Uranium Repartitioning during Microbial Driven Reductive Transformation of U(VI)-Sorbed Schwertmannite and Jarosite.

This study exposes U(VI)-sorbed schwertmannite and jarosite to biotic reductive incubations under field-relevant conditions and examines the changes in aqueous and solid-phase speciation of U, Fe, and S as well as associated microbial communities over 180 days. The chemical, X-ray absorption spectroscopy, X-ray diffraction, and microscopic data demonstrated that the U(VI)-sorbed schwertmannite underwent a rapid reductive dissolution and solid-phase transformation to goethite, during which the surface-sorbed U(VI) was partly reduced and mostly repartitioned to monomeric U(VI)/U(IV) complexes by carboxyl and phosphoryl ligands on biomass or organic substances. Furthermore, the microbial data suggest that these processes were likely driven by the consecutive developments of fermentative and sulfate- and iron- reducing microbial communities. In contrast, the U(VI)-sorbed jarosite only stimulated the growth of some fermentative communities and underwent very limited reductive dissolution and thus, remaining in its initial state with no detectable mineralogical transformation and solid-phase U reduction/repartitioning. Accordingly, these two biotic incubations did not induce increased risk of U reliberation to the aqueous phase. These findings have important implications for understanding the interactions of schwertmannite/jarosite with microbial communities and colinked behavior and fate of U following the establishment of reducing conditions in various acidic and U-rich settings.

Storage and Distribution of Organic Carbon and Nutrients in Acidic Soils Developed on Sulfidic Sediments: The Roles of Reactive Iron and Macropores.

In a boreal acidic sulfate-rich subsoil (pH 3-4) developing on sulfidic and organic-rich sediments over the past 70 years, extensive brownish-to-yellowish layers have formed on macropores. Our data reveal that these layers ("macropore surfaces") are strongly enriched in 1 M HCl-extractable reactive iron (2-7% dry weight), largely bound to schwertmannite and 2-line ferrihydrite. These reactive iron phases trap large pools of labile organic matter (OM) and HCl-extractable phosphorus, possibly derived from the cultivated layer. Within soil aggregates, the OM is of a different nature from that on the macropore surfaces but similar to that in the underlying sulfidic sediments (C-horizon). This provides evidence that the sedimentary OM in the bulk subsoil has been largely preserved without significant decomposition and/or fractionation, likely due to physiochemical stabilization by the reactive iron phases that also existed abundantly within the aggregates. These findings not only highlight the important yet underappreciated roles of iron oxyhydroxysulfates in OM/nutrient storage and distribution in acidic sulfate-rich and other similar environments but also suggest that boreal acidic sulfate-rich subsoils and other similar soil systems (existing widely on coastal plains worldwide and being increasingly formed in thawing permafrost) may act as global sinks for OM and nutrients in the short run.

Microbial Necromass, Lignin, and Glycoproteins for Determining and Optimizing Blue Carbon Formation.

Coastal wetlands contribute to the mitigation of climate change through the sequestration of "blue carbon". Microbial necromass, lignin, and glycoproteins (i.e., glomalin-related soil proteins (GRSP)), as important components of soil organic carbon (SOC), are sensitive to environmental change. However, their contributions to blue carbon formation and the underlying factors remain largely unresolved. To address this paucity of knowledge, we investigated their contributions to blue carbon formation along a salinity gradient in coastal marshes. Our results revealed decreasing contributions of microbial necromass and lignin to blue carbon as the salinity increased, while GRSP showed an opposite trend. Using random forest models, we showed that their contributions to SOC were dependent on microbial biomass and resource stoichiometry. In N-limited saline soils, contributions of microbial necromass to SOC decreased due to increased N-acquisition enzyme activity. Decreases in lignin contributions were linked to reduced mineral protection offered by short-range-ordered Fe (FeSRO). Partial least-squares path modeling (PLS-PM) further indicated that GRSP could increase microbial necromass and lignin formation by enhancing mineral protection. Our findings have implications for improving the accumulation of refractory and mineral-bound organic matter in coastal wetlands, considering the current scenario of heightened nutrient discharge and sea-level rise.

