for examining changes in soil carbon stocks in response to different crop management? A case study

Measures of particulate organic carbon (POC), humus organic carbon (HOC), and resistant organic carbon (ROC) (primarily char) are often used to represent the active, slow, and inert carbon pools used in soil carbon models. However, these fractions are difficult to measure directly, and mid infrared (MIR) spectroscopic techniques are increasingly being investigated to quantify these fractions and total organic carbon (TOC). This study examined the change in MIR-predicted pools of TOC, POC, HOC, and ROC in response to different crop management between two time periods (1981 and 2008) in a long-term wheat cropping trial in Queensland, Australia. The aims were (i) to assess the ability of MIR to detect changes in carbon stocks compared with direct measurements of TOC (LECO-TOC); and (ii) to assess how well the behaviour of POC, HOC, and ROC corresponded with the active, slow, and inert conceptual carbon pools. Significant declines in carbon stocks were observed over time using both LECO-TOC and MIR-predicted stocks of TOC, POC, HOC, and ROC, although MIR-TOC under-estimated loss by 27–30% compared with LECO-TOC. The decline in MIR-POC and MIR-HOC was consistent with the expected behaviour of the active and slow conceptual pools; however, the decline in ROC was not consistent with that of the inert pool. In addition, MIR measurements did not accurately detect differences in the rate of carbon loss under different crop management practices.

A Cytosolic Acyltransferase Contributes to Triacylglycerol Synthesis in Sucrose-Rescued Arabidopsis Seed Oil Catabolism Mutants

Triacylglycerol (TAG) levels and oil bodies persist in sucrose (Suc)-rescued Arabidopsis (Arabidopsis thaliana) seedlings disrupted in seed oil catabolism. This study set out to establish if TAG levels persist as a metabolically inert pool when downstream catabolism is disrupted, or if other mechanisms, such as fatty acid (FA) recycling into TAG are operating. We show that TAG composition changes significantly in Suc-rescued seedlings compared with that found in dry seeds, with 18:2 and 18:3 accumulating. However, 20:1 FA is not efficiently recycled back into TAG in young seedlings, instead partitioning into the membrane lipid fraction and diacylglycerol. In the lipolysis mutant sugar dependent1and the β-oxidation double mutant acx1acx2 (for acyl-Coenzyme A oxidase), levels of TAG actually increased in seedlings growing on Suc. We performed a transcriptomic study and identified up-regulation of an acyltransferase gene, DIACYLGLYCEROL ACYLTRANSFERASE3 (DGAT3), with homology to a peanut (Arachis hypogaea) cytosolic acyltransferase. The acyl-Coenzyme A substrate for this acyltransferase accumulates in mutants that are blocked in oil breakdown postlipolysis. Transient expression in Nicotiana benthamiana confirmed involvement in TAG synthesis and specificity toward 18:3 and 18:2 FAs. Double-mutant analysis with the peroxisomal ATP-binding cassette transporter mutant peroxisomal ABC transporter1 indicated involvement of DGAT3 in the partitioning of 18:3 into TAG in mutant seedlings growing on Suc. Fusion of the DGAT3 protein with green fluorescent protein confirmed localization to the cytosol of N. benthamiana. This work has demonstrated active recycling of 18:2 and 18:3 FAs into TAG when seed oil breakdown is blocked in a process involving a soluble cytosolic acyltransferase.

Uncertainty analyses for calibrating a soil carbon balance model to agricultural field trial data in Sweden and Kenya

How do additional data of the same and/or different type contribute to reducing model parameter and predictive uncertainties? Most modeling applications of soil organic carbon (SOC) time series in agricultural field trial datasets have been conducted without accounting for model parameter uncertainty. There have been recent advances with Monte Carlo-based uncertainty analyses in the field of hydrological modeling that are applicable, relevant and potentially valuable in modeling the dynamics of SOC. Here we employed a Monte Carlo method with threshold screening known as Generalized Likelihood Uncertainty Estimation (GLUE) to calibrate the Introductory Carbon Balance Model (ICBM) to long-term field trail data from Ultuna, Sweden and Machang’a, Kenya. Calibration results are presented in terms of parameter distributions and credibility bands on time series simulations for a number of case studies. Using these methods, we demonstrate that widely uncertain model parameters, as well as strong covariance between inert pool size and rate constant parameters, exist when root mean square simulation errors were within uncertainties in input estimations and data observations. We show that even rough estimates of the inert pool (perhaps from chemical analysis) can be quite valuable to reduce uncertainties in model parameters. In fact, such estimates were more effective at reducing parameter and predictive uncertainty than an additional 16 years time series data at Ultuna. We also demonstrate an effective method to jointly, simultaneously and in principle more robustly calibrate model parameters to multiple datasets across different climatic regions within an uncertainty framework. These methods and approaches should have benefits for use with other SOC models and datasets as well.

