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The stock of organic carbon contained within a soil represents the balance between inputs and losses. Inputs are defined by the ability of vegetation to capture and retain carbon dioxide, effects that management practices have on the proportion of captured carbon that is added to soil and the application organic amendments. The proportion of organic amendment carbon retained is defined by its rate of mineralisation. In this study, the rate of carbon mineralisation from 85 different potential soil organic amendments (composts, manures, plant residues and biosolids) was quantified under controlled environmental conditions over a 547 day incubation period. The composition of each organic amendment was quantified using nuclear magnetic resonance and mid- and near-infrared spectroscopies. Cumulative mineralisation of organic carbon from the amendments was fitted to a two-pool exponential model. Multivariate chemometric algorithms were derived to allow the size of the fast and slow cycling pools of carbon to be predicted from the acquired spectroscopic data. However, the fast and slow decomposition rate constants could not be predicted suggesting that prediction of the residence time of organic amendment carbon in soil would likely require additional information related to soil type, environmental conditions, and management practices in use at the site of application.

Soil organic carbon (C) is an essential component of the global C cycle. Processes that control its composition and dynamics over large scales are not well understood. Thus, our understanding of C cycling is incomplete, which makes it difficult to predict C gains and losses due to changes in climate, land use and management. Here we show that controls on the composition of organic C, the particulate, humus and resistant fractions, and the potential vulnerability of C to decomposition across Australia are distinct, scale-dependent and variable. We used machine-learning with 5,721 topsoil measurements to show that, continentally, the climate, soil properties (for example, total nitrogen and pH) and elevation are dominant controls. However, we found that such general assessments disregard underlying region-specific controls that affect the distribution of the organic C fractions and vulnerability. This can lead to misinterpretations that prejudice our understanding of soil C processes and dynamics. Regionally, climate is mediated through interactions with soil properties, mineralogy and topography. In some regions, climate is uninfluential. These results highlight the need for regional assessments of soil C dynamics and more local parameterization of biogeochemical and Earth system models. Our analysis propounds the development of region-specific strategies for effective C management and climate change mitigation. Soil geochemistry can be more important than climate in controlling carbon storage, its composition as well as stability, but controls are distinct, scale-dependent and variable, according to an analysis of topsoil measurements across Australia.

Conversion of soils supporting native vegetation to agricultural production has led to a loss of soil carbon stocks. Replacing a portion of the lost stocks will sequester atmospheric carbon with the concurrent benefit of enhancing soil sustainability. The ability of the fine fraction of soils (≤50-µm fraction) to adsorb organic carbon (OC) is considered a key mechanism capable of stabilising soil OC against loss. The difference between the current and maximum concentrations of OC in the soil fine fraction (FFC) has been termed the ‘saturation deficit’ (SatDef) and used to define the potential for a soil to sequester carbon. For New Zealand surface 0–15 cm soil layers, pedotransfer functions have been derived to quantify the soil carbon SatDef. The ability of combining infrared spectroscopy (IR) with partial least squares regression (PLSR) to derive predictive algorithms for soil properties included in these pedotransfer functions, the capacity of the soil fine fraction to stabilise carbon and the SatDef of the soil fine fraction were assessed in this study. A total of 168 air-dried and finely ground New Zealand surface soils representative of the major soil orders used for agricultural production were included. Principal components analysis of IR spectra showed a grouping by soil order that was related to mineralogy. Predictive IR/PLSR algorithms were derived for specific surface area, pyrophosphate-extractable aluminium, the FFC content, the 90th quantile regression of FFC and the SatDef of the fine fraction (R2 values ≥0.85; ratio of performance to interquartile range values ≥2.9). The results indicate that IR/PLSR provides a rapid and cost-effective mechanism for deriving information related to the amount of FFC in soils and the SatDef of the fine fraction. The IR/PLSR approach could be used to define the potential of soils to sequester carbon and identify the soil types to target for carbon sequestration technologies. The approach would also generate valuable data for soil carbon in national inventories or national soil condition monitoring programs.