High potential of stable carbon sequestration in phytoliths of China's grasslands.

Phytolith carbon (C) sequestration plays a key role in mitigating global climate change at a centennial to millennial time scale. However, previous estimates of phytolith-occluded carbon (PhytOC) storage and potential in China's grasslands have large uncertainties mainly due to multiple data sources. This contributes to the uncertainty in predicting long-term C sequestration in terrestrial ecosystems using Earth System Models. In this study, we carried out an intensive field investigation (79 sites, 237 soil profiles [0-100 cm], and 61 vegetation assessments) to quantify PhytOC storage in China's grasslands and to better explore the biogeographical patterns and influencing factors. Generally, PhytOC production flux and soil PhytOC density in both the Tibetan Plateau and the Inner Mongolian Plateau had a decreasing trend from the Northeast to the Southwest. The aboveground PhytOC production rate in China's grassland was 0.48 × 106 t CO2 a-1 , and the soil PhytOC storage was 383 × 106 t CO2 . About 45% of soil PhytOC was stored in the deep soil layers (50-100 cm), highlighting the importance of deep soil layers for C stock assessments. Importantly, the Tibetan Plateau had the greatest contribution (more than 70%) to the PhytOC storage in China's grasslands. The results of multiple regression analysis indicated that altitude and soil texture significantly influenced the spatial distribution of soil PhytOC, explaining 78.1% of the total variation. Soil phytolith turnover time in China's grasslands was mainly controlled by climatic conditions, with the turnover time on the Tibetan Plateau being significantly longer than that on the Inner Mongolian Plateau. Our results offer more accurate estimates of the potential for phytolith C sequestration from ecological restoration projects in degraded grassland ecosystems. These estimates are essential to parameterizing and validating global C models.

Storage, patterns and influencing factors for soil organic carbon in coastal wetlands of China.

Soil organic carbon (SOC) in coastal wetlands, also known as "blue C," is an essential component of the global C cycles. To gain a detailed insight into blue C storage and controlling factors, we studied 142 sites across ca. 5000 km of coastal wetlands, covering temperate, subtropical, and tropical climates in China. The wetlands represented six vegetation types (Phragmites australis, mixed of P. australis and Suaeda, single Suaeda, Spartina alterniflora, mangrove [Kandelia obovata and Avicennia marina], tidal flat) and three vegetation types invaded by S. alterniflora (P. australis, K. obovata, A. marina). Our results revealed large spatial heterogeneity in SOC density of the top 1-m ranging 40-200 Mg C ha-1 , with higher values in mid-latitude regions (25-30° N) compared with those in both low- (20°N) and high-latitude (38-40°N) regions. Vegetation type influenced SOC density, with P. australis and S. alterniflora having the largest SOC density, followed by mangrove, mixed P. australis and Suaeda, single Suaeda and tidal flat. SOC density increased by 6.25 Mg ha-1 following S. alterniflora invasion into P. australis community but decreased by 28.56 and 8.17 Mg ha-1 following invasion into K. obovata and A. marina communities. Based on field measurements and published literature, we calculated a total inventory of 57 × 106 Mg C in the top 1-m soil across China's coastal wetlands. Edaphic variables controlled SOC content, with soil chemical properties explaining the largest variance in SOC content. Climate did not control SOC content but had a strong interactive effect with edaphic variables. Plant biomass and quality traits were a minor contributor in regulating SOC content, highlighting the importance of quantity and quality of OC inputs and the balance between production and degradation within the coastal wetlands. These findings provide new insights into blue C stabilization mechanisms and sequestration capacity in coastal wetlands.

Distribution, sources, and decomposition of soil organic matter along a salinity gradient in estuarine wetlands characterized by C:N ratio, δ13 C-δ15 N, and lignin biomarker.