Changes in clay-associated organic matter quality in a C depletion sequence as measured by differential thermal analyses

Land use changes result in significant decreases in soil organic matter stocks due to enhanced mineralization attributed to increased tillage, and due to decreased organic matter inputs. The current paradigm of soil organic matter dynamics suggests that decreasing organic matter stocks are also associated with shifts in organic matter quality to more resistant fractions as the more labile pools are decomposed. The objectives of the current study were to characterize changes in clay-associated organic matter quality in a soil C depletion sequence in terms of thermal properties, and to thermally characterize peroxide-resistant organic matter. Clay-sized fractions were isolated from a sequence of soils ranging from native forest to long-term bare fallow, and analyzed using thermogravimetry (TG) and differential scanning calorimetry (DSC) before and after hydrogen peroxide treatment. Differential scanning calorimetry traces had exothermic maxima near 285 and 333 °C, consistent with analyses of chemically extracted organic matter and humic substances reported in the literature. Peak fitting analyses showed that these maxima consisted of several hidden peaks, but their interpretation is problematic. The qualitative shifts in the thermal properties of clay-associated organic matter with differing land use observed in the DSC traces were quantified using cultivation and fallow treatment to forest ratios. Thermogravimetric mass loss ratios were greater in the thermally labile (180–310 °C) exothermic region than in the more thermally resistant (310–450 °C) exothermic region. Similarly, ratios of peak heights and areas for the fitted DSC peaks were higher for the 324 °C peak, compared to peaks at 257 and 284 °C. The higher ratios indicate that the more thermally resistant organic matter has been retained and the more thermally labile organic matter is lost. The observed shift in the distribution of organic matter from thermally labile to thermally resistant fractions from forest to long-term bare fallow clay samples is consistent with the current paradigm of decomposition consisting of a shift to more biologically resistant fractions with increasing time under cultivation and decreasing organic matter inputs. These results suggest that the thermal properties of clay-associated organic matter are related to their biological decomposability. Hydrogen peroxide treatment of clay samples removed approximately 87% of the initial organic C in all samples, and peroxide-resistant organic matter was found to be more thermally stable than the whole. However, thermal analyses of the peroxide-resistant fraction did not show any changes in thermal properties with decreases in total C content. Rather than isolating a biologically resistant pool of organic matter, peroxide-resistant organic matter may isolate an inert pool of soil organic matter with thermal properties consistent with the presence of black carbon, which remains unchanged by changes in land use.

CN-SIM—a model for the turnover of soil organic matter. I. Long-term carbon and radiocarbon development

A novel computer model is presented which describes the flow of C and N in the soil. It employs a structure with conceptual compartments. Organic matter is represented by seven different compartments, two for added matter, two for soil microbial biomass, one for microbial residues, one for native (‘humified’) organic matter, and one for inert organic matter. The latter pool represents both truly inert matter, and matter with negligible turnover in a time-span of decades to a century. This paper describes the parameterisation and performance of this model on selected long-term field carbon and radiocarbon data from United Kingdom, Sweden and Denmark. Previously unpublished radiocarbon data series from Denmark are included. Statistical methods were employed to estimate parameters, and obtain proximate confidence intervals for these parameters. Simulations in good agreement with measured values could be achieved, using the same set of parameters on all sites. It was demonstrated that the inert pool might constitute any amount between approx. 10 and 50% of total soil C, so that modelling cannot be used as a tool to obtain narrow estimates for this pool.