Developing a routine and cost effective capability for measuring soil organic carbon (SOC) content and composition will allow identification of land management practices with a potential to maintain or enhance SOC stocks. Coupling SOC content data and mid-infrared (MIR) spectra through the application of partial least-squares regression (PLSR) analyses has been used to develop such a prediction capability. The objective of this study was to determine whether MIR/PLSR analyses provide accurate estimates of the content and composition of SOC that can be used to quantify SOC stocks and its potential vulnerability to loss. Soil was collected from a field trial incorporating a range of land use (pasture, arable cropping and bare fallow) and tillage (intensive, minimum and no tillage) treatments over a nine-year period. The SOC content was measured by dry combustion analysis. Particulate organic carbon was separated from other forms of carbon on the basis of particle size (SOC in the >50µm fraction). Resistant organic carbon was quantified using solid-state 13C nuclear magnetic resonance. The MIR/PLSR algorithms were successfully developed to predict the natural logarithms of the contents of SOC and POC in the collected soils. With initial calibration, a single MIR analysis could be used in conjunction with PLSR algorithms to predict the content of SOC and its allocation to component fractions. The MIR/PLSR predicted SOC contents provided reliable estimates of the impact of agricultural management on the 0–25-cm SOC stocks, as well as an indication of the vulnerability of SOC to loss. Development of this capability will facilitate the rapid and cost effective collection of SOC content data for detecting the impact of agricultural management treatments on SOC stocks, composition and potential vulnerability to change.

Coastal habitats including saltmarshes and mangrove forests can accumulate and store significant blue carbon stocks, which may persist for millennia. Despite this implied stability, the distribution and structure of intertidal‐supratidal wetlands are known to respond to changes imposed by geomorphic evolution, climatic, sea level, and anthropogenic influences. In this study, we reconstruct environmental histories and biogeochemical conditions in four wetlands of similar contemporary vegetation in SE Australia. The objective is to assess the importance of historic factors to contemporary organic carbon (C) stocks and accumulation rates. Results from the four cores—two collected from marine‐influenced saltmarshes (Wapengo marine site (WAP‐M) and Port Stephens marine site (POR‐M)) and two from fluvial influenced saltmarshes (Wapengo fluvial site (WAP‐F) and Port Stephens fluvial site (POR‐F))—highlight different environmental histories and preservation conditions. High C stocks are associated with the presence of a mangrove phase below the contemporary saltmarsh sediments in the POR‐M and POR‐F cores. 13C nuclear magnetic resonance analyses show this historic mangrove root C to be remarkably stable in its molecular composition despite its age, consistent with its position in deep sediments. WAP‐M and WAP‐F cores did not contain mangrove root C; however, significant preservation of char C (up to 46% of C in some depths) in WAP‐F reveals the importance of historic catchment processes to this site. Together, these results highlight the importance of integrating historic ecosystem and catchment factors into attempts to upscale C accounting to broader spatial scales. Multiple ecosystem shifts were observed within the surface 1 m of blue carbon sedimentsPreservation of inputs from prior ecosystem formation can make large contributions to contemporary carbon stocksHeterogeneity of historical influence needs to be accounted for in regional assessments of carbon storage

In this paper, we present a framework for a space–time observation system for soil organic carbon (STOS-SOC). We propose that the RothC model be embedded within the STOS-SOC, which is driven by satellite-derived inputs and readily available geospatial inputs, such as digital soil maps. In particular, advances in remote sensing have enabled the development of satellite products that represent key inputs into soil carbon models, examples being evapotranspiration and biomass inputs to soil, which characterise space–time variations in management and land use. Starting from an initial calibrated base for prediction, as new observations are acquired, data assimilation techniques could be used to optimise calibration algorithms and predicted model outputs. We present initial results obtained from the implementation of the proposed STOS-SOC approach to the 1445-km2 Cox’s Creek catchment in northern New South Wales, Australia. Our results showed that use of satellite-derived biomass inputs with a MODIS satellite product (MOD17A3) improved the accuracy of simulations by 16% compared with carbon inputs derived through other methods normally adopted in the spatialisation of the RothC model. We further discuss the possibility of improving the capabilities of the STOS-SOC for future applications.