Despite increasing recognition of the critical role of coastal wetlands in mitigating climate change, sea-level rise, and salinity increase, soil organic carbon (SOC) sequestration mechanisms in estuarine wetlands remain poorly understood. Here, we present new results on the source, decomposition, and storage of SOC in estuarine wetlands with four vegetation types, including single Phragmites australis (P, habitat I), a mixture of P. australis and Suaeda salsa (P + S, habitat II), single S. salsa (S, habitat III), and tidal flat (TF, habitat IV) across a salinity gradient. Values of δ13 C increased with depth in aerobic soil layers (0-40 cm) but slightly decreased in anaerobic soil layers (40-100 cm). The δ15 N was significantly enriched in soil organic matter at all depths than in the living plant tissues, indicating a preferential decomposition of 14 N-enriched organic components. Thus, the kinetic isotope fractionation during microbial degradation and the preferential substrate utilization are the dominant mechanisms in regulating isotopic compositions in aerobic and anaerobic conditions, respectively. Stable isotopic (δ13 C and δ15 N), elemental (C and N), and lignin composition (inherited (Ad/Al)s and C/V) were not completely consistent in reflecting the differences in SOC decomposition or accumulation among four vegetation types, possibly due to differences in litter inputs, root distributions, substrate quality, water-table level, salinity, and microbial community composition/activity. Organic C contents and storage decreased from upstream to downstream, likely due to primarily changes in autochthonous sources (e.g., decreased onsite plant biomass input) and allochthonous materials (e.g., decreased fluvially transported upland river inputs, and increased tidally induced marine algae and phytoplankton). Our results revealed that multiple indicators are essential to unravel the degree of SOC decomposition and accumulation, and a combination of C:N ratios, δ13 C, δ15 N, and lignin biomarker provides a robust approach to decipher the decomposition and source of sedimentary organic matter along the river-estuary-ocean continuum.

A Combined X-ray Absorption and Mössbauer Spectroscopy Study on Fe Valence and Secondary Mineralogy in Granitoid Fracture Networks: Implications for Geological Disposal of Spent Nuclear Fuels.

Underground repository in crystalline bedrock is a widely accepted solution for long-term disposal of spent nuclear fuels. During future deglaciations, meltwater will intrude via bedrock fractures to the depths of future repositories where O2 left in the meltwater could corrode metal canisters and enhance the migration of redox-sensitive radionuclides. Since glacial meltwater is poor in reduced phases, the quantity and (bio)accessibility of minerogenic Fe(II) in bedrock fractures determine to what extent O2 in future meltwater can be consumed. Here, we determined Fe valence and mineralogy in secondary mineral assemblages sampled throughout the upper kilometer of fractured crystalline bedrock at two sites on the Baltic Shield, using X-ray absorption and Mössbauer spectroscopic techniques that were found to deliver matching results. The data point to extensive O2-consuming capacity of the bedrock fractures, because Fe(II)-rich phyllosilicates were abundant and secondary pyrite was dispersed deep into the bedrock with no overall increase in Fe(II) concentrations and Fe(II)/Fe(III) proportions with depth. The results imply that repeated Pleistocene deglaciations did not cause a measurable decrease in the Fe(II) pool. In surficial fractures, largely opened during glacial unloading, ferrihydrite and illite have formed abundantly via oxidative transformation of Fe(II)-rich phyllosilicates and recently exposed primary biotite/hornblende.

Organic matter composition and stability in estuarine wetlands depending on soil salinity.

