Browsing by Author "Hobley, E"

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    Comparing the stability and chemistry of soil organic carbon protected via pyrogenesis, aggregation and mineral-association
    (International Union of Soil Science, 2014-06-23) Hobley, E; Willgoose, GR; Frisia, S; Jacobsen, GE
    We investigated the influence of soil texture and mineralogy on soil organic carbon (SOC) stability in two native soils of different texture and mineralogy from the Southern Highlands of NSW, Australia. To do this, a heavy-textured (clayey) soil and coarse-textured (sandy) soil were sampled at various depths to bedrock. The bulk samples were then fractionated into different particle-sizes and SOC content and stability measured using elemental analysis and radiocarbon analysis. Diffuse-reflectance infrared Fourier Transform spectroscopy was applied to infer molecular chemistry and variability in the soils, and investigate to SOC chemical structures associated with shifts in radiocarbon content of the soils. In both soils, the highest SOC content was found in the finest fraction, indicating that particle-size is a dominant control on SOC retention, independent of soil texture. In contrast, the mechanisms of SOC stabilization varied between the two soils, which we attribute to the differences in mineralogy and texture. In the coarse-textured soil, the chemical recalcitrance of charcoal was found to be the dominant stabilization mechanism in most of the soil profile, and the chemical recalcitrance of other aromatic structures may have contributed to SOC stability in subsoils. In the clayey soil, the most important stabilization mechanism throughout the soil profile was aggregation, which was centuries older than the mineral-associated organic matter in the soil. SOC was highly correlated with radiocarbon content and depth in both soils, so that SOC turnover may be limited by substrate availability at depths near bedrock in the soils. Comparing the radiocarbon ages of the two soils, the most stable carbon was (1) C stored in charcoal, followed by (2) C occluded within aggregates consisting of highly-charged clay minerals, (3) C associated with highly-charged clay minerals and Fe/Mn oxides, and (4) C associated with lowly-charged silicates or sandy aggregates. Our results indicate that there is a disconnect between SOC storage and SOC stability. Our findings have implications for SOC sequestration schemes, namely that trade-offs exist between enhancing SOC storage and enhancing SOC stability, and that texture and mineralogy should be considered when tailoring these schemes within an ecosystem.
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    Environmental and site factors controlling the vertical distribution and radiocarbon ages of organic carbon in a sandy soil
    (Springer, 2013-04-16) Hobley, E; Willgoose, G; Frisia, S; Jacobsen, GE
    Soil organic carbon (SOC) content and radiocarbon concentration were measured in three particle-size fractions and charcoal fragments at four depths to bedrock in a sandy soil from SE Australia. SOC content declined with depth for all fractions. The enrichment factors of SOC showed that the finest particles are most important for SOC storage throughout the soil profile, and their importance for SOC storage increased with depth. In the topsoil, all particle-size fractions contained modern SOC. In contrast, charcoal from this depth gave radiocarbon ages of 85–165 years Before Present (BP). This difference was more pronounced at 30–60 cm, where the charcoal was dated at 2,540 years BP, over 12 times as old as the youngest fraction at that depth. These results confirm charcoal as a highly stable form of SOC. The radiocarbon data in the topsoil and near bedrock indicate that neither microaggregation nor mineral association is important for SOC stability in this soil. At intermediate sampling depths, the mid-sized fraction was the oldest. We believe that this is the result of charcoal accumulation in this fraction, inducing a shift in radiocarbon age. However, near bedrock (100–120 cm), radiocarbon concentration did not differ significantly between fractions, despite greater SOC retention in smaller fractions. In addition, radiocarbon ages at 100–120 cm indicate that charcoal is not present at this depth. We propose that environmental and soil conditions (substrate limitation, water and oxygen availability, and temperature) are responsible for the stabilization of SOC at this depth, where SOC concentrations were very low (0.1–0.3 %). Our results demonstrate that, although fine particles retain more SOC than coarse ones, they do not stabilize SOC in this sandy soil. Instead, environmental (bushfires and climate) and site factors (soil texture and soil mineralogy) control the distribution and stability of SOC throughout the soil profile. © 2013, Springer-Verlag.
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    Land-use affects the radiocarbon age, storage and depth distribution of soil organic carbon in Eastern Australia
    (European Geosciences Union, 2015-01-01) Hobley, E; Wilson, B; Hua, Q
    Land-use has been shown to affect soil organic carbon (SOC) storage, with natural systems generally storing larger quantities of SOC than anthropogenically managed systems in surface soils. However, these effects are often difficult to detect deeper in the soil profile. Little is known regarding the effects of land-use on the radiocarbon age of SOC, both at the surface and deeper in the soil profile. We investigated the storage, radiocarbon content and depth distribution of soil organic carbon from across the state of NSW, Australia. A total of 100 profiles were analysed for total SOC concentration at numerous depths (up to 1 m) and a machine learning approach implementing tree ensemble methods was used to identify the key drivers of SOC depth distribution. Surface SOC storage was strongly associated with climate (predominately precipitation, to a lesser degree relative humidity and temperature), whereas SOC depth distribution was predominately influenced by land-use, soil type and to a lesser extent temperature. A subset of 12 soil profiles from a range of climate zones were analysed for radiocarbon content with a view to contrasting three land-use systems: natural, cleared/grazed and cropped. Radiocarbon content was affected strongly by land-use, with effects most pronounced at depth. Native systems appeared to have the youngest carbon throughout the profile, with cropped and grazed systems having older SOC. Radiocarbon content was also strongly associated with SOC content. Our results indicate that natural systems act as a carbon pump into the soil, injecting young, fresh organic carbon which is vertically distributed throughout the profile. In contrast, managed systems are deprived of this input and are depleted in SOC at all depths, leading to higher radiocarbon ages throughout the profile. © Author(s) 2015.
