Techniques for the dierentiation of carbon types present in lignite-rich mine soils

Abstract The objective of this study was to assess high energy UV photo-oxidation

Geochemistry (delta13C, delta15N, 13C NMR) and residence times (14C and OSL) of soil organic matter from red-brown earths of South Australia: implications for soil genesis

Soil forming processes important to the development of Red-Brown Earths (duplex soils) in southeastern Australia have been investigated by a combination of techniques, including isotopic (δ13C, δ15N, 14C), spectroscopic (13C NMR, MIR), optically stimulated luminescence dating (OSL) and phytolith analyses. A distinct increase in clay content, corresponding changes in the abundance of major elements, as well as changes in organic chemistry (13C NMR), stable isotope trends (δ13C, δ15N), and phytolith abundance, are apparent in the transition from the very sandy A horizon to the clayey B horizon in three soil profiles from the Coonawarra–Padthaway region of South Australia. These structural and chemical changes between the A and the B horizons are associated with an abrupt increase in both 14C (bulk soil organic matter) and OSL burial ages of individual quartz grains. While previous interpretations have promoted the formation of duplex red-brown earths as due to clay illuviation, we propose a two-stage soil formation, which may be related to paleoclimatic changes during and after the Last Glacial Maximum. Our data suggest that a major part of the A horizon was aeolian derived and was deposited over the last 10,000 years, whereas much of the B horizon, although originally aeolian, has been extensively modified over much longer periods of time (tens of thousands of years). These results indicate the influence of different substrates (sandy versus clayey), process and time for formation as well as paleoclimatic history on the physical properties of the soil and the chemical characteristics of the organic matter within the soil profile

Synthesis and characterisation of laboratory-charred grass straw (Oryza sativa) and chestnut wood (Castanea sativa) as reference materials for black carbon quantification

We synthesised large (~2 kg) quantities of two chars for use as commercially available reference materials for the quantification of black carbon (BC). We pyrolysed chestnut wood (Castanea sativa) and grass straw (Oryza sativa) at 450 °C under a N2 atmosphere, which mimics the oxygen-free conditions on the inside of burning material at a moderate burning temperature. The charred materials were dominated by aromatic carbon (~70%), had low H/C (~0.7) and O/C (~0.3) ratios and low surface areas (2–6 m² g⁻¹). Isotopic changes on charring were small (≤0.3‰). In these respects, the synthesised chars were similar to chars produced at low temperature (<500 °C) in natural fires and thus may prove to be appropriate materials for calibrating BC quantification methods. Both chars have been used in a comparative study of BC quantification

13C-depleted charcoal from C3 and C4 grasses and the role of occluded carbon in phytoliths

The δ13C values of plants and corresponding charred materials from wood, C3 and C4 grasses, derived from natural burning and laboratory combustion were obtained to determine whether there was a significant difference in δ13C of grass-derived char (C3 and C4) compared with wood-derived (C3) char. Our data showed that there is an up to 8‰ 13C-depletion in C4-derived chars from natural burning but there was no significant isotopic change in chars from wood or C3 grasses. We suggest that this 13C-depletion in C4-derived chars is due to protected organic matter in silicate structures (phytoliths), which were found to be depleted by up to 9‰. Analysis of this protected carbon by Py-GCMS indicated the presence of low relative amounts of n-alkanes. However, 13C-NMR data suggested that a significant portion of the phytolith-occluded material was composed of simple carbohydrates (O-alkyl carbon) and that alkyl carbon (lipid material) constituted a minor fraction. These isotopic and spectroscopic data have important implications for the calculation of the proportions of C3- versus C4-derived charred organic matter in modern as well as geological studies

Comparative analysis of black carbon in soils

Black carbon (BC), produced by incomplete combustion of fossil fuels and vegetation, occurs ubiquitously in soils and sediments. BC exists as a continuum from partly charred material to highly graphitized soot particles, with no general agreement on clear-cut boundaries of definition or analysis. In a comparative analysis, we measured BC forms in eight soil samples by six established methods. All methods involved removal of the non-BC components from the sample by thermal or chemical means or a combination of both. The remaining carbon, operationally defined as BC, was quantified via mass balance, elemental composition or by exploiting benzenecarboxylic acids as molecular markers or applying C-13 MAS NMR (magic angle spinning nuclear magnetic resonance) spectroscopy. BC concentrations measured for individual samples vary over 2 orders of magnitude (up to a factor of 571). One possible explanation for this wide range of results is that the individual BC methods rely on operational definitions with clearcut but different boundaries and developed for specific scientific questions, whereas BC represents a continuum of materials with widely contrasting physicochemical properties. Thus the methods are inherently designed to analytically determine different parts of the continuum, and it is crucial to know how measurements made by different techniques relate to each other. It is clear from this preliminary comparative analysis that a collection of BC reference materials should be established as soon as possible 1) to ensure long-term intralaboratory and interlaboratory data quality and 2) to facilitate comparative analyses between different analytical techniques and scientific approaches. [References: 17

Recent Vegetation Changes In Central Queensland, Australia: Evidence From δ\u3csup\u3e13\u3c/sup\u3eC and \u3csup\u3e14\u3c/sup\u3eC Analyses of Soil Organic Matter