Coastal wetlands are key players in mitigating global climate change by sequestering soil organic matter. Soil organic matter consists of less stable particulate organic matter (POM) and more stable mineral-associated organic matter (MAOM). The distribution and drivers of MAOM and POM in coastal wetlands have received little attention, despite the processes and mechanisms differ from that in the upland soils. We explored the distribution of POM and MAOM, their contributions to SOM, and the controlling factors along a salinity gradient in an estuarine wetland. In the estuarine wetland, POM C and N were influenced by soil depth and vegetation type, whereas MAOM C and N were influenced only by vegetation type. In the estuarine wetland, SOM was predominantly in the form of MAOM (> 70 %) and increased with salinity (70 %-76 %), leading to long-term C sequestration. Both POM and MAOM increased with SOM, and the increase rate of POM was higher than that of MAOM. Aboveground plant biomass decreased with increasing salinity, resulted in a decrease in POM C (46 %-81 %) and N (52 %-82 %) pools. As the mineral amount and activity, and microbial biomass decreased, the MAOM C (2.5 %-64 %) and N pool (8.6 %-59 %) decreased with salinity. When evaluating POM, the most influential factors were microbial biomass carbon (MBC) and dissolved organic carbon (DOC). Key parameters, including MBC, DOC, soil salinity, soil water content, aboveground plant biomass, mineral content and activity, and bulk density, were identified as influencing factors for both MAOM abundance. Soil water content not only directly controlled MAOM, but together with salinity also indirectly regulated POM and MAOM by controlling microbial biomass and aboveground plant biomass. Our findings have important implications for improving the accumulation and increased stability of soil organic matter in coastal wetlands, considering the global sea level rise and increased frequency of inundation.

Iron­sulfur geochemistry and acidity retention in hydrologically active macropores of boreal acid sulfate soils: Effects of mitigation suspensions of fine-grained calcite and peat.

Acid sulfate soils discharge large amounts of sulfuric acid along with toxic metals, deteriorating water quality and ecosystem health of recipient waterbodies. There is thus an urgent need to develop cost-effective and sustainable measures to mitigate the negative effects of these soils. In this study, we flushed aseptically-prepared MQ water (reference) or mitigation suspensions containing calcite, peat or a combination of both through 15-cm-thick soil cores from an acid sulfate soil field in western Finland, and investigated the geochemistry of Fe and S on the surfaces of macropores and in the solid columnar blocks (interiors) of the soil columns. The macropore surfaces of all soil columns were strongly enriched in total and HCl-extractable Fe and S relative to the interiors, owing to the existence of abundant Fe oxyhydroxysulfates (schwertmannite and partly jarosite) as yellow-to-brownish surface-coatings. The dissolution/hydrolysis of Fe oxyhydroxysulfates (predominantly jarosite) on the macropore surfaces of the reference columns, although being constantly flushed, effectively buffered the permeates at pH close to 4. These results suggest that Fe oxyhydroxysulfates accumulated on the macropore surfaces of boreal acid sulfate soils can act as long-lasting acidification sources. The treatments with mitigation suspensions led to a (near-)complete conversion of jarosite to Fe hydroxides, causing a substantial loss of S. In contrast, we did not observe any recognizable evidence indicating transformation of schwertmannite. However, sulfate sorbed by this mineral might be partially lost through anion-exchange processes during the treatments with calcite. No Fe sulfides were found in the peat-treated columns. Since Fe sulfides can support renewed acidification events, the moderate mineralogical changes induced by peat are desirable. In addition, peat materials can act as toxic-metal scavengers. Thus, the peat materials used here, which is relatively cheap in the boreal zone, is ideal for remediating boreal acid sulfate soils and other similar jarosite-bearing soils.

A nationwide acid sulfate soil study - A rapid and cost-efficient approach for characterizing large-scale features.

Acid sulfate soils are sulfide-rich soils that pose a notable environmental risk as their strong acidity and low pH mobilizes metals from soil minerals leading to both acidification and metal contamination of the surrounding environment. In this study a rapid and cost-efficient approach was developed to resolve the main distribution patterns and geochemical features of acid sulfate soils throughout coastal plains stretching for some 2000 km in eastern, southern, and western Sweden. Of the investigated 126 field sites, 47 % had acid sulfate soils including 33 % active, 12 % potential, and 2 % pseudo acid sulfate soils. There were large regional variations in the extent of acid sulfate soils, with overall much higher proportions of these soils along the eastern coastal plains facing the Baltic Sea than the western coastal plains facing the Kattegatt/Skagerrak (Atlantic Ocean). The sulfur concentrations of the soil's parent material, consisting of reduced near-pH neutral sediments, were correlated inversely both with the minimum pH of the soils in situ (rS = -0.65) and the pH after incubation (oxidation) of the reduced sediments (rS = -0.77). This indicated the importance of sulfide levels in terms of both present and potential future acidification. Hence, the higher proportion of acid sulfate soils in the east was largely the result of higher sulfur concentrations in this part of the country. The study showed that the approach was successful in identifying large-scale spatial patterns and geochemical characteristics of importance for environmental assessments related to these environmentally unfriendly soils.

