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This study describes iron (Fe) pipestems formed around and within root channels during the development of a homogeneous unripe sulfidic sediment into a ripe soil where redox‐ or pH‐active elements segregate. Pipestems and adjacent soil material samples were characterised by chemical and mineralogical analyses and scanning electron microscopy (SEM) associated with energy dispersive spectroscopy (EDS). Pipestems occurred at 60–190 cm and at 30–130 cm in a young Finnish soil and a more mature Australian soil, respectively, while sulfidic materials were found below 170–180 cm. The pipestems consisted of mineral grains cemented together by Fe precipitate. There was an up to 24‐fold enrichment of Fe and an up to 13‐fold enrichment of sulfur (S) in the Finnish pipestems compared to the adjacent soil material, whereas the corresponding enrichment in the Australian soil was up to 27‐fold for Fe and up to 8‐fold for S. In the Australian pipestem matrices, the Fe concentration was as high as 40% compared to 14% in the Finnish ones. It was estimated that about 70 ton ha−1 S had been mobilised from the sulfidic material in the Finnish soil at the depth of 50–150 cm. Part of S has leached out but a substantial amount remained in the soil constituting the stock of retained acidity. Almost all pipestems contained a new solid phase precipitated within the former cortex cells of the plant roots. In the Finnish samples, this precipitate consisted of jarosite and schwertmannite. After oxidative exhaustion of sulfidic material in the surrounding soil, these metastable minerals are gradually hydrolysed, associated with leaching of S. In the mature Australian soil, most of these minerals had already been transformed to goethite. Pipestems formed after roots in unripe soil are sites for synthesis and hydrolysis of minerals and serve as routes for atmospheric oxygen into the reduced subsoil and for soluble reaction products to exit the soil. Pipestems have an important role in the ripening of the soil profile. [ABSTRACT FROM AUTHOR]

Accumulation of sulfidic sediments in freshwater environments is a relatively recent phenomenon and an emerging environmental issue. In the present study, benthic sediments along short (~250 m) reaches of an inland freshwater river in south-eastern Australia were examined to determine the abundance and vertical distribution of fine- grained organic sulfidic sediments, identified by acid volatile sulfide (AVS) and chromium-reducible sulfur (SCR)1 contents. Sulfidic sediments (up to 404 mmol kg-1 SCR) preferentially accumulated in zones immediately overlying coarse sandy bed material. Conversely, where bed material was clay or silt dominated, comparatively limited sulfidic sediment had accumulated (where AVS and SCR were not detected). This suggests that the hydraulic conductivity of the underlying bed material could play a role in the formation of sulfidic sediments and that the overlying water column is not the sole source of SO42- . Evidence suggests that accumulation of sulfidic materials occurred preferentially downstream of channel obstructions, such as submerged logs or in scour pools. However, sediment accumulation was not limited to lower-energy parts of the channel, as would be expected for fine-grained organic sediments. Evidence of reworking, burial or sulfide formation at depth highlights the dynamism of the system and its differences to many coastal systems where these sediments are commonly found. [ABSTRACT FROM AUTHOR]

Phytolith carbon (C) sequestration has been identified to be a relatively stable and efficacious mechanism to securely reduce atmospheric carbon dioxide (CO2) concentration. As a fast-growing and Si-accumulating plant, moso bamboo (Phyllostachys pubescens) can produce large quantities of phytolith-occluded carbon (PhytOC). In this study, we sampled the mature leaves, leaf litter and soils from moso bamboo stands located in Qingshan (30°13′N, 119°47′E; Lin'an, China) to study topographic effects on phytolith and PhytOC production in moso bamboo ecosystems. Our results showed that the phytolith contents of mature leaves and leaf litter in moso bamboo stand decreased with the slope steepness as follows: 2–5° > 5–15° > 15–35°, and showed an increasing tendency with the slope position as follows: lower slope < middle slope < upper slope. The PhytOC production flux also showed the same trend as that for phytoliths and ranged from 10.32 to 16.16 kg CO2ha−1year−1. Our data indicate that topography, especially slope position and slope steepness, should be considered as some key factors to enhance bamboo phytolith C sequestration during future bamboo afforestation and reforestation. [ABSTRACT FROM PUBLISHER]

