Size-density Fractions Research Articles

The Analysis of Coal Fines Separation Precision Exposed to Changeable Hydrodynamic Parameters of Jig Work

In technology of coal fines beneficiation in Poland mainly fines jigging processes are in use. In case of steam coal fines beneficiation it is till 80% of the whole amount of produced assortments, while in case of coking coal fines it is 100%. The necessary condition of not homogenous feed separation which is directed to beneficiation process in pulsating water stream is a sufficient liberation of particles. The stratification of particles in working bed causes that particles of certain size, density and shape gather in individual layers in working bed of jig. The introduction of sufficient amount of additional water determines appropriate liberation of particles group, which generates partition into concentrate and tailings. The paper presents the results of sampling of industrial jig used for the beneficiation of coal fines by three various settings of additional amount of water under sieve which is directed to jigging. These amounts were equal to 35, 50 and 70 [m 3 /h]. Collected samples of separation products were then sieved into narrow particle size fractions and divided into density fractions. In such narrow size-density fractions the coordinates of partition curves were calculated for tailings of hard coal fines, which were subsequently approximated by means of Weibull distribution function. The separation precision measured by separation density, probable error and imperfection were determined on the basis of obtained model separation curves. The evaluation of separation effects was performed for a wide particle size fraction: feed directed to jigging process and narrow particle size fractions. The analysis of separation results in size-density fractions allowed to determine the influence of particle size change on the value of probable error. The results of separation precision in size-density fractions were compared with effects of separation of wide particle fraction, i.e. feed directed to jigging process.

Evaluation of the Liberation Characteristic of Coarse Middling Coking Coal by Size-Density Fractionation

Evaluation of the Liberation Characteristic of Coarse Middling Coking Coal by Size-Density Fractionation

Residual nitrogen from slurry and mineral fertiliser two years after application: Fractionation and plant availability

Fertilisation with animal manure has a long-standing tradition as a way to close nutrient cycles on farms. However, the fate of nitrogen (N) from manure in the soil beyond the year of application remains poorly understood. The aim of this research was to understand the residual N fertiliser value of mineral fertiliser (Min) and cattle slurry (Slu) in relation to the partitioning of N from the fertilisers in different soil physical fractions. To this end, we characterised the fate of 15N-labelled Min and Slu in soil organic matter (SOM) physical fractions two years after field application and related it to plant uptake of residual N. A secondary objective was to compare two fractionation methods with respect to accuracy and easiness, namely a combined density and particle size fractionation, separating five fractions, and a simple particle size fractionation, separating two fractions. All fractions were analysed for 15N abundance. The residual N fertiliser value was determined as the uptake of 15N by ryegrass (Lolium multiflorum) during 6 weeks. Furthermore, we deduced the source of 15N taken up by the plants from changes in the percentage of N derived from labelled fertilisers in each of the SOM physical fractions obtained by simple size fractionation before and after the pot experiment. Two years after application, most 15N was found in the fractions <20 μm, for both fractionation methods, i.e. in the mineral-associated organic matter (MAOM). The 15N recovery in these fractions in percent of the quantity of 15N originally applied as fertiliser tended to be higher for Slu (density-size fractionation: 19.8 ± 6.6%; simple size fractionation: 25.1 ± 6.6%) than for Min (density-size fractionation: 12.0 ± 2.1%; simple size fractionation: 16.8 ± 2.6%). Irrespective of the fertiliser type, about 2% of applied 15N was available to ryegrass plants two years after application. Out of these 2%, most 15N in the plants originated from MAOM, suggesting that the availability of the N stored in MAOM could be larger than previously thought. This finding is in line with the emerging view that MAOM is a dynamic fraction that plays an important role in the N cycle.