Effect of weathering on abundance and release of potentially toxic elements in soils developed on Lower Cambrian black shales, P. R. China.

This paper examines the geochemical features of 8 soil profiles developed on metalliferous black shales distributed in the central parts of the South China black shale horizon. The concentrations of 21 trace elements and 8 major elements were determined using ICP-MS and XRF, respectively, and weathering intensity (W) was calculated according to a new technique recently proposed in the literature. The data showed that the black shale soils inherited a heterogeneous geochemical character from their parent materials. A partial least square regression model and EF(bedrock) (enrichment factor normalized to underlying bedrock) indicated that W was not a major control in the redistribution of trace metals. Barium, Sn, Cu, V, and U tended to be leached in the upper soil horizons and trapped by Al and Fe oxides, whereas Sb, Cd, and Mo with negative EF values across the whole profiles may have been leached out during the first stage of pedogenesis (mainly weathering of black shale). Compared with the Chinese average soils, the soils were strongly enriched in the potentially toxic metals Mo, Cd, Sb, Sn, U, V, Cu, and Ba, among which the 5 first listed were enriched to the highest degrees. Elevated concentrations of these toxic metals can have a long-term negative effect on human health, in particular, the soils in mining areas dominated by strongly acidic conditions. As a whole, the black shale soils have much in common with acid sulfate soils. Therefore, black shale soils together with acid sulfate soils deserve more attention in the context of metal exposure and human health.

Arsenic in the water and agricultural crop production system: Bangladesh perspectives.

The presence of high levels of carcinogenic metalloid arsenic (As) in the groundwater system of Bangladesh has been considered as one of the major environmental disasters in this region. Many parts of Bangladesh have extensively reported the presence of high levels of arsenic in the groundwater due to both geological and anthropogenic activities. In this paper, we reviewed the available literature and scientific information regarding arsenic pollution in Bangladesh, including arsenic chemistry and occurrences. Along with using As-rich groundwater as a drinking-water source, the agricultural activities and especially irrigation have greatly depended on the groundwater resources in this region due to high water demands for ensuring food security. A number of investigations in Bangladesh have shown that high arsenic content in both soil and groundwater may result in high levels of arsenic accumulation in different plants, including cereals and vegetables. This review provides information regarding arsenic accumulation in major rice varieties, soil-groundwater-rice arsenic interaction, and past arsenic policies and plans, as well as previously implemented arsenic mitigation options for both drinking and irrigation water systems in Bangladesh. In conclusion, this review highlights the importance and necessity for more in-depth studies as well as more effective arsenic mitigation action plans to reduce arsenic incorporation in the food chain of Bangladesh.

The response of metal mobilization and redistribution to reoxygenation in Baltic Sea anoxic sediments.

To bring life back to anoxic coastal and sea basins, reoxygenation of anoxic/hypoxic zones has been proposed. This research focuses on the metals released during the oxidization of sediments from two locations in the anoxic Eastern Gotland Basin under a laboratory-scale study. Triplicate experimental cores and reference cores were collected from the North and South Eastern Gotland Basins. The oxygenation of the water column took place over a 96-hour experiment in a dark and 5 °C environment. In 12 and 24 hour intervals, the surface waters were exchanged and, over time, analyzed for pH, electroconductivity (EC), total organic carbon (TOC), soluble metal concentrations, and the top samples (0-10 cm) were analyzed with 3-step (E1: water-soluble, E2: exchangeable, and E3: organic-bound) sequential chemical extraction (SCE). Results show stable pH and decreasing EC in the column waters. The EC indicates that metals are released in the initial phases (12 h) of reoxygenation for both sites. Arsenic, Ba, Co, Mn, Rb, U, K, Sr, and Mo are released into the water column during the 96 hour experiment, and based on the calculations for the entire East Gotland Basin, would mean 8, 50, 0.55, 734, 53, 27, 347,178, 3468, and 156 µg L-1 are released, respectively. Elements Mn, Mo, U, and As are released in higher concentrations during the experiment than previously measured in the Eastern Gotland Basin, which provides vital information for future proposed remediation and natural geochemical processes with their known environmental impacts. The SCE results show that redox-sensitive metals (Mn, U, and Mo) are released in the highest concentrations into the solution. The relationship between the highest released metals (beside redox-sensitive) into solution over the oxygenation and their initial abundant phase is noticed, where the smallest released concentrations belong to K < Rb < Sr in E2, and As