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Controversy surrounds the green credentials of hydroelectricity because of the potentially large emission of greenhouse gases (GHG) from associated reservoirs. However, limited and patchy data particularly for China is constraining the current global assessment of GHG releases from hydroelectric reservoirs. This study provides the first evaluation of the CO and CH emissions from China's hydroelectric reservoirs by considering the reservoir water surface and drawdown areas, and downstream sources (including spillways and turbines, as well as river downstream). The total emission of 29.6 Tg CO/year and 0.47 Tg CH/year from hydroelectric reservoirs in China, expressed as CO equivalents (eq), corresponds to 45.6 Tg COeq/year, which is 2-fold higher than the current GHG emission (ca. 23 Tg COeq/year) from global temperate hydropower reservoirs. China's average emission of 70 g COeq/kWh from hydropower amounts to 7 % of the emissions from coal-fired plant alternatives. China's hydroelectric reservoirs thus currently mitigate GHG emission when compared to the main alternative source of electricity with potentially far great reductions in GHG emissions and benefits possible through relatively minor changes to reservoir management and design. On average, the sum of drawdown and downstream emission including river reaches below dams and turbines, which is overlooked by most studies, represents the equivalent of 42 % of the CO and 92 % of CH that emit from hydroelectric reservoirs in China. Main drivers on GHG emission rates are summarized and highlight that water depth and stratification control CH flux, and CO flux shows significant negative relationships with pH, DO, and Chl-a. Based on our finding, a substantial revision of the global carbon emissions from hydroelectric reservoirs is warranted. [ABSTRACT FROM AUTHOR]

Background and aims: Phytolith occluded carbon (PhytOC) is of interest for isotope studies, dating of sediments and the capture and storage of carbon. Many methodologies have been used for the isolation of phytoliths from plant material; however, there are wide disparities in the PhytOC contents when determined by different methodologies. In this study we examine the utility of the two main methods used for quantifying PhytOC. Methods: These methods are: (1) a microwave digestion followed by a Walkley-Black digestion, and (2) HSO/HO. Results: Method (1) produced PhytOC values over 50 times higher than those acquired by method (2). SEM examination indicated that the differences were likely due to shattering of the phytoliths by method (2) allowing consumption by the acid and peroxide of PhytOC . Conclusion: These results indicate that for the samples analysed here: 1] the modified microwave method allowed the total PhytOC to be measured, 2] the HSO/HO method allowed the PhytOC within the tightly packed silica matrix to be measured, and 3] the PhytOC retained within the phytolith cavities could possibly be calculated by subtracting 2] from 1]. For the samples analysed here most of the PhytOC resided in the phytolith cavities. [ABSTRACT FROM AUTHOR]

Phytolith Occluded Carbon (PhytOC) has recently been demonstrated to be an important long-term terrestrial carbon fraction. The aim of this study was to examine the rates of silica accumulation and carbon bio-sequestered within the silica phytoliths of the leaf and stem material of wheat (Triticum sp.) cultivars. The phytolith content of 53 wheat cultivars sourced from 25 countries around the world and grown on a single trial site was first isolated and the PhytOC content then determined. The data shows that the phytolith occluded carbon content of the wheat cultivars ranged from 0.06% to 0.60% of dry leaf and stem biomass: a range of 1,000%. The data also demonstrates that it is the efficiency by which carbon is encapsulated within silica rather than the quantity of silica accumulated by the plant that is the most important factor in determining the relative PhytOC yields. The potential phytolith carbon bio-sequestration rates in the leaf and stem components of these wheat cultivars ranged up to 0.246 t-e-CO hay. These phytolith carbon bio-sequestration rates indicate a substantial potential (~50 million t-e-CO y) exists for increasing the rate of secure carbon bio-sequestration in wheat crops using existing cultivars. [ABSTRACT FROM AUTHOR]

Solid phase Fe and S fractions were examined in an acid sulfate soil (ASS) wetland undergoing remediation via tidal inundation. Considerable diagenetic enrichment of reactive Fe(III) oxides (HCl- and dithionite-extractable) occurred near the soil surface (0-0.05 m depth), where extremely large concentrations up to 3534 μmol/g accounted for ~90% of the total Fe pool. This major source of reactive Fe exerts a substantial influence on S cycling and the formation, speciation and transformation of reduced inorganic S (RIS) in tidally inundated ASS. Under these geochemical conditions, acid volatile sulfide (AVS; up to 57 μmol/g) and elemental sulfur (S; up to 41 μmol/g) were the dominant fractions of RIS in near surface soils. AVS-S to pyrite-S ratios exceeded 2.9 near the surface, indicating that abundant reactive Fe favoured the accumulation of AVS minerals and S over pyrite. This is supported by the significant correlation of poorly crystalline Fe with AVS-S and S-S contents ( r = 0.83 and r = 0.85, respectively, P < 0.01). XANES spectroscopy provided direct evidence for the presence of a greigite-like phase in AVS-S measured by chemical extraction. While the abundant reactive Fe may limit the transformation of AVS minerals and S to pyrite during early diagenesis (~5 years), continued sulfidisation over longer time scales is likely to eventually lead to enhanced sequestration of S within pyrite (with a predicted 8% pyrite by mass). These findings provide an important understanding of sulfidisation processes occurring in reactive Fe-enriched, tidally inundated ASS landscapes. [ABSTRACT FROM AUTHOR]