Chronological changes in soil biogeochemical properties of the glacier foreland of Midtre Lovénbreen, Svalbard, attributed to soil-forming factors

Glacier forelands provide an excellent opportunity to investigate vegetation succession and soil development along the chronosequence; however, there are few studies on soil biogeochemical changes from environmental factors, aside from time. This study aimed to investigate soil development and biogeochemical changes in the glacier foreland of Midtre Lovénbreen, Svalbard, by considering various factors, including time. Eighteen vegetation and soil variables were measured at 38 different sampling sites of varying soil age, depth, and glacio-fluvial activity. Soil organic matter (SOM) was quantitatively measured, and the compositional changes in SOM were determined following size-density fractionation. In the topsoil, the soil organic carbon (SOC) and total nitrogen (N) content was found to increase along the soil chronosequence and were highly correlated with vegetation-associated variables. These findings suggest that plant-derived material was the main driver of the light fraction of SOM accumulation in the topsoil. The heavy fractions of SOM were composed of microbially transformed organic compounds, eventually contributing to SOM stabilization within short 90-yr deglaciation under harsh climatic conditions. In addition to time, the soil vertical profiles showed that other environmental parameters, also affected the soil biogeochemical properties. The high total phosphorous (P) content and electrical conductivity in the topsoil were attributed to unweathered subglacial materials and a considerable amount of inorganic ions from subglacial meltwater. The high P and magnesium content in the subsoil were attributed to parent materials, while the high sodium and potassium content in the surface soil were a result of sea-salt deposition. Glacio-fluvial runoff hampered ecosystem development by inhibiting vegetation development and SOM accumulation. This study emphasizes the importance of considering various soil-forming factors, including parent/subglacial materials, aeolian deposition, and glacio-fluvial runoff, as well as soil age, to obtain a comprehensive understanding of the ecosystem development in glacier forelands.

Changes in the hydrophobic-hydrophilic properties of the organic matter of the chernozems of the Kamennaya Steppe

Soil samples and physical size-density fractions isolated from them (silt particle size less than 1 µm, light fraction (LF) with a density of less than 2 g/cm3 and a fraction of the residue) of ordinary chernozem were studied in three contrasting variants of the experimental fields of the Kamennaya Steppe agrolandscape of the Voronezh region: mowed steppe, long-term permanent bare fallow and permanent corn – the main differences of which are in tillage (cultivated and not cultivated lands) and in the supply/absence of plant residues and root secretions. The LF content changes in the series: “mowed steppe” &gt; “permanent corn” &gt; “permanent bare fallow”, which corresponds to the direction of changes in the total carbon content of the soil and a decrease in the value of the contact angle of wetting (CA) of the surface of the solid phase of the studied chernozems. The determination of the total C and N content revealed the change in the qualitative and quantitative composition of the size-density fractions for different land use cases. Chromatographic fractionation of alkaline extractions of humus substances (HS) of chernozem samples and size-density fractions revealed an increase in the degree of hydrophilicity of HS while simultaneously increasing the hydrophobicity of the solid phase surface and the carbon content in the soil. HS of LF of the “mowed steppe” turned out to be by 63% more hydrophilic than HS of LF of “permanent bare fallow” and by 47% more hydrophilic than HS of LF of “permanent corn”. While the hydrophilicity of the HS silt differed by 16 and 27%, respectively. The hydrophilicity of the HS of the original soil in the plot of the “mowed steppe” was by 41% higher than the hydrophilicity of the HS in the soil in the plot of “permanent bare fallow” and by 24% higher than in the soil of the plot of “permanent corn”. In addition, changes in the hydrophilicity of HS of size-density fractions are more intense than the HS of the soil, so the change in the degree of hydrophilicity of HS of size-density fractions is an indicator of soil degradation under different agrogenic pressue.