Influence of site conditions and management on different soil organic matter (som) pools

Basing on both a site gradient as well as a gradient in management intensities the influence of these factors on different SOM pools was investigated: (A) a very easily decomposable C pool, (B) a C pool which is related to clay fraction and representing a relatively inert pool, (C) a C pool which is not, or only loosely connected to the mineral soil part representing the intermediates between the extreme C pools (A) and (B). The decomposable C pool was measured by the parameter of hot water extractable carbon (HW-C). The clay related and not clay related SOM pools were isolated by a combined fractionation according to both particle sizes (clay fraction < 1 μm and 1 – 2 μm) and specific densities (light fraction < 1.8 g/cm3 and 1.8 – 2.0 g/cm3). Fertilization increases easily decomposable SOM pools reflected in C content of a hot water extractable SOM fraction and in mass of specific light SOM fractions but to a certain extent the C pool associated with clay minerals as well. At comparable management (fertilization) levels transformation conditions at a site (temperature and precipitation) have an influence on the accumulation of the decomposable SOM pool (effective biological transformation time – WMZ)

Entwicklung der organischen bodensubstanz (obs) auf löss in abhängigkeit von unterschiedlicher düngung am beispiel des bodenbildungsversuches in Halle/Saale: Development of soil organic matter (som) on loess in dependence of different fertilization e.g. of the soil formation trial at Halle/Saale

In 1948, a model trial to study soil formation was arranged at Halle/Saale by K. Schmalfuß. Loess with very low content of soil organic matter was used as substrate. The aim was to investigate the soil formation processes influenced by the P and N mineral fertilization. After 30 years, crop yields reached a characteristic, approximately constant level corresponding to the different fertilization treatments and cultivated plants. Recent comparisons of the highest N with the no-N fertilizer treatment show that the mean shoot dry weight per year has a 2.5-fold higher but the Corg content in the topsoil only a 1.2-fold higher value. An accumulation of alkaline-soluble organic soil substances and a decrease of hot water soluble carbon compounds were observed in the highest fertilizer treatment as well as in the treatments without N or P fertilization. Carbon compounds were apparently enriched in the inert carbon pool of the topsoil. During soil formation, the cation exchange capacity and the content of easily soluble P compounds increased, lime content decreased and the C/N ratio became narrower. The results suggest that these parameters are continuing to change and the steady state of the level of organic soil substances and of the transformation processes between C pools in the topsoil has not yet been reached.

Stabilization of carbon from maize in a sandy soil in a long‐term experiment

SummaryThe sequestration of carbon (C) in soil is not completely understood, and quantitative information about the amounts of organic carbon in the various fractions and their rates of turnover could improve understanding. We aimed (i) to quantify the amounts of C derived from maize at various depths in the soil in a long‐term field experiment with and without fertilization using 13C/12C analysis, (ii) to model changes in the organic C, and (iii) to compare measured and modelled pools of C. The organic C derived from the maize was measured in soil samples collected to a depth of 65 cm from four plots, two of which had been under continuous maize and two under continuous rye during long‐term field experiments with NPK and without fertilization. The fractionation procedures included particle‐size fractionation and extractions in water and in pyrophosphate solution. We used the Rothamsted Carbon Model to model the dynamics of the carbon from 13C data. The amounts of C derived from maize in the Ap horizon after 39 years of continuous maize cropping were 9.5% of the total organic C (where unfertilized) and 14.0% where NPK had been applied. Fertilization did not affect the residence time of carbon in the soil. The amounts of C derived from maize in water extracts were 21% of the total organic C (where unfertilized) and 22% where NPK had been applied. The extracts that were soluble in pyrophosphate and insoluble in acid were depleted in C from maize (the amounts were 5% and 7% of the total organic C, respectively). The results of the 13C natural abundance technique were used to model the dynamics of the organic C. Both the total organic C and the C derived from maize in the particle‐size fraction 0–63 μm agreed well with the total and maize‐derived sums of the model pools ‘inert organic matter’, ‘humified organic matter’ and ‘microbial biomass’. The model suggested that 64% (unfertilized) or 53% (NPK) of the organic C in the Ap horizon were inert. Only one of three published equations to determine the size of the inert pool agreed well with these model results.

Effect of Metabolites of γ-Aminobutyric Shunt on Activities of NAD- and NADP-Isocitrate Dehydrogenases and Aconitate Hydratase from Higher Plants

We studied the influence of metabolites of γ-aminobutyric shunt of the tricarboxylic acid cycle on the activities of aconitate hydratase (EC.4.2.1.3) as well as NAD- and NADP-specific isocitrate dehydrogenases (EC.1.1.1.41 and EC.1.1.1.42, respectively) using purified enzyme preparations from pea leaves (Pisum sativum L.) and maize scutellum (Zea mays L.). γ-Aminobutyric acid and succinate proved to have no significant effect on these enzymes, while 0.1–0.2 mM glutamate markedly activated NADP isocitrate dehydrogenase from both sources, especially at unsaturating concentrations of the substrate. Succinic semialdehyde stimulated the activities of aconitate hydratase and NADP isocitrate dehydrogenase. The obtained data point to a similar pattern of the effect of intermediates of γ-aminobutyric shunt on the studied enzymatic activities for both photosynthetic tissues (pea leaves) and those with acidifying, transport, and digestive functions (maize scutellum). However, the absence of pronounced control effects of most metabolites on the studied enzymes allows us to assign them to a relatively inert pool of metabolites.