The importance of soil organic carbon (SOC) in maintaining soil health is well understood. However, there is growing interest in studying SOC with an emphasis on quantifying its changes in space and time. This is because of the potential for soil to be used to sequester atmospheric C. There are many issues which make this difficult, for example shortcomings in sampling designs, and differences in vertical and lateral sampling supports between surveys, particularly if legacy data are used as the baseline survey. In this study, we systematically work through these issues and show how a protocol can be developed using design-based and model-based statistical approaches to estimate changes in SOC in space and time at different spatial supports. We demonstrate this protocol in a small subcatchment in the upper Namoi valley for estimating the change in SOC over time, whereby the baseline dataset was collected during 1999–2001 and is compared with a dataset from November 2010. The results from both design-based and model-based approaches revealed a drop in SOC across the catchment between the two survey periods. A 0.26% drop in SOC was reported globally across the catchment. Nevertheless, the change in SOC reported for both approaches was not statistically significant.

Quantifying the content and composition of soil carbon in the laboratory is time-consuming, requires specialised equipment and is therefore expensive. Rapid, simple and low-cost accurate methods of analysis are required to support current interests in carbon accounting. This study was completed to develop national and state-based models capable of predicting soil carbon content and composition by coupling diffuse reflectance mid-infrared (MIR) spectra with partial least-squares regression (PLSR) analyses. Total, organic and inorganic carbon contents were determined and MIR spectra acquired for 20495 soil samples collected from 4526 locations from soil depths to 1m within Australia’s agricultural regions. However, all subsequent MIR/PLSR models were developed using soils only collected from the 0–10, 10–20 and 20–30cm depth layers. The extent of grinding applied to air-dried soil samples was found to be an important determinant of the variability in acquired MIR spectra. After standardisation of the grinding time, national MIR/PLSR models were developed using an independent test-set validation approach to predict the square-root transformed contents of total, organic and inorganic carbon and total nitrogen. Laboratory fractionation of soil organic carbon into particulate, humus and resistant forms was completed on 312 soil samples. Reliable national MIR/PLSR models were developed using cross-validation to predict the contents of these soil organic carbon fractions; however, further work is required to enhance the representation of soils with significant contents of inorganic carbon. Regional MIR/PLSR models developed for total, organic and inorganic carbon and total nitrogen contents were found to produce more reliable and accurate predictions than the national models. The MIR/PLSR approach offers a more rapid and more cost effective method, relative to traditional laboratory methods, to derive estimates of the content and composition of soil carbon and total nitrogen content provided that the soils are well represented by the calibration samples used to build the predictive models.

The use of subtropical perennial grasses in temperate grazing systems is increasingly being promoted for production and environmental benefits. This study employed a combination of elemental and stable isotope analyses to explore whether pastures sown to either kikuyu (Pennisetum clandestinum) or a combination of panic (Panicum maximum) and Rhodes grass (Chloris gayana) could increase soil organic carbon (SOC) levels in five regions across southern Australia. Carbon was sequestered under kikuyu at a rate of 0.90±0.25MgCha-1year-1 along the south coast of Western Australia. Lower but still significant sequestration rates were found for kikuyu in South Australia (0.26±0.13MgCha-1year-1). No changes in SOC were found for panic–Rhodes grass pasture systems in the northern district of Western Australia. Additionally, we found no changes in SOC when kikuyu-based pastures were established on formerly cropped paddocks in the Namoi Catchment of New South Wales. Stable isotope results corroborated these findings and suggested that, where SOC has accumulated, the gains have been dominated by SOC derived from the perennial vegetation and have been concentrated in the upper 10cm of soil.

Perennial grass pastures are being increasingly adopted, but little is known about the flows of carbon (C) from photosynthesis into soil organic matter (SOM) that could be used for calculations in carbon accounting. Repeat-pulse labelling of perennial grass pastures (kikuyu and Rhodes grass) with 14C in the field in Western Australia was used to trace the allocation of C to SOM fractions and to determine the stability of each fraction over an extended period. For kikuyu, >40% of the 14C fed to the plants was allocated belowground within 10 days of labelling, and after 1 year half of this remained. Allocation of 14C belowground under Rhodes grass ranged between 20 and 24% of 14C applied and remained constant for up to 6 months. At least 90% of the 14C belowground was found in the surface 300mm of soil. The allocation of 14C to the coarse (50µm–2mm) and fine (<50µm) SOM fractions was similar in magnitude for the two grasses and remained stable through time. It was estimated that in 1 year ~1tCha-1 was assimilated into the coarse+fine SOM fractions under kikuyu. However, Rhodes grass was not uniformly distributed across the paddock, thereby reducing the estimates of assimilation of C belowground in these systems to one-tenth of that under kikuyu. Data obtained will help validate plant–soil models for assessing rates of C sequestration under perennial pastures.