Organo-mineral associations largely contribute to the stabilization of century-old pyrogenic organic matter in cropland soils

Understanding the processes underlying carbon (C) stability in soils is of utmost importance in the context of climate change. In this setting, biochar is often studied for its persistence in soils and reported to have positive impacts on soil fertility. Whilst recent research has mostly focused on the short-term effects of biochar amendments to soil, a better understanding of its long-term persistence in soils is needed.Our study focuses on agricultural soils enriched in charcoal residues, produced in preindustrial kiln sites ca. 220 years ago, as a proxy for aged biochar. Our aim is to better understand the processes governing the long-term persistence of pyrogenic organic matter (PyOM) in cultivated soils. To achieve this, we focus on i) the effect of PyOM on soil aggregation, ii) its distribution amongst soil fractions and iii) its chemical and thermal properties. For this purpose, we combined a soil particle size-density fractionation with elemental and thermal analyses on topsoil samples collected in charcoal enriched (CHAR) and adjacent reference (REF) soils in a conventionally cropped field in Wallonia (Belgium).The presence of charcoal in soils resulted in a 91 ± 34% higher C content in CHAR soils than REF soils, of which 84 to 94% was PyOM and 6 to 16% was additional non-PyOM. In CHAR soils, macroaggregation was promoted at the expense of microaggregation (CHARMACRO = 48.9 ± 12.8; CHARMICRO = 25.5 ± 8.3 g 100 g−1 soil) whereas these were similar in REF soils (REFMACRO = 41.3 ± 9.5; REFMICRO = 36.0 ± 8.7 g 100 g−1 soil). Elemental and thermal analyses revealed that PyOM did not only occur as free light fractions but was also occluded in aggregates or sorbed onto mineral phases (56.4 ± 22.9% of total PyOM) suggesting PyOM may be further stabilized through organo-mineral associations. Century-old PyOM showed increased H:C and O:C atomic ratios compared to recently pyrolyzed OM, particularly in occluded as opposed to free light fractions, vouching for the functionalization of its surfaces. Furthermore, regardless of high C contents in CHAR soils, similar C:N ratios between studied soils in mineral dominated fractions suggested increased amounts of N-rich non-PyOM in charcoal rich soils. Our results demonstrate that, over centuries in cultivated soils, PyOM plays an active role in aggregation patterns. We conclude that century old PyOM is broken down from coarse to fine particles, and through functionalization of its surfaces contributes to organo-mineral associations. Beyond its intrinsic chemical recalcitrance, the long-term persistence of biochar is enhanced by occlusion and sorption processes.

Soil organic matter as affected by the conversion of natural tropical rainforest to monoculture rubber plantations under acric ferralsols

Land-use change (LUC) in the tropics, such as the exponential rate of conversion of natural habitats into intense monocultures focusing on cash-crop cultivation, is a major causal factor of global environmental change. To understand the effects of LUC on soil organic matter (SOM) stability and the dynamics of C and N within SOM fractions, we measured the C and N content and δ13C and δ15N in fractions of different aggregate- and density-size of acric ferralsols in tropical rainforests and rubber plantations. The proportion of macroaggregates, heavy and light fractions significantly decreased after LUC. The results showed that, in general, conversion of tropical rainforest to rubber plantation significantly decreased the C and N content in bulk soil and the aggregate- and density-size to 20 cm soil depth. The decrease in C and N content in bulk soil was mainly driven by decreasing C and N associated with macroaggregates and light fractions, which accounted for > 50%. We found significant correlations among mean weight diameter, aggregate-, and density-size fractions C, N, and C/N ratios. The conversion of tropical rainforest to rubber plantation significantly enriched soil δ13C while depleting δ15N. Enrichment of δ13C in rubber plantations could be explained by the mixing of old and fresh C. We conclude that, C and N dynamics within SOM fractions were greatly affected by LUC and the δ13C and δ15N signature confirms the changes in SOM stability after forest conversion. We suggest that planting intercrops within rubber monocultures may improve SOM accumulation, soil aggregation, and C and N sequestration.