Silicon Affects Plant Stoichiometry and Accumulation of C, N, and P in Grasslands.

Silicon (Si) plays an important role in improving soil nutrient availability and plant carbon (C) accumulation and may therefore impact the biogeochemical cycles of C, nitrogen (N), and phosphorus (P) in terrestrial ecosystems profoundly. However, research on this process in grassland ecosystems is scarce, despite the fact that these ecosystems are one of the most significant accumulators of biogenic Si (BSi). In this study, we collected the aboveground parts of four widespread grasses and soil profile samples in northern China and assessed the correlations between Si concentrations and stoichiometry and accumulation of C, N, and P in grasses at the landscape scale. Our results showed that Si concentrations in plants were significantly negatively correlated (p < 0.01) with associated C concentrations. There was no significant correlation between Si and N concentrations. It is worth noting that since the Si concentration increased, the P concentration increased from less than 0.10% to more than 0.20% and therefore C:P and N:P ratios decreased concomitantly. Besides, the soil noncrystalline Si played more important role in C, N, and P accumulation than other environmental factors (e.g., MAT, MAP, and altitude). These findings indicate that Si may facilitate grasses in adjusting the utilization of nutrients (C, N, and P) and may particularly alleviate P deficiency in grasslands. We conclude that Si positively alters the concentrations and accumulation of C, N, and P likely resulting in the variation of ecological stoichiometry in both vegetation and litter decomposition in soils. This study further suggests that the physiological function of Si is an important but overlooked factor in influencing biogeochemical cycles of C and P in grassland ecosystems.

Silicon Effects on Biomass Carbon and Phytolith-Occluded Carbon in Grasslands Under High-Salinity Conditions.

Changes in climate and land use are causing grasslands to suffer increasingly from abiotic stresses, including soil salinization. Silicon (Si) amendment has been frequently proposed to improve plant resistance to multiple biotic and abiotic stresses and increase ecosystem productivity while controlling the biogeochemical carbon (C) cycle. However, the effects of Si on plant C distribution and accumulation in salt-suffering grasslands are still unclear. In this study, we investigated how salt ions affected major elemental composition in plants and whether Si enhanced biomass C accumulation in grassland species in situ. In samples from the margins of salt lakes, our results showed that the differing distance away from the shore resulted in distinctive phytocoenosis, including halophytes and moderately salt-tolerant grasses, which are closely related to changing soil properties. Different salinity (Na+/K+, ranging from 0.02 to 11.8) in plants caused negative effects on plant C content that decreased from 53.9 to 29.2% with the increase in salinity. Plant Si storage [0.02-2.29 g Si m-2 dry weight (dw)] and plant Si content (0.53 to 2.58%) were positively correlated with bioavailable Si in soils (ranging from 94.4 to 192 mg kg-1). Although C contents in plants and phytoliths were negatively correlated with plant Si content, biomass C accumulation (1.90-83.5 g C m-2 dw) increased due to the increase of Si storage in plants. Plant phytolith-occluded carbon (PhytOC) increased from 0.07 to 0.28‰ of dry mass with the increase of Si content in moderately salt-tolerant grasses. This study demonstrates the potential of Si in mediating plant salinity and C assimilation, providing a reference for potential manipulation of long-term C sequestration via PhytOC production and biomass C accumulation in Si-accumulator dominated grasslands.