Soil organic carbon (OC) exists as a diverse mixture of organic materials with different susceptibilities to biological decomposition. Computer simulation models constructed to predict the dynamics of soil OC have dealt with this diversity using a series of conceptual pools differentiated from one another by the magnitude of their respective decomposition rate constants. Research has now shown that the conceptual pools can be replaced by measureable fractions of soil OC separated on the basis of physical and chemical properties. In this study, an automated protocol for allocating soil OC to coarse (>50µm) and fine (≤50µm) fractions was assessed. Automating the size fractionation process was shown to reduce operator dependence and variability between replicate analyses. Solid-state 13C nuclear magnetic resonance spectroscopy was used to quantify the content of biologically resistant poly-aryl carbon in the coarse and fine size fractions. Cross-polarisation analyses were completed for coarse and fine fractions of 312 soils, and direct polarisation analyses were completed for 38 representative fractions. Direct polarisation analyses indicated that the resistant poly-aryl carbon was under-represented in the cross-polarisation analyses, on average, by a factor of ~2. Combining this under-representation with a spectral analysis process allowed the proportion of coarse- and fine-fraction OC existing as resistant poly-aryl C to be defined. The content of resistant OC was calculated as the sum of that found in the coarse and fine fractions. Contents of particulate and humus OC were calculated after subtracting the resistant OC from the coarse and fine fractions, respectively. Across the 312 soils analysed, substantial variations in the contents of humus, particulate, and resistant carbon were noted, with respective average values of 9.4, 4.0, and 4.5g fraction C/kg soil obtained. When expressed as a proportion of the OC present in each soil, the humus, particulate, and resistant OC accounted for 56, 19, and 26%, respectively. The nuclear magnetic resonance analyses also indicated that the use of a 50-µm sieve to differentiate particulate (>50µm) from humus (≤50µm) forms of OC provided an effective separation based on extents of decomposition. The procedures developed in this study provided a means to differentiate three biologically significant forms of soil OC based on size, extent of decomposition, and chemical composition (poly-aryl content).

Here we take advantage of the stable carbon isotope shift that occurs when a C4 plant is sown into a soil previously dominated by C3 vegetation, to explore the movement and fate of newly sequestered soil organic carbon (SOC) following establishment of subtropical perennial pastures in temperate regions of Australia. In kikuyu-based pastures up to 33 years of age, SOC accumulated exclusively in the coarse size fraction (>50μm) in the sandy soils of southern Western Australia. In South Australian loams, regardless of pasture age, new SOC was found to accumulate in both the coarse and fine (<50μm) size fractions. These differential results suggest that in soils with low clay content, new SOC remains in an unprotected form that is highly vulnerable to loss through decomposition and erosion. The Rothamsted Carbon Model, modified to track changes in stable isotopes, was able to represent the changes in total SOC stocks in both regions; however, the model over-predicted the incorporation of the new C4-SOC into the soil. This difference between data and model output could be reconciled if a greater proportion of new SOC is rapidly mineralised without being incorporated into any sort of stabilised pool.

Organic carbon and nitrogen found in soils are subject to a range of biological processes capable of generating or consuming greenhouse gases (CO2, N2O and CH4). In response to the strong impact that agricultural management can have on the amount of organic carbon and nitrogen stored in soil and their rates of biological cycling, soils have the potential to reduce or enhance concentrations of greenhouse gases in the atmosphere. Concern also exists over the potential positive feedback that a changing climate may have on rates of greenhouse gas emission from soil. Climate projections for most of the agricultural regions of Australia suggest a warmer and drier future with greater extremes relative to current climate. Since emissions of greenhouse gases from soil derive from biological processes that are sensitive to soil temperature and water content, climate change may impact significantly on future emissions. In this paper, the potential effects of climate change and options for adaptation and mitigations will be considered, followed by an assessment of future research requirements. The paper concludes by suggesting that the diversity of climate, soil types, and agricultural practices in place across Australia will make it difficult to define generic scenarios for greenhouse gas emissions. Development of a robust modelling capability will be required to construct regional and national emission assessments and to define the potential outcomes of on-farm management decisions and policy decisions. This model development will require comprehensive field datasets to calibrate the models and validate model outputs. Additionally, improved spatial layers of model input variables collected on a regular basis will be required to optimise accounting at regional to national scales.