Intensification enhances litter carbon and nitrogen decomposition dynamics in subtropical grazinglands

AbstractIncreased grazing land management intensification through the selection of productive species and the use of N affects the amount and quality of C and N inputs to the soil. This study evaluated the short‐term impacts of litter quality on litter decomposition and C and N distribution among soil size–density fractions. A 168‐d litter bag mesocosm experiment and a 120‐d incubation study evaluated the decomposition of three perennial species [rhizoma peanut (Arachis glabrata Benth., RP), bahiagrass (Paspalum notatum Flügge; BG), and saw‐palmetto (Serenoa repens Bartr., SP)] on grazing land soils with contrasting C concentrations (3.4 and 18 g C kg−1; Buck Island Ranch and Ona, respectively). Soil did not affect litter decomposition. There was less remaining biomass at the end of incubation for RP and BG (58%) than SP (79%). Nitrogen immobilization occurred on SP litter, probably because of its high initial lignin/N and fiber‐bound N/N ratios (43 and 384 g kg−1 organic matter, respectively). Litter had no effect on soil C and N concentrations in size–density fractions; however, greater proportions of C and N (∼47 and 59%, respectively) were found in the light‐free fraction. Nitrogen addition promoted C and N accumulation in the mineral fraction. These results suggest plant litter chemical characteristics played a more important role on litter decomposition than soil C and N concentrations. Management intensification through changes in plant species and N fertilization had positive effects on C and N dynamics in coarse‐textured soils, with enhanced decomposability of cultivated forage species relative to native vegetation.

Evaluation of jig work on the basis of granulometric analysis of particle size fractions of benefication products in purpose of process optimization

The analysis of hard coal jigging was conducted because of optimal recovery of useful fraction in concentrate and not useful fraction in tailings. The evaluation of the process was conducted on the basis of industrial sampling of coal dust jig and performed granulometric and densimetric analyses of collected products samples in laboratory conditions. In selected size-density fractions of partition products the yields of products were calculated and the amounts of ash and total sulfur were determined. On the basis of the results of granulometric, densimetric and chemical analyses of obtained size-density fractions, balance of partition products and appropriate calculations the beneficiation curves were created which allowed to evaluate the process because of optimization of device work in purpose of achieving higher recovery by possibly highest concentrate quality.The evaluation of hard coal beneficiation results was done in the paper because of optimal recovery of organic phase in concentrate and mineral components in tailings in aspect of process monitoring. The analysis of obtained characteristics of ability to beneficiation was performed in purpose of selecting optimal technological factors for the needs of optimization of device work.

Soil faunal effect on plant litter decomposition in mineral soil examined by two in-situ approaches: Sequential density-size fractionation and micromorphology

Potentially significant effects of soil fauna on plant litter decomposition are difficult to quantify by the commonly used litter bag approach due to physical constraints of meshed bag and inevitable loss of litter fragments through the mesh. We thus tested the applicability of two bag-free approaches, physical fractionation and micromorphological analysis, to examine litter decomposition in an Andosol under a temperate humid climate. In-situ decomposition of two winter cover crop litter, C-rich wild oat (WO) and N-rich hairy vetch (HV), was compared with a conventionally farmed (CF) field which received no litter as a control. The surface soils (Ap holizons) samples taken at 0, 2, and 8 weeks after the litter incorporation were separated by density to recover a low-density fraction (LF, <1.6 g cm−3) followed by its size fractionation via wet sieving (4.0, 2.0, 1.0, 0.5, and 0.1 mm mesh sizes) based on the idea that litter fragmentation occurs in parallel with decomposition and that the size of the faunal excrements depends on that of the organism. While the LF total mass did not show clear change over the eight weeks in all treatments, its size distribution exhibited a significant decline in >4 mm size fraction and concurrent increases in 0.5–2 mm fractions primarily as macrofauna excrements in the WO field. While the loss due to its fragmentation below 0.1 mm is possible, the identified excrements (1–2 mm size fractions) produced over eight-weeks accounted for 4.6 ± 0.2% and 5.5 ± 0.4%, of the C and N present in the WO litter on a ground area basis, and 0.67 ± 0.01% and 0.43 ± 0.01% of the surface soil C and N, respectively. Micromorphology of thin soil sections confirmed the abundance of larvae (Coleoptera) excrements that were mainly comprised of WO stems. In contrast, the faunal excrements found in HV soil were largely from mesofauna (mites). The significant difference found between the litter decomposition patterns was partly attributable to the faunal activity. Specifically, the decomposition of the tough tissues of WO was promoted by macrofauna, while that of HV legume litter was associated with mesofauna without any indications of macrofaunal influence. The physical fractionation based approach we developed here allowed to estimate the production rate of macrofaunal excrements under the field condition for the first time, which can help to elucidate the impact of soil fauna on biogeochemical cycling.