Holocene carbon accumulation in lakes of the current east Asian monsoonal margin: Implications under a changing climate.

Carbon (C) present in lake sediments is an important global sink for CO2; however, an in-depth understanding of the impact of climate variability and the associated changes in vegetation on sediment C dynamics is still lacking. A total of 13 lakes were studied to quantify the influence of climate and vegetation on the reconstructed Holocene C accumulation rate (CAR) in lake sediments of the modern East Asian monsoonal margin. The corresponding paleoclimate information was assessed, including the temperature (30-90°N in the Northern Hemisphere) and precipitation (indicated by the δ18O of the Sanbao, Dongge, and Hulu caves). The Holocene vegetation conditions were inferred by pollen records, including arboreal pollen/non-arboreal pollen and pollen percentages. The results showed that the peak CAR occurred during the mid-Holocene, coinciding with the strongest period of the East Asian summer monsoon and expansion of forests. Lakes in the temperate steppe (TS) regions had a mean CAR of 13.41 ± 0.88 g C m-2 yr-1, which was significantly greater than the CARs of temperate desert (TD) and highland meadow/steppe (HMS; 6.76 ± 0.29 and 7.39 ± 0.73 g C m-2 yr-1, respectively). The major influencing factor for the TS sub-region was vegetation dynamics, especially the proportion of arboreal vegetation, while temperature and vegetation coverage were more important for the HMS. These findings indicate that C accumulation in lake sediments is linked with climate and vegetation changes over long timescales; however, there was notable spatial heterogeneity in the CARs, such as opposing temporal changes and different major influencing factors among the three sub-regions during the mid-Holocene. Aridification and forest loss would decrease C storage. However, prediction of C accumulation remains difficult because of the spatial heterogeneity in CARs and the interaction between the CAR and various factors under future climate change conditions.

Geochemical controls on dispersion of U and Th in Quaternary deposits, stream water, and aquatic plants in an area with a granite pluton.

The weathering of U and/or Th rich granite plutons, which occurs worldwide, may serve as a potentially important, but as yet poorly defined source for U and Th in (sub-)surface environments. Here, we assessed the impact of an outcrop of such granite (5 km in diameter) and its erosional products on the distribution of U and Th in four nemo-boreal catchments. The results showed that (i) the pluton was enriched in both U and Th; and (ii) secondary U and Th phases were accumulated by peat/gyttja and in other Quaternary deposits with high contents of organic matter. Movement of the ice sheet during the latest glaciation led to dispersal of U- and Th-rich materials eroded from the pluton, resulting in a progressive increase in dissolved U and Th concentrations, as well as U concentrations in aquatic plants with increasing proximity to the pluton. The accumulation of U in the aquatic plants growing upon the pluton (100-365 mg kg-1, dry ash weight) shows that this rock represents a long-term risk for adjacent ecosystems. Dissolved pools of U and Th were correlated with those of dissolved organic matter (DOM) and were predicted to largely occur as organic complexes. This demonstrates the importance of DOM in the transport of U and Th in the catchments. Large fractions of Ca2UO2(CO3)30(aq) were modeled to occur in the stream with highest pH and alkalinity and thus, explain the strongly elevated U concentrations and fluxes in this particular stream. In future climate scenarios, boreal catchments will experience intensified runoff and warmer temperature that favor the production of hydrologically accessible DOM and alkalinity. Therefore, the results obtained from this study have implications for predicting the distribution and transport of Th and U in boreal catchments, especially those associated with U and/or Th rich granite plutons.

Impact of grassland degradation on the distribution and bioavailability of soil silicon: Implications for the Si cycle in grasslands.