The importance of bogs in the catchment hydrology of the Australian Alps has been long recognised but little studied. Damaged bogs are thought to be the source of dried peats now common throughout the Alps. We described the characteristics of a bog peat and a dried peat, to better understand the relationship between the two. Standard chemical properties of peat were measured: pH, loss on ignition, gravimetric contents of carbon and nitrogen, and electrical conductivity. We also measured the concentrations of total and plant-available elements, and the chemical composition of the organic carbon, leading to a measure of the extent of decomposition. The results suggest that this is a typical Sphagnum bog peat?low pH, high carbon content?and the distribution of carbon groups and other elements reflect the stable water source of this groundwater-fed Sphagnum bog. The properties of the dried peat were most similar to the catotelm (lower layer) of the bog peat. This resemblance, combined with decades of field observations, indicates that dried peat may form from the catotelm of bog peat, after the acrotelm (upper layer) dries and erodes. These results have implications for the management of Alps catchments, and further studies are needed to ascertain the hydrologic and carbon cycling roles of organic soils in the Australian Alps.

Sulfonylurea residues have been found to inhibit the growth of some legume crops and pastures in seasons following application. Negative effects of these herbicides on symbiotic nitrogen fixation by legume crops and pastures have been demonstrated. Reductions in nitrogen fixation may result from a direct effect of the herbicide on rhizobial growth and/or an indirect effect on plant growth. In this study the influence of chlorsulfuron on the growth of chickpea rhizobia [Mesorhizobium ciceri (CC1192)], the growth of chickpea plants, and the extent of nodulation and nitrogen fixation by the chickpea/rhizobia symbiosis were examined. In vitro studies (in yeast mannitol broth and a defined medium) showed that chlorsulfuron applied at double the recommended field application rate did not influence the growth of chickpea rhizobia. An experiment using 14C-labelled chlorsulfuron was conducted to determine if rhizobial cells exposed to chlorsulfuron could deliver the herbicide to the point of root infection and nodule formation. Approximately 1% of the herbicide present in the rhizobial growth medium remained with the cell/inoculum material after rinsing with 1/4 strength Ringer?s solution. This was considered unlikely to affect chickpea growth, nodulation, or nitrogen fixation. A pot experiment was used to define the influence of chlorsulfuron on the growth, nodulation, and nitrogen fixation of chickpeas. The presence of chlorsulfuron in the soil reduced the nodulation and nitrogen fixation of the chickpea plants. Pre-exposing rhizobia to chlorsulfuron before inoculating them into pots with germinating chickpea seeds, reduced the number of nodules formed by 51%. Exposure of chickpeas and chickpea rhizobia to chlorsulfuron can adversely affect the formation and activity of symbiotic nitrogen-fixing nodules, even when only the rhizobial inoculant is exposed briefly to the herbicide.