Soil aggregation and soil fraction associated carbon under different vegetation types in a complex landscape

Land cover change has been shown to affect soil characteristics and soil organic carbon (SOC) storage. However, little is known about the driving factors associated with soil carbon (C) stabilisation in complex landscapes. This study was conducted to investigate the effects of both inherent soil characteristics and recent vegetation cover change on soil aggregation and soil fraction associated C in a complex landscape. The specific objectives were: (1) determine bulk soil properties and SOC saturation deficit; (2) quantify soil aggregation, soil size-density fractions, and soil fraction associated C; and (3) identify the factors that influence soil fractions and soil fraction associated C in two adjacent catchments differing in vegetation cover, Central Otago, New Zealand. Catchment GH1 (n = 17 plots) was dominated by tussock grassland and native shrubs. Catchment GH2 (n = 21 plots) was converted from tussock grassland into a pine forest in 1981. The catchments differed in soil texture (e.g. sand content GH1: 62.9%, GH2: 50.7%, P = 0.007), soil SOC stocks (GH1: 5.0 kg C m−2, GH2: 4.3 kg C m−2, P = 0.04), mean weight diameter (MWD) (GH1: 782.3 µm, GH2: 736.5 µm, P = 0.002), and proportion of the macroaggregate heavy fraction (macro_HF) (GH1: 72%, GH2: 55%, P = 0.01). No significant differences were found in SOC saturation deficit (GH1: 14.8 mg g−1, GH2: 13.1 mg g−1, P = 0.13). Dominant vegetation cover explained 21% of the variation in MWD in GH1, whereas silt+clay C content explained 31.6% of the MWD variation in GH2. The macro_HF fraction was negatively correlated with the proportion of silt+clay. Our findings illustrate that physical and chemical soil characteristics are important drivers in such a complex landscape and may have masked the effect of recent vegetation change on soil aggregation, and soil fraction associated C.

Short-term impacts of high levels of nitrogen fertilization on soil carbon dynamics in a tropical pasture

Bermudagrass (Cynodon spp.) is widely recognized by its ability to respond positively to nitrogen (N) fertilization. Nitrogen often promotes shoot and root growth and, consequently, increase carbon (C) inputs into the soil via above- and below-ground biomass. The objective of this 2-yr field study was to assess the short-term effects of increased N levels on soil C dynamics of a Tifton 85 bermudagrass pasture grown in a tropical soil. Treatments consisted of five levels of N fertilization (0, 60, 120, 180 and 240 kg ha−1 N) applied at each harvest for a total of four harvest events per year. Soil samples were collected at the 0 to 0.1 and 0.1 to 0.2 m depths for determination of soil C content. The latter was also fractionated into free-light (FLF), particulate organic carbon (POC), and mineral-associated organic carbon (C-min) fractions. Results indicated that N levels showed no effect on total C content and distribution among the various size-density fractions. The top layer (0–0.1 m) was more sensitive to changes in soil C for more recalcitrant fractions (POC and C-min) relative to 0.1–0.2 m layer. Our findings suggest that acceleration of the decomposition rate induced by N fertilization may have offset the potential accumulation of soil C associated with high inputs of above- and below-ground biomass, but additional research is warranted to better elucidate long-term impacts of nitrogen fertilization on soil C dynamics in grassland of tropical regions.