This study examines whether changes from grassy to woody vegetation coincide with the initiation of European farming practices (last 50–150 years) in central Queensland, using a site where historical records suggest invasion of woody vegetation (Acacia sp.). Soil samples were taken along a transect, spanning the transition between C4-dominated grassland and C3-dominated woodland. We utilize δ13C and 14C data of bulk and size-separated soil organic matter (SOM) to infer the time course of changes in vegetation and soil carbon stocks. δ13C values of bulk SOM indicate a shift from grass (C4)- to tree (C3)-derived carbon in the woodland compared with the grassland. The δ13C analyses of the size-separates showed that most of the labile, C3-derived carbon was present in the particulate organic carbon (POC) fraction (\u3e53 μm) (down to 30 cm), whereas δ13C values in the \u3c53 μm fraction showed greater C4 contributions. The δ13C values of the POC and the \u3c53 μm fractions in the grassland and transition zone were significantly different, inferring a relatively recent change from a C4- to a C3-dominated system (\u3c100 years). This interpretation is supported by 14C data as the 14C activity of both the \u3e200 and 53–200 μm fraction was greater than 108% modern (pMC), indicating that most of the C3-derived POC fractions were formed during the past ∼45 years. This “bomb-derived” carbon (\u3e100 pMC) was present to a depth of 30 cm. Thus, 14C and δ13C data from the size fractions indicate that much of the vegetation change at this site occurred over the last 50 years. We also found that the thickened site had greater C storage in above-ground biomass and soil carbon stocks compared with the grassy site. Preliminary modelling of changes in carbon stocks, using the Roth-C model, support that the change from grassland to woodland probably occurred \u3c100 years ago

Black carbon increases cation exchange capacity in soils

Black Carbon (BC) may significantly affect nutrient retention and play a key role in a wide range of biogeochemical processes in soils, especially for nutrient cycling. Anthrosols from the Brazilian Amazon (ages between 600 and 8700 yr BP) with high contents of biomass-derived BC had greater potential cation exchange capacity (CEC measured at pH 7) per unit organic C than adjacent soils with low BC contents. Synchrotron-based near edge X-ray absorption fine structure (NEXAFS) spectroscopy coupled with scanning transmission X-ray microscopy (STXM) techniques explained the source of the higher surface charge of BC compared with non-BC by mapping cross-sectional areas of BC particles with diameters of 10 to 50 μm for C forms. The largest cross-sectional areas consisted of highly aromatic or only slightly oxidized organic C most likely originating from the BC itself with a characteristic peak at 286.1 eV, which could not be found in humic substance extracts, bacteria or fungi. Oxidation significantly increased from the core of BC particles to their surfaces as shown by the ratio of carboxyl-C/aromatic-C. Spotted and non-continuous distribution patterns of highly oxidized C functional groups with distinctly different chemical signatures on BC particle surfaces (peak shift at 286.1 eV to a higher energy of 286.7 eV) indicated that non-BC may be adsorbed on the surfaces of BC particles creating highly oxidized surface. As a consequence of both oxidation of the BC particles themselves and adsorption of organic matter to BC surfaces, the charge density (potential CEC per unit surface area) was greater in BC-rich Anthrosols than adjacent soils. Additionally, a high specific surface area was attributable to the presence of BC, which may contribute to the high CEC found in soils that are rich in BC. © Soil Science Society of America

Comparison of quantification methods to measure fire-derived (black/elemental) carbon in soils and sediments using reference materials from soil, water, sediment and the atmosphere

Black carbon (BC), the product of incomplete combustion of fossil fuels and biomass (called elemental carbon (EC) in atmospheric sciences), was quantified in 12 different materials by 17 laboratories from different disciplines, using seven different methods. The materials were divided into three classes: (1) potentially interfering materials, (2) laboratory-produced BC-rich materials, and (3) BC-containing environmental matrices (from soil, water, sediment, and atmosphere). This is the first comprehensive intercomparison of this type (multimethod, multilab, and multisample), focusing mainly on methods used for soil and sediment BC studies. Results for the potentially interfering materials (which by definition contained no fire-derived organic carbon) highlighted situations where individual methods may overestimate BC concentrations. Results for the BC-rich materials (one soot and two chars) showed that some of the methods identified most of the carbon in all three materials as BC, whereas other methods identified only soot carbon as BC. The different methods also gave widely different BC contents for the environmental matrices. However, these variations could be understood in the light of the findings for the other two groups of materials, i.e., that some methods incorrectly identify non-BC carbon as BC, and that the detection efficiency of each technique varies across the BC continuum. We found that atmospheric BC quantification methods are not ideal for soil and sediment studies as in their methodology these incorporate the definition of BC as light-absorbing material irrespective of its origin, leading to biases when applied to terrestrial and sedimentary materials. This study shows that any attempt to merge data generated via different methods must consider the different, operationally defined analytical windows of the BC continuum detected by each technique, as well as the limitations and potential biases of each technique. A major goal of this ring trial was to provide a basis on which to choose between the different BC quantification methods in soil and sediment studies. In this paper we summarize the advantages and disadvantages of each method. In future studies, we strongly recommend the evaluation of all methods analyzing for BC in soils and sediments against the set of BC reference materials analyzed here