The Agricultural Production Systems Simulator (APSIM) suite of models was used to predict dynamics in water and nitrogen in soil, as well as the growth and yield of sequential crops of wheat and barley in pasture?wheat?barley rotations, between 1995 and 1997 at Roseworthy, South Australia. The NWHEAT model satisfactorily predicted above-ground dry matter, leaf area index and grain yields for both crops in rotations with either grassy (Grass) or medic (Medic) pastures, including the lack of significant response of yield to nitrogen fertiliser applied to wheat at sowing. Simulation data for soil water, from SOILWAT2, was consistent with measured data. Simulation with SOILN2, however, largely underestimated soil nitrogen, due to excessive uptake by the simulated wheat during the season when nitrogen was abundant and water supply readily available. Thus, the soil nitrate had to be reset at sowing for the following barley crop; simulated soil nitrate agreed with the measured data in this season when this nutrient was low. For most variables of crop growth and soil water, the simulated data were mostly within 2 standard errors of the measured means. Prediction of grain protein was underestimated in all cases, including where nitrogen in the shoot was overestimated. This was possibly due to inadequate remobilisation of nitrogen from the straw and roots to the grain by the simulated crop. A satisfactory prediction of dry matter, grain yield and grain weight was obtained for wheat when the models were extended to other trials at Roseworthy (Lower North), Minnipa (Upper Eyre Peninsula) and Wunkar (Murray Mallee), based on limited soil data. Long-term simulations of wheat yields showed that, with early sowing in the Lower North, median wheat yield increased by 50 kg/ha for every kilogram of nitrogen applied at sowing, up to a maximum nitrogen rate of 50 kg/ha. In the drier districts of the Upper Eyre Peninsula and the Murray Mallee, nitrogen fertiliser of no more than 25 kg/ha, applied at sowing, was enough to achieve yield benefits in any given season. At these drier sites, crop failures occurred in 5% (Upper Eyre Peninsula) and 10% (Murray Mallee) of the seasons simulated. Median sowing dates from these simulations were 15 May for the Lower North, 30 May for the Upper Eyre Peninsula and 24 May for Murray Mallee. This suggested that sowing could be conducted at least a week earlier than currently practised in the 3 environments. This study demonstrated the capability of APSIM to predict growth and grain yield of wheat and barley, as well as the associated dynamics of soil water in the main cereal belts of South Australia.

Experimental estimates of amounts of foliage nitrogen (N) fixed in Australian pastures range from 2 to 284 kg N/ha.year for annual and perennial legumes growing in temperate and tropical environments. Differences in the amounts of N2 fixed relate primarily to the legume content and net productivity of pastures. On average, close to 20–25 kg of shoot N are fixed for every tonne of legume herbage dry matter produced across a wide range of environments. Strategies likely to improve the potential for N2 fixation include: (i) rhizobial inoculation at time of first sowing a new legume species; (ii) amelioration of nutritional problems (applications of superphosphate or lime); (iii) manipulation of pasture composition (herbicide applications to remove grasses in annual pastures in the year prior to cropping); and (iv) including lucerne to offset the year-to-year variability in N2 fixation inputs from annual legumes. However, pasture response to such management treatments and the subsequent availability of soil mineral N may be modified by livestock effects on nutrient cycling, pasture productivity and botanical composition. Conclusions about the relative size of the contributions of fixed N to the N economies of Australian farming systems depend on whether or not estimates of fixed N are included for nodulated roots. Thus residual net inputs of fixed N after each year of a legume-based pasture are generally rated sufficient to balance the N removed by at least 1 subsequent wheat crop provided estimates of below-ground N are included in calculations. Pasture type influences the duration of subsequent rotational benefits and while residual effects on mineral N are commonly exhausted within 2 years after an annual legume-based pasture phase, N carry-over following lucerne generally lasts considerably longer.

The influence of three cropping treatments on the quantitative and qualitative composition of the carbohydrate fraction of a silt loam soil and its water-stable aggregates was assessed. Three cropping treatments were considered: bromegrass (Bromus inermisLeyss.) grown continuously for 15 yr (B15), grain-corn (Zea maysL.) grown continuously using conventional tillage for 15 yr (C15), and grain-corn grown continuously for 13 yr followed by 2 yr of bromegrass (B13C2). Six water-stable aggregate size fractions ranging from 8.00–4.70 to 0.25–0.10 mm in diameter were separated from the soil by wet sieving. The carbohydrate fraction was extracted by acid hydrolysis, reduced, acetylated to alditol acetates and analyzed by temperature-programmed capillary gas chromatography. Total carbohydrate content was not significantly influenced by cropping treatment but was observed to increase as aggregate size decreased irrespective of cropping treatment. The carbohydrate content in the different aggregate size fractions was positively correlated with both organic carbon and clay contents. With the exception of xylose and mannose, the monomeric sugars behaved similarly to total carbohydrates. Based on the magnitude of the ratio of the content of mannose to that of arabinose plus xylose, the contribution of plant carbohydrates to the total carbohydrate fraction increased as aggregate size decreased in the B15treatment, but decreased in the C13B2and C15treatments. No significant correlations existed between either the total carbohydrate content or the content of individual sugars, and the stability of aggregates in the soils from the different cropping treatments. Key words: Wet aggregate stability, soil organic matter, soil structure, crop rotation, corn, bromegrass