A new methodology of floatability tests on Polish copper ores based on grain size-density fractions

Ores difficult to enrich require the appropriate methodology of tests on enrichment for the preparation of ore for flotation tests and precise mineralogical analytics. A feature which should differentiate the separated ore samples for flotation tests is not only the size of grains, but also their density. These features reflect the characteristics of real industrial streams directed for flotation. The methodology for separating narrow grain classes by means of sieves applied to date did not guarantee meeting these separation conditions. The article presents a new methodology for examining the copper ore enrichment based on, instead of testing samples in grain classes, testing grain size-density fractions. To conduct these tests, the hydraulic classification in the field of centrifugal force which ensures the separation of grain size-density fractions varied in terms of granulation and density was used. Samples prepared in this way were subjected to flotation tests.

Anthropogenic N deposition increases soil organic matteraccumulation without altering its biochemical composition.

Accumulating evidence indicates that future rates of atmospheric N deposition have the potential to increase soil C storage by reducing the decay of plant litter and soil organic matter (SOM). Although the microbial mechanism underlying this response is not well understood, a decline in decay could alter the amount, as well as biochemical composition of SOM. Here, we used size-density fractionation and solid-state 13 C-NMR spectroscopy to explore the extent to which declines in microbial decay in a long-term (ca. 20yrs.) N deposition experiment have altered the biochemical composition of forest floor, bulk mineral soil, as well as free and occluded particulate organic matter. Significant amounts of organic matter have accumulated in occluded particulate organic matter (~20%; oPOM); however, experimental N deposition had not altered the abundance of carboxyl, aryl, alkyl, or O/N-alkyl C in forest floor, bulk mineral soil, or any soil fraction. These observations suggest that biochemically equivalent organic matter has accumulated in oPOM at a greater rate under experimental N deposition, relative to the ambient treatment. Although we do not understand the process by which experimental N deposition has fostered the occlusion of organic matter by mineral soil particles, our results highlight the importance of interactions among the products of microbial decay and the chemical and physical properties of silt and clay particles that occlude organic matter from microbial attack. Because oPOM can reside in soils for decades to centuries, organic matter accumulating under future rates of anthropogenic N deposition could remain in soil for long periods of time. If temperate forest soils in the Northern Hemisphere respond like those in our experiment, then unabated deposition of anthropogenic N from the atmosphere has the potential to foster greater soil C storage, especially in fine-texture forest soils.

DRIFT spectroscopy combined with sodium hypochlorite oxidation reveals different organic matter characteristics in density-size fractions of organically managed soils

The objective of this study was to characterize the organic matter (OM) in density-size fractions of soil samples from a commercial organic farm using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The soil samples were separated by density fractionation with a sodium iodide (Nal) solution (1.6 g cm–3) into free particulate OM (fPOM), occluded particulate OM (oPOM), heavy particulate OM (hPOM), and mineral-associated OM (MAOM) fractions. The OM characterization by DRIFT spectroscopy was the difference in spectra obtained before and after sodium hypochlorite (NaClO) oxidation. However, the infrared absorption bands derived from the soil mineral matrix interfered with the detection of the absorption bands of polysaccharides. An increase in the amount of organic C under organic management was observed for all the density-size fractions, but the functional group composition of the NaClO-oxidizable OM differed among the fractions. The NaClO-oxidizable OM in the fPOM fraction was characterized by a high lignin content, whereas the oPOM fraction had high amounts of aliphatic compounds and lignin. The hPOM fraction contained less lignin and more proteinous materials, and the MAOM fraction was rich in proteinous materials. This study demonstrates that DRIFT spectroscopy combined with NaClO oxidation is a powerful tool for characterizing the relatively unstable OM in soils.