Marker concentration patterns of labelled leaf and stem particles in the rumen of cattle grazing bermuda grass (Cynodon dactylon) analysed by reference to a raft model.

Large (>1600 microm), ingestively masticated particles of bermuda grass (Cynodon dactylon L. Pers.) leaf and stem labelled with 169Yb and 144Ce respectively were inserted into the rumen digesta raft of heifers grazing bermuda grass. The concentration of markers in digesta sampled from the raft and ventral rumen were monitored at regular intervals over approximately 144 h. The data from the two sampling sites were simultaneously fitted to two pool (raft and ventral rumen-reticulum) models with either reversible or sequential flow between the two pools. The sequential flow model fitted the data equally as well as the reversible flow model but the reversible flow model was used because of its greater application. The reversible flow model, hereafter called the raft model, had the following features: a relatively slow age-dependent transfer rate from the raft (means for a gamma 2 distributed rate parameter for leaf 0.0740 v. stem 0.0478 h(-1)), a very slow first order reversible flow from the ventral rumen to the raft (mean for leaf and stem 0.010 h(-1)) and a very rapid first order exit from the ventral rumen (mean of leaf and stem 0.44 h(-1)). The raft was calculated to occupy approximately 0.82 total rumen DM of the raft and ventral rumen pools. Fitting a sequential two pool model or a single exponential model individually to values from each of the two sampling sites yielded similar parameter values for both sites and faster rate parameters for leaf as compared with stem, in agreement with the raft model. These results were interpreted as indicating that the raft forms a large relatively inert pool within the rumen. Particles generated within the raft have difficulty escaping but once into the ventral rumen pool they escape quickly with a low probability of return to the raft. It was concluded that the raft model gave a good interpretation of the data and emphasized escape from and movement within the raft as important components of the residence time of leaf and stem particles within the rumen digesta of cattle.

Changes in chemical nature of soil organic carbon in Vertisols under wheat in south-eastern Queensland

The impact of cropping and cultivation (up to 50 years) on the nature and pool structure of organic C in two different soil types was investigated using a combination of physical and chemical fractionations and solid-state 13 C NMR spectroscopy. NMR spectroscopy revealed that aryl C contributed significantly to the organic C in the Waco soil (Pellustert) but not in the Langlands-Logie soil (Chromustert). The aryl C content of both soils was largely preserved despite the significant decrease in total organic C, following cultivation, although other organic forms appeared to rapidly decline at similar rates to one another. High energy UV photo-oxidation along with solid-state 13 C NMR spectroscopy demonstrated that the aryl C was mainly charcoal (char) in the &lt;53 mm fraction of the soils which appeared to be highly resistant to microbial decomposition. Char C content of the Waco soil remained near 6.0 g C/kg soil and near 2.0 g C/kg soil for the Langlands-Logie soil. This char was evident to a depth of at least 30 cm in both soils. Fractionation yielded 4 organic C fractions: particulate organic C, humic C, char C, and physically protected C. By equating these fractions to the resistant plant material (particulate organic C), humic pool (humic C), and inert pool (char C) of the RothC soil C turnover model and comparing a number of simulations with measured fractions, we showed that the inert pool equated well with the measured char C. The measured particulate organic C fraction was of an appropriate size to represent the resistant plant material pool of the model but appeared to have a much slower turnover rate. Similarly, the measured humic pool was of a similar size to that required by the model but was more labile (faster turnover rate) than that used in the RothC model. This may be due to a combination of the labile proteinaceous nature of this pool and its lower than expected protection by physical association with the smectitic clay matrix.