Changes in cropping practices can alter soil structural characteristics. The purpose of this study was to assess the extent and rate at which bromegrass (Bromus inermisLeyss) and continuous grain-corn (Zea maysL.) influence soil aggregation and various soil binding agents. Five cropping treatments ranging from 15 yr of continuous grain-corn to 15 yr of continuous bromegrass production were investigated. Soil aggregation and the nature of soil binding agents were determined by measuring the water-stable aggregate size distribution subsequent to the application of six chemical pretreatments — control, distilled water, 0.08 M sodium azide, 0.08 M sodium chloride, 0.08 M sodium periodate, and 0.02 M sodium pyrophosphate. A simple power function was used to describe the relationship between the cumulative percentage by weight of water-stable aggregates and aggregate diameter. The exponent in the power function provided a measure of the influence of cropping treatment and chemical pretreatment across the whole range of aggregate sizes. The size and stability of aggregates increased with increasing domination of bromegrass in the cropping history. Materials resistant to soaking in water contributed to the increased stability under bromegrass. Materials sensitive to periodate and pyrophosphate anions contributed to the stabilization of aggregates from all cropping treatments. However, these materials did not appear to account for the differences in structural stability observed across cropping treatments. The rate of structural deterioration induced by grain-corn production under conventional tillage appeared to be faster than the rate of structural enhancement promoted by bromegrass production. Key words: Polysaccharides, periodates, pyrophosphate, corn, bromegrass, aggregate stability

Information on rates of change of the structure of different soils under different cropping systems is vital to research in soil conservation. Much data on soil structure cannot be used to provide information on rates of change because of an inability to relate measurements at a given time to cropping history. A conceptual model is presented which facilitates quantification of cropping history in relation to soil structure. The model relates structural form, structural stability, stabilizing materials, biological factors and forces which may be exerted on the soil. Soil and crop management practices influence the nature and magnitude of forces involved, biological factors, and the level of stabilizing materials. The model is applied, for illustrative purposes, to five different cropping treatments involving the production of corn and bromegrass grown for different lengths of time over a 15–yr period. Functions to describe the accumulation or loss of stabilizing material under the two different cropping systems are assumed and functions describing the relations between calculated levels of stabilizing materials and structural stability, and between structural stability and structural form are then presented. The model provides a framework for expressing the impact of soil and crop management practices on soil structure and includes variables which could be used for assessing the "quality" of soil under current conditions. It also provides a method for characterizing the susceptibility of soils to structural change arising from soil and crop management practices. Extensive research is required to evaluate the model, however potential applications justify the effort which will be required. Key words: Tensile strength, wet aggregate stability, compaction index, corn, bromegrass, soil conservation

SummaryFeldspathic agglomerates and related tuff-like rocks occurring at the Bukusu carbonatite complex were long thought to have been deposited as pyroclastics during a later (Elgon) period of volcanism. Evidence is presented for a reinterpretation of these rocks as vent agglomerates and tuffisites emplaced in the central diatreme of this large, pre-Elgon, complex.

AimStress fracture aetiology is often multifactorial and laboratory blood tests (LBT) can unmask underlying metabolic bone risk factors and disorders. Coeliac disease (CD) is associated with low bone mineral density and an increased risk of fractures.1In addition, there are rare reports of occult CD presenting with stress fractures.2Anti-tissue transglutaminase antibody (TTG) testing has a high sensitivity and specificity for CD and is used as a screening test.3This report examines the incidence of undiagnosed CD in patients presenting with stress fractures to a Sport and Exercise Medicine (SEM) clinic.MethodsA retrospective analysis of 100 consecutive patients with radiologically proven stress fractures presenting to a single tertiary NHS SEM clinic was performed. Age, gender, fracture site, co-morbidities, TTG result and subsequent investigations were examined. Records were reviewed to confirm LBT, including TTG, had been performed at the time of diagnosis.ResultsSeventy patients (70%) were female and mean age was 37 years (range 18–69). Metatarsal (35%) and tibial (21%) fractures were most common. TTG was performed in 85 patients. Two patients were excluded due to pre-existing CD. Five patients (5/83 (6%), mean age 38 years (28–57), 80% female) had a positive TTG; three of whom had CD confirmed by endoscopic biopsy and two are awaiting investigation. Four patients with a positive TTG underwent dual energy X-ray absorptiometry, with osteopenia (T-Score between −1.0 and −2.5) found in 75% of cases, although only one had a Z-score less than -2.0.ConclusionIn this cohort, the incidence of undiagnosed CD was between 3.6% to 6%, with a prevalence between 5% to 7%, approximately 5-fold higher than UK population estimates. We recommend that TTG screening should be performed in all patients presenting with stress fractures to identify underlying CD. Further work is required to confirm this association and elucidate potential underlying mechanisms.ReferencesHeikkila¨ K, Pearce J, Ma¨ki M, et al. Celiac disease and bone fractures: a systematic review and meta-analysis. J Clin Endocrinol Metab2015;100(1):25–34.Gilbody J, Trevett M. Coeliac disease presenting with bilateral fibular stress fractures. Foot Ankle Surg2009;15:96–100.3. Downey L, Houten R, Murch S, Longson D, Group GD. Recognition, assessment, and management of coeliac disease: summary of updated NICE guidance. BMJ2015;351:h4513.