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    Relationship between land-use, soil organic carbon depth distribution and radiocarbon age New South Wales, Australia
    (University of New South Wales and Australian Nuclear Science and Technology Organisation, 2015-07-09) Hobley, E; Wilson, B; Hua, Q
    Not provided to ANSTO Library.
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    Soil organic carbon in eastern Australia
    (American Geophysical Union (AGU), 2016-12-14) Hobley, E; Baldock, JA; Hua, Q; Wilson, B
    We investigated the drivers of SOC dynamics and depth distribution across eastern Australia using laboratory analyses (CN, fractionation, radiocarbon) coupled with modelling and machine learning. At over 1400 sites, surface SOC storage was driven by precipitation, whereas SOC depth distribution (0-30 cm) was influenced by land-use. Based upon these findings, 100 sites were selected for profile analysis (up to 1 m) of SOC and its component fractions - particulate (POC), humus (HOC) and resistant (ROC) organic carbon. Profile SOC content was modelled using an exponential model describing surface SOC content, SOC depth distribution and residual SOC at depth and the drivers of these parameters investigated via machine learning. Corroborating previous findings, surface SOC content was highly influenced by rainfall, whereas SOC depth distribution was influenced by land-use. At depth, site properties were the most important predictors of SOC. Cropped sites had significantly lower SOC content than native and grazed sites at depth, indicating that land-use influences SOC content throughout the profile. The machine learning algorithms identified depth as the key control on the proportion of all three fractions down the profile: POC decreased whereas HOC increased with increasing depth. POC was strongly linked with total SOC but HOC and ROC were driven more by climate and soil physico-chemical properties. Human influences (land-use and management) were not important to the fractions, implying that the controls humans can exert on SOC stability may be limited. A subset of 12 soil profiles was analysed for 14C. Radiocarbon content was affected strongly by land-use, with effects most pronounced at depth. Native systems had the youngest carbon down the profile, cropped systems had the oldest SOC. All fractions reacted to land-use change down the soil profile, indicating a lack of stability when the whole profile is viewed. These results indicate that natural systems act as a carbon pump into the soil, injecting young, fresh organic carbon down the entire profile. In contrast, managed systems are deprived of this input and are depleted in SOC at all depths. Our results strongly suggest that SOC storage in the region is input driven. © AGU 2016
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    Stability and storage of soil organic carbon in a heavy-textured Karst soil from south-eastern Australia
    (CSIRO publishing, 2014-05-09) Hobley, E; Willgoose, GR; Frisia, S; Jacobsen, GE
    Both aggregation and mineral association have been previously found to enhance soil organic carbon (SOC) storage (the amount of organic C retained in a soil), and stability (the length of time organic C is retained in a soil). These mechanisms are therefore attractive targets for soil C sequestration. In this study, we investigate and compare SOC storage and stability of SOC associated with fine minerals and stored within aggregates using a combination of particle-size fractionation, elemental analysis and radiocarbon dating. In this heavy-textured, highly aggregated soil, SOC was found to be preferentially associated with fine minerals throughout the soil profile. By contrast, the oldest SOC was located in the coarsest, most highly aggregated fraction. In the topsoil, radiocarbon ages of the aggregate-associated SOC indicate retention times in the order of centuries. Below the topsoil, retention times of aggregate-SOC are in the order of millennia. Throughout the soil profile, radiocarbon dates indicate an enhanced stability in the order of centuries compared with the fine mineral fraction. Despite this, the radiocarbon ages of the mineral-associated SOC were in the order of centuries to millennia in the subsoil (30–100 cm), indicating that mineral-association is also an effective stabilisation mechanism in this subsoil. Our results indicate that enhanced SOC storage does not equate to enhanced SOC stability, which is an important consideration for sequestration schemes targeting both the amount and longevity of soil carbon. © 2014 CSIRO Publishing
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    Vertical distribution of charcoal in a sandy soil: evidence from DRIFT spectra and field emission scanning electron microscopy
    (Wiley Online Library, 2014-09-12) Hobley, E; Willgoose, GR; Frisia, S; Jacobsen, GE
    This study uses diffuse reflectance infrared Fourier Transform (DRIFT) spectrometry and field emission scanning electron microscopy to investigate the vertical distribution of charcoal in a sandy soil from SE Australia. The soil was sampled to bedrock (120 cm) at varying depths and bulk samples were fractionated into three particle-sizes: macro- (2000–200 µm), micro- (200–60 µm) and mineral-associated organic matter (MAOM, < 60 µm). Charcoal was isolated from 0–30 and 30–60-cm depths. Soil charcoal was detected by using a DRIFT band centred at 1590 cm−1 and scanning electron microscopy combined with energy dispersive spectroscopy. Charcoal content as a proportion of soil organic carbon (SOC) was estimated with linear regressions of cumulative DRIFT bands. At 0–30 cm, charcoal content as a portion of SOC did not differ significantly between particle-size fractions, constituting 5–26% of SOC. At a depth of 30–60 cm, charcoal constituted 19–39% of SOC in the fractions. At 60–100 cm, charcoal was only detectable in the mid-sized fraction, where it constituted about 17% of SOC. These results support our previous hypothesis of charcoal enrichment in the micro-fraction inducing a greater SOC stability in this fraction as inferred from radiocarbon ages (Hobley et al., 2013). Our findings indicate that DRIFT spectra can be used to detect the presence and amount of charcoal in soil, which may prove to be a simple and low-cost alternative to more laborious and costly detection methods.© 2014, British Society of Soil Science.