Persistent C in Soils

There is general agreement that in many soils some C compounds persist for centuries and perhaps millenia, particularly deep in the soil profile. Various chemistries and mechanisms of stabilization of C compounds have been proposed to explain their longevity in soils. There is evidence that some part of the reason for the persistence of C in soils is related to a range of interactions with mineral surfaces and secretion within the mineral matrix, but in recent years charcoal has emerged as a major contributor to the so called 'inert pool' of C in soils. Finely divided charcoal appears to be a major constituent of many Australian soils (Skjemstad et al., 1996). It is not currently possible to measure quantitatively the charcoal present in soils but combinations of controlled high energy ultra-violet photo-oxidation with CPMAS 13C NMR and scanning electron microscopy allow a good assessment of the charcoal present in soil and soil fractions. Charcoal is relatively resistant to ultraviolet photo-oxidation which essentially removes other plant and microbially derived organic materials as CO2. Thus the charcoal can be concentrated and often recognised by scanning electron microscopy because it has a wood-like morphology. Such samples exhibit strong NMR signals in the 130 ppm region because of the strong aromaticity. The strong aromaticity of 'humic acid' fractions also results in signals in the 130 ppm region and it is quite likely that a significant proportion of the aromatic components in humic acids originate in charcoal. In a limited range of soils as much as one third of the organic C appears to be charcoal and a relationship between charcoal in soil and previous histories of fire are being slowly established. For example, the controlled burning of vegetation to maintain grasslands on volcanic ash soils in Japan has established a clear relationship between fires and the chemistry of the soil organic matter (Golchin et al., 1997a,b). Soils subject to fire for decades were richer in aromatic components which were concentrated in humic acid fractions. Studies by solid state ~3C CPMAS Bloch decay and proton spin relaxation editing (PSRE) indicate an accumulation of charcoal after decades of burning. The Bloch decay also showed that in soils containing significant amounts of aromatic components charcoal is likely to be present and also that the amount of aromatic material present is likely to be seriously underestimated by ~3C CPMAS NMR. Unfortunately, we know little about the dynamics of charcoal in soils, or about the significance of charcoal in the global C cycle. However, considerations of the global situation with respect to charcoal and related materials indicates that it is not inert although we have no good information of its l o n g e v i t y (Waters et al . , u n p u b l i s h e d ) . Microbiologically condensed ring compounds are very resistant to oxidation but limited data suggest that charcoal may not be a major sink for C in the global C cycle. A second group of C compounds which are resistant biochemically are aliphatic C chains. During 13C NMR studies with metal probes we have concluded that domains of aliphatic materials occur in soils (Smernik and Oades, submitted to Geoderma). These domains are hydrophobic aliphatic components probably nanometers in size. Currently we do not know their origin or dynamics but they represent part of the colloidal fraction in some soils. Currently we are unable to separate these components from the alkyl-nitrogen rich materials which are concentrated in the colloidal or clay fractions of soils.

Dietary fat saturation effects on low-density-lipoprotein concentrations and metabolism in various animal models

Saturated vegetable oils (coconut, palm, and palm kernel oil) and fats (butter and lard) are hypercholesterolemic relative to monounsaturated and polyunsaturated vegetable oils. The increase in plasma low-density-lipoprotein-cholesterol (LDL-C) concentrations associated with consumption of saturated vegetable oils and fats is largely explained by a decrease in hepatic LDL receptor activity and an increase in the LDL-C production rate. Hepatic LDL receptor activity may be regulated by the messenger RNA concentration of the LDL receptor. The decrease in hepatic LDL receptor activity with saturated fat feeding is associated with decreased hepatic sterol O-acyltransferase activity and, therefore, a reduced inert pool of cholesteryl ester. A putative regulatory pool of cholesterol is increased with saturated fat feeding and suppresses LDL receptor activity, possibly through hepatic messenger RNA regulation. For most studies, an independent effect of a vegetable oil or fat could not be ascertained because there was no neutral control and at least two of the test oils or fats were varied. Animal data for the effects of individual fatty acids on plasma LDL-C concentrations and metabolism are sparse. The evidence suggests that caproic acid (6:0), caprylic acid (8:0), and capric acid (10:0) are neutral with respect to their LDL-C-raising properties and their ability to modulate LDL metabolism. Lauric acid (12:0), myristic acid (14:0), and palmitic acid (16:0) are approximately equivalent in their LDL-C-raising potential by reducing hepatic LDL receptor activity and increasing the LDL-C production rate, apparently via modulation of sterol O-acyltransferase activity. Stearic acid (18:0) appears to be neutral in its LDL-C-raising potential and how it affects LDL metabolism.