Measurement of dispersible clay is important for the diagnosis of structural stability problems in soil. However, clay dispersibility is known to change with water content and time. The purpose of the present study was to determine how incubation of sodic soil under different water content regimes influences clay dispersibility. Two topsoils (depth 0-0·1 m), one sodic [exchangeable sodium percentage (ESP) 9 · 7] and the other non-sodic (ESP 3·8), were collected from an experimental pasture at Kyabram, Victoria, and 2 soils, a sodic topsoil (depth 0-0·1 m, ESP 6·9) and the corresponding subsoil (depth 0·2-0 m, ESP 25·7), were collected from a cropped field at Two Wells, South Australia. The soils were incubated for 264 days in a split-plot design. The main treatments were soil type and incubation water content: continuously air-dry, continuously wet (-50 kPa), or with wet/dry cycles. The subtreatment was water content at analysis: air dry or wet (-50 kPa). Clay dispersion was greater when measured on wet soils than dry soils, irrespective of water contents during the prior incubation. Electrical conductivity increased, and sodium adsorption ratio (SAR), pH, and organic carbon content decreased as a function of the time for which the soils were wet. In the Kyabram soils that were wet when analysed, easily dispersible clay content increased with SAR. Decreases in moderately dispersible clay under the wetting/drying regime were not related to electrolyte composition, and were attributed to particle rearrangement and cementation. The decreases in clay dispersibility with time occurred despite net losses of carbohydrate and aliphatic materials. An implication of the work is that the decomposition of soil organic matter, even in the absence of fresh additions, may reduce clay dispersion in sodic soils by altering electrolyte concentration and composition. sodium adsorption ratio, organic matter, pH, water content.

Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy has become an important tool for examining the chemical structure of natural organic materials and the chemical changes associated with decomposition. In this paper, solid-state 13C NMR data pertaining to changes in the chemical composition of a diverse range of natural organic materials, including wood, peat, composts, forest litter layers, and organic materials in surface layers of mineral soils, were reviewed with the objective of deriving an index of the extent of decomposition of such organic materials based on changes in chemical composition. Chemical changes associated with the decomposition of wood varied considerably and were dependent on a strong interaction between the species of wood examined and the species composition of the microbial decomposer community, making the derivation of a single general index applicable to wood decomposition unlikely. For the remaining forms of natural organic residues, decomposition was almost always associated with an increased content of alkyl C and a decreased content of O-alkyl C. The concomitant increase and decrease in alkyl and O-alkyl C contents, respectively, suggested that the ratio of alkyl to O-alkyl carbon (A/O-A ratio) may provide a sensitive index of the extent of decomposition. Contrary to the traditional view that humic substances with an aromatic core accumulate as decomposition proceeds, changes in the aromatic region were variable and suggested a relationship with the activity of lignin-degrading fungi. The A/O-A ratio did appear to provide a sensitive index of extent of decomposition provided that its use was restricted to situations where the organic materials were derived from a common starting material. In addition, the potential for adsorption of highly decomposable materials on mineral soil surfaces and the impacts which such an adsorption may have on bioavailability required consideration when the A/O-A ratio was used to assess the extent of decomposition of organic materials found in mineral soils. Additional keywords: composts, forest litter, peat, soil organic matter, wood, chemical composition.