The rates of carbon bio-sequestration within silica phytoliths of the leaf litter of 10 economically important bamboo species indicates that (a) there is considerable variation in the content of carbon occluded within the phytoliths (PhytOC) of the leaves between different bamboo species, (b) this variation does not appear to be directly related to the quantity of silica in the plant but rather the efficiency of carbon encapsulation by the silica. The PhytOC content of the species under the experimental conditions ranged from 1.6% to 4% of the leaf silica weight. The potential phytolith carbon bio-sequestration rates in the leaf-litter component for the bamboos ranged up to 0.7 tonnes of carbon dioxide (CO2) equivalents (t-e-CO2) ha−1 yr−1 for these species. Assuming a median phytolith carbon bio-sequestration yield of 0.36 t-e-CO2 ha−1 yr−1, the global potential for bio-sequestration via phytolith carbon (from bamboo and/or other similar grass crops) is estimated to be ∼1.5 billion t-e-CO2 yr−1, equivalent to 11% of the current increase in atmospheric CO2. The data indicate that the management of vegetation such as bamboo forests to maximize the production of PhytOC has the potential to result in considerable quantities of securely bio-sequestered carbon. [ABSTRACT FROM AUTHOR]

The potential to reduce emissions from agriculture and increase the amount of carbon captured in soils is currently being examined by researchers in a number of countries. This paper describes a process of carbon capture and long-term storage using silica phytoliths and, provides the results of a study of this process on newly planted and ratooned sugarcane varieties. Our results indicate that a) there was significant variation in the phytolith occluded carbon (PhytOC) content of different varieties, b) this did not appear to be directly related to the quantity of silica in the plant but rather the efficiency of carbon encapsulation by individual varieties and c) it was possible to accurately quantify this carbon fraction prior to its incorporation into soil. The carbon content of the varieties tested under the particular suite of environmental conditions for which they were grown ranged from 0.12 te-CO2 ha-y−1 to 0.36 te-CO2 ha-y−1. This PhytOC process provides an approach which reduces emissions from agriculture for the long-term (millennia), as opposed to many other soil organic carbon fractions that may decompose over a much shorter time. Moreover, the ability to quantify PhytOC prior to its incorporation into the soil will provide a distinct practical advantage for the quantification of this carbon form over other soil carbon fractions in emerging emissions trading and offset markets. [ABSTRACT FROM AUTHOR]

Grain-size distribution is a fundamental tool for interpreting sedimentary units within depositional systems. The techniques assessed in this study are commonly used to determine grain-size distributions for sand-dominated sediments. However, the degree of consistency and differences in interpretation when using a combination of grain-size methods have not yet been assessed systematically for sand-dominated fluvial sediments. Results obtained from laser diffraction, X-ray attenuation and scanning electron microscopy grain-size analysis techniques were compared with those obtained from the traditional sieve/hydrometer method. Scanning electron microscopy was shown to provide an inaccurate quantitative analysis of grain-size distributions because of difficulties in obtaining representative samples for examination. The X-ray attenuation method is unsuitable for sand-dominated sediments because of its upper size range of only 300 μm. The consistently strong correlation between the laser diffraction results and the sieve/hydrometer results shows that these methods are comparable for sand-dominated fluvial sediments. Provided that sample preparation is consistent, the latter two methods can be used together within a study of such sediments while maintaining a high degree of accuracy. These results indicate that data for sand-dominated fluvial sediments gained from the long-established sieve/hydrometer method can be compared with confidence to those obtained by modern studies using laser diffraction techniques. [ABSTRACT FROM AUTHOR]

Although iron stains are a commonly described feature of marine sandgrains, the micromorphology and mineralogy of these features has notbeen determined previously. Light microscopy, scanning electron microscopy and X-ray diffractometry were used in this study to examine the grain surface features responsible for the iron staining of quartzitic beach sand along the coastline of southeastern Australia. These iron stains consist of iron oxide enriched phyllosilicate clay coatings. Iron oxides coating directly onto the quartz grains were not observed indicating that the phyllosilicate clay coatings are a co-requisite for iron staining. The main iron oxide mineral present in these features was goethite with minor amounts of hematite. The phyllosilicate clays in these coatings are kaolinite and illite. These platy claysare embedded into the quartz sand and are usually oriented with their flat surfaces perpendicular to that of the grain surface. [ABSTRACT FROM AUTHOR]