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• Soil water content (SWC) is determined as the outcomes of land use changes. • A significant correlation occurred between δ13C and δ15N values in wetlands, but underwent considerable changes after wetland conversion. • The shifts in organic matter sources over time and distinct mechanisms of degradation and preservation of SOM were underscore in wetlands following land use changes. Land use change in wetlands leads to significant losses of soil organic matter (SOM). Stable carbon (C) and nitrogen (N) isotopes offer insights into changes in C3/C4 vegetation, SOM sources, and decomposition processes. Yet, predicting the spatial–temporal dynamics of SOM contents and isotopes under land use changes remains challenging. This study delves into the effects of land use changes on soil organic carbon (SOC), total nitrogen (TN), δ13C and δ15N values, and soil physico-chemical properties and lignin phenols. Our results highlight the significance of soil water content (SWC) in determining the outcomes of land use changes. The conversion of wetland to cropland, forestland and construction land, led to notable reductions in SOC contents (8.71–56.33 %), and TN contents (7.87–37.12 %). Wetland conversion resulted in an enrichment of 13C and 15N abundance, with wetlands exhibiting the lowest δ13C (−25.57 to –22.89 ‰) and δ15N (2.66 to 6.67 ‰) values. A significant correlation occurred between δ13C and δ15N values in wetlands, but underwent considerable changes after wetland conversion. Key parameters, including bulk density (BD), C:N, the acid-to-aldehyde of vanillyl ((Ad/Al)v), lignin content (Λ 8), and total phosphorus (TP), were identified as influencing factors for both SOC and TN contents. When evaluating δ13C values, the most influential factors included silt, C:N, SOC, sand, and BD. These indicate the importance of soil chemical group (from 41 % to 21 %) in elucidating δ13C values declined, while lignin group's (from 9 % to 28 %) importance increased from topsoil to subsoil. The acid-to-aldehyde of syringyl ((Ad/Al)s), Λ 8 , C:N, BD and the cinnamyl-to-vanillyl ratio (C/V) were identified as the primary factors influencing δ15N values, with chemical group accounting for 36 % and lignin group for 48 % in topsoil, while physical group dominated 42 % in subsoil. Our findings underscore the shifts in SOM sources and distinct mechanisms of degradation/preservation of SOM following land use changes. [ABSTRACT FROM AUTHOR]

Abstract Croplands are subjected to nutrient loss mainly due to agricultural harvest. Silicon has beneficial effect on alleviating nutrient imbalance-induced stress. Addition of crop residue biochars to cropland can import both silicon (Si) and nutrients (e.g. nitrogen, phosphorus and potassium) directly and enhance their availability. Nevertheless, how the concentrations of Si and nutrients vary among the biochars derived from different feedstocks, and how crop Si and nutrients respond to addition of biochars to croplands have not yet been clarified comprehensively and quantitatively. Total and essentially available Si and nutrients in crop residue biochars and their relationships with crop Si and nutrient uptake were investigated by using data collected from peer reviewed papers. Biochars derived from rice husk, rice straw, corn stover, sugarcane residues, and wheat straw, which were produced by thermal pyrolysis at 150–900 °C under oxygen-limited conditions, averagely contained 20.03% (n = 10), 12.39% (n = 16), 10.25% (n = 7), 7.40% (n = 9), and 3.34% (n = 3) of total Si, respectively. By contrast, crop residue biochars contained, on average, 1.23% nitrogen (n = 461), 0.32% phosphorus (n = 209), 0.56% sulfur (n = 187), 2.73% potassium (n = 197), 1.17% calcium (n = 123), and 0.54% magnesium (n = 111), which largely depended on and varied widely with their feedstocks and pyrolysis conditions. On average, 32.6%–54.9% of the total Si and nutrients (excluding nitrogen) in crop residue biochars were essentially available. Hence, addition of crop residue biochars to croplands may contribute a considerable amount of total and available Si and nutrients, except available inorganic nitrogen. The increasing amounts of Si and nutrient input with addition of biochars had positive and statistically significant (p < 0.05) relationships with the increment of crop Si and nutrient uptake, respectively. In conclusion, addition of crop residue biochars can be beneficial to sustainable agriculture system through concerting Si and nutrient cycling in croplands. Graphic abstract Figure: Silicon (Si)-rich and nutrient-rich crop residue biochars have the potential to enhance or stabilize yields via the increment of Si and nutrient uptake by crops, while enhancing systems' resilience to climate, plus resistance to unfavorable habitats. Unlabelled Image Highlights • Silicon (Si) and nutrients usually coexisted in crop residue biochars. • Crop residue biochars had higher Si and nutrient concentrations than the feedstocks. • Si and nutrient input via crop residue biochars improved crop Si and nutrient uptake. • Crop residue biochars can concert Si and nutrient cycles for sustainable agriculture. [ABSTRACT FROM AUTHOR]