Carbon storage in soil size-density fractions after 20 years of compost fertilization

Fractionation by particle size provides a rough differentiation between young active, and older intermediate and passive soil organic matter. Soil samples from three treatments of a 20 years` fertilization experiment, C2 which had been fertilized with 10 t ha -1 (wet wt.) compost per year on average, N2 with mineral N fertilizer at 32 kg N ha -1 year -1 , and the unfertilized control (O) were subjected to particle size fractionation and to density fractionation. After low-energy sonication the samples were separated into the size fractions coarse sand (200-200 µm), fine sand (200-63 µm) silt (63-2 µm) and clay (2-0,1 µm). Density fractionation using Na-polytungstate with 1.8 g cm -3 density was applied to separate particulate organic matter (POM) from the sand-sized fraction. Compost fertilization resulted in an increase in C org in all size and density fractions. In total, the C org content was 10 % higher with compost fertilization than in the unfertilized control. Approximately 40 % of the additional soil carbon was located in the POM, 56 % in the silt-sized fraction and 3 % in the clay-sized fraction. With mineral N fertilization the sum of C org contents of all fractions was about the same as without fertilization, with an increase of POM-C org and a decrease of C org in the silt and clay-sized fraction. Keywords: fractionation, particulate organic matter, POM, soil organic matter

Carbon storage potential in size–density fractions from semi-natural grassland ecosystems with different productivities over varying soil depths

Researchers have increasingly recognised a profound need for more information on SOC stocks in the soil and the factors governing their stability and dynamics. Many questions still remain unanswered about the interplay between changes in plant communities and the extent to which changes in aboveground productivity affect the carbon dynamics in soils through changes in its quantity and quality. Therefore, the main aim of this research was to examine the SOC accumulation potential of semi-natural grasslands of different productivities and determine the distribution of SOM fractions over varying soil depth intervals (0–10, 10–20, 20–30 30–50 50–80 and 80+cm). SOM fractionation was considered as a relative measure of stability to separate SOM associated with clay minerals from SOM of specific light densities less than 2gcm−3 (size-density fractionation). Two clay-associated fractions (CF1, <1μm; and CF2, 1–2μm) and two light fractions (LF1, <1.8gcm−3; and LF2, 1.8–2.0gcm−3) were separated. The stability of these fractions was characterised by their carbon hot water extractability (CHWE) and stable carbon isotope composition. In the semi-natural grasslands studied, most OC was stored in the top 30cm, where turnover is rapid. Effects of low productivity grasslands became only significantly apparent when fractional OC contributions of total SOM was considered (CF1 and LF1). In deeper soil depths OC was largely attributed to the CF1 fraction of low productivity grasslands. We suggest that the majority of OM in deeper soil depth intervals is microbially-derived, as evidenced by decreasing C/N ratios and decreasing δ13C values. The hot water extraction and natural δ13C abundance, employed here allowed the characterisation of SOM stabilisation properties, however how climatic changes affect the fate of OM within different soil depth intervals is still unknown.

Short-term bioavailability of carbon in soil organic matter fractions of different particle sizes and densities in grassland ecosystems

The quality, stability and availability of organic carbon (OC) in soil organic matter (SOM) can vary widely between differently managed ecosystems. Several approaches have been developed for isolating SOM fractions to examine their ecological roles, but links between the bioavailability of the OC of size–density fractions and soil microbial communities have not been previously explored. Thus, in the presented laboratory study we investigated the potential bioavailability of OC and the structure of associated microbial communities in different particle-size and density fractions of SOM. For this we used samples from four grassland ecosystems with contrasting management intensity regimes and two soil types: a Haplic Cambisol and a typical Chernozem. A combined size–density fractionation protocol was applied to separate clay-associated SOM fractions (CF1, <1μm; CF2, 1–2μm) from light SOM fractions (LF1, <1.8gcm−3; LF2, 1.8–2.0gcm−3). These fractions were used as carbon sources in a respiration experiment to determine their potential bioavailability. Measured CO2-release was used as an index of substrate accessibility and linked to the soil microbial community structure, as determined by phospholipid fatty acids (PLFA) analysis.Several key factors controlling decomposition processes, and thus the potential bioavailability of OC, were identified: management intensity and the plant community composition of the grasslands (both of which affect the chemical composition and turnover of OC) and specific properties of individual SOM fractions. The PLFA patterns highlighted differences in the composition of microbial communities associated with the examined grasslands, and SOM fractions, providing the first broad insights into their active microbial communities. From observed interactions between abiotic and biotic factors affecting the decomposition of SOM fractions we demonstrate that increasing management intensity could enhance the potential bioavailability of OC, not only in the active and intermediate SOM pools, but also in the passive pool.