Grassland ecosystems play an important role in the global terrestrial silicon (Si) cycle, and Si is a beneficial element and structural constituent for the growth of grasses. In previous decades, grasslands have been degraded to different degrees because of the drying climate and intense human disturbance. However, the impact of grassland degradation on the distribution and bioavailability of soil Si is largely unknown. Here, we investigated vegetation and soil conditions of 30 sites to characterize different degrees of degradation for grasslands in the agro-pastoral ecotone of northern China. We then explored the impact of grassland degradation on the distribution and bioavailability of soil Si, including total Si and four forms of noncrystalline Si in three horizons (0-10, 10-20 and 20-40 cm) of different soil profiles. The concentrations of noncrystalline Si in soil profiles significantly decreased with increasing degrees of degradation, being 7.35 ±â€¯0.88 mg g-1, 5.36 ±â€¯0.39 mg g-1, 3.81 ±â€¯0.37 mg g-1 and 3.60 ±â€¯0.26 mg g-1 in non-degraded, lightly degraded, moderately degraded and seriously degraded grasslands, respectively. Moreover, the storage of noncrystalline Si decreased from higher than 40 t ha-1 to lower than 23 t ha-1. The corresponding bioavailability of soil Si also generally decreased with grassland degradation. These processes may not only affect the Si pools and fluxes in soils but also influence the Si uptake in plants. We suggest that grassland degradation can significantly affect the global grassland Si cycle. Grassland management methods such as fertilizing and avoiding overgrazing can potentially double the content and storage of noncrystalline Si in soils, thereby enhancing the soil Si bioavailability by >17%.

Comparison of boreal acid sulfate soil microbial communities in oxidative and reductive environments.

Due to land uplift after the last ice age, previously stable Baltic Sea sulfidic sediments are becoming dry land. When these sediments are drained, the sulfide minerals are exposed to air and can release large amounts of metals and acid into the environment. This can cause severe ecological damage such as fish kills in rivers feeding the northern Baltic Sea. In this study, five sites were investigated for the occurrence of acid sulfate soils and their geochemistry and microbiology was identified. The pH and soil chemistry identified three of the areas as having classical acid sulfate soil characteristics and culture independent identification of 16S rRNA genes identified populations related to acidophilic bacteria capable of catalyzing sulfidic mineral dissolution, including species likely adapted to low temperature. These results were compared to an acid sulfate soil area that had been flooded for ten years and showed that the previously oxidized sulfidic materials had an increased pH compared to the unremediated oxidized layers. In addition, the microbiology of the flooded soil had changed such that alkalinity producing ferric and sulfate reducing reactions had likely occurred. This suggested that flooding of acid sulfate soils mitigates their environmental impact.

Arsenic removal from contaminated brackish sea water by sorption onto Al hydroxides and Fe phases mobilized by land-use.

This study examines the spatial and temporal distribution patterns of arsenic (As) in solid and aqueous materials along the mixing zone of an estuary, located in the south-eastern part of the Bothnian Bay and fed by a creek running through an acid sulfate (AS) soil landscape. The concentrations of As in solution form (<1 kDa) increase steadily from the creek mouth to the outer estuary, suggesting that inflowing seawater, rather than AS soil, is the major As source in the estuary. In sediments at the outer estuary, As was accumulated and diagenetically cycled in the surficial layers, as throughout much of the Bothnian Bay. In contrast, in sediments in the inner estuary, As concentrations and accumulation rates showed systematical peaks at greater depths. These peaks were overall consistent with the temporal trend of past As discharges from the Rönnskär smelter and the accompanied As concentrations in past sea-water of the Bothnian Bay, pointing to a connection between the historical smelter activities and the sediment-bound As in the inner estuary. However, the concentrations and accumulation rates of As peaked at depths where the smelter activities had already declined, but a large increase in the deposition of Al hydroxides and Fe phases occurred in response to intensified land-use in the mid 1960's and early 1970's. This correspondence suggests that, apart from the inflowing As-contaminated seawater, capture by Al hydroxides, Fe hydroxides and Fe-organic complexes is another important factor for As deposition in the inner estuary. After accumulating in the sediment, the solid-phase As was partly remobilized, as reflected by increased pore-water As concentrations, a process favored by As(V) reduction and high concentrations of dissolved organic matter.