Characterization of Soil Organic Matter in Aggregates and Size-Density Fractions by Solid State 13C CPMAS NMR Spectroscopy

Understanding the changes in soil organic matter (SOM) composition during aggregate formation is crucial to explain the stabilization of SOM in aggregates. The objectives of this study were to investigate (i) the composition of SOM associated with different aggregates and size-density fractions and (ii) the role of selective preservation in determining the composition of organic matter in aggregate and size-density fractions. Surface soil samples were collected from an Alfisol on the Northern Tablelands of NSW, Australia, with contrasting land uses of native pasture, crop–pasture rotation and woodland. Solid-state 13C cross-polarization and magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectroscopy was used to determine the SOM composition in macroaggregates (250–2000 μm), microaggregates (53–250 μm), and <53-μm fraction. The chemical composition of light fraction (LF), coarse particulate organic matter (cPOM), fine particulate organic matter (fPOM), and mineral-associated soil organic matter (mSOM) were also determined. The major constituent of SOM of aggregate size fractions was O-alkyl carbon, which represented 44–57% of the total signal acquired, whereas alkyl carbon contributed 16–27%. There was a progressive increase in alkyl carbon content with decrease in aggregate size. Results suggest that SOM associated with the <53-μm fraction was at a more advanced stage of decomposition than that of macroaggregates and microaggregates. The LF and cPOM were dominated by O-alkyl carbon while alkyl carbon content was high in fPOM and mSOM. Interestingly, the relative change in O-alkyl, alkyl, and aromatic carbon between aggregates and SOM fractions revealed that microbial synthesis and decomposition of organic matter along with selective preservation of alkyl and aromatic carbon play significant roles in determining the composition of organic matter in aggregates.

Soil organic carbon characteristics relating to geomorphology near Vestre Lovénbreen moraine in Svalbard

Soil organic carbon (SOC) in the Arctic is vulnerable to climate change. However, research on SOC stored in the high Arctic regions is currently very limited. Thus, this study was aimed at understanding the distribution and characteristics of SOC with respect to geomorphology and vegetation in Svalbard. In August 2011, soil samples were collected near the Vestre Lov<TEX>$\acute{e}$</TEX>nbreen moraine. Sampling sites were chosen according to altitude (High, Mid, and Low) and differences in levels of vegetation establishment. Vegetation coverage, aboveground biomass, and SOC contents were measured, and density-size fractionation of SOC was conducted. The SOC content was the highest in the Mid site (<TEX>$126.9mg\;g^{-1}$</TEX>) and the lowest in the High site (<TEX>$32.1mg\;g^{-1}$</TEX>), although aboveground biomass and vegetation coverage were not different between these two sites. The low SOC content measured at the High site could be related to a slower soil development following glacial retreat. On the other hand, the Low site contained a high amount of SOC despite having low vegetative cover and a high ratio of sand particles. These incompatible relationships between SOC and vegetation in the Low site might be associated with past site disturbances such as runoff from snow/glacier melting. This study showed that geomorphological features combined with glacier retreat or melting snow/glacier effects could have affected the SOC distribution and vegetation establishment in the high Arctic.