Ecosystem water use efficiency (WUE) reflects the intimately coupled relationship between the carbon and water cycles in terrestrial ecosystems. However, the inter-annual variation and its drivers of WUE are poorly understood at regional/global scale, due to either limited data availability or uncertainties in current data streams. In this study, we used process-based models simulated gross primary productivity (GPP) and evapotranspiration (ET) data to estimate the ecosystem WUE (eWUE, GPP/ET) in China for 1979–2012. The eWUE estimates were validated against eddy covariance-based values from 35 flux towers. The inter-annual variation of the eWUE was quantified and its responses to annual precipitation (AP), annual mean temperature (AMT), and annual mean leaf area index (AMLAI) were analyzed. The key findings were as follows. (i) The mean annual eWUE over China was 1.48 ± 1.04 g C kg −1 H 2 O and had a slightly increasing but not significant trend (7.32 × 10 −4  g C kg −1 H 2 O yr −1 , p  < 0.05) from 1979 to 2012. (ii) The spatial distribution of the eWUE trend showed large spatial variability. ∼21.4% and ∼19.0% of vegetated land in China had significant increasing and decreasing trends (Mann-Kendall test, p  < 0.1), respectively. The increasing eWUE was mainly found in the northeast, southwest, and central areas of China, while the decreasing eWUE was mostly distributed in west China. (iii) The inter-annual variation of the spatially averaged annual eWUE was negatively correlated with that of AP and AMT, and positively correlated with that of AMLAI. In ∼41.4%, ∼9.9%, and ∼3.1% of vegetated land in China the inter-annual variation of eWUE was dominated by the inter-annual variations of AP, AMT, and AMLAI, respectively. In most land of north China and west China the inter-annual variation of eWUE was dominated by the inter-annual variation of AP, while in central, east and south China all the AP, AMT, AMLAI, and other drivers played important roles. [ABSTRACT FROM AUTHOR]

Abstract The terrestrial biogeochemical silicon (Si) cycle may significantly influence the stabilization of soil organic carbon (SOC), and thus plays an important role in regulating the global carbon (C) balance and climate change. Processes involved in Si-enhanced SOC stability at a decadal or centennial scale include protection of SOC through amorphous Si and interactions of Si-iron and Si-aluminum. Strategic manipulation of the Si cycle in terrestrial ecosystems offers a new opportunity to enhance soil C sequestration. Rock powder amendment, establishment of Si-rich plant buffer strips and innovative management practices that return Si-rich biomass materials to soil can be implemented as strategies to enhance soil C sequestration through regulating the terrestrial Si cycle. However, quantifying (i) the contribution of different processes to the enhancement of soil dissolved Si and secondary Si minerals, (ii) the relative importance of different SOC stabilization mechanisms, and (iii) the potential and cost of different measures has not been attempted. [ABSTRACT FROM AUTHOR]

[Display omitted] • SOC stocks of VCEs were estimated and projected at a 10 m spatial resolution. • Climate and soil salinity are critical for estimating SOC stocks in the VCEs. • Warming would decrease SOC stocks of VCEs by enhancing SOC decomposition. • Increasing precipitation would increase SOC stocks by reducing soil salinity. • SOC stocks of VCEs in the Bohai Rim could reduce ∼ 12 % − 19 % between 2041 and 2100. Vegetated coastal ecosystems (VCEs) have much higher organic carbon sequestration rates and storage capacities than most other earth surface ecosystems, and thus play a vital role in blue carbon sequestration for climate change mitigation. However, soil organic carbon (SOC) stock assessment in VCEs at a large regional-scale remain problematic, and the controlling mechanisms for SOC remain poorly understood. Here, we estimated current and future SOC stocks of VCEs in the Bohai Rim Region, China at a spatial resolution of 10 m, using a data-driven method based on multi-source data; and investigated the key environmental and anthropogenic controls. The total SOC stocks and SOC density of VCEs in the Bohai Rim Region were estimated to be approximately 62.7 Tg carbon and about 68.8 Mg ha−1, respectively (in the upper 1 m of soil). We found that climate and soil salinity are the most critical environmental controls of SOC stocks within VCEs in the region. Increasing temperature and soil salinity would increase SOC decomposition and decrease plant productivity, resulting in a decrease in SOC stocks. Conversely, increasing precipitation would increase SOC stocks by reducing soil salinity. We projected that the potential losses of SOC stocks due to the combined effects of climate changes, sea-level rise, and anthropogenic disturbances would be ∼ 12.2 % between 2041 and 2060 under two Shared Socioeconomic Pathway (SSP) scenarios (SSP245 and SSP585), extending to 19.3 % between 2081 and 2100. The decreases in SOC stocks would occur mainly in the higher latitude and colder regions. The study demonstrated the potential significant reduction in SOC stocks in coastal ecosystems and provided an important framework for better understanding the blue carbon ecosystems. [ABSTRACT FROM AUTHOR]

Carbon (C) occlusion within phytoliths (PhytOC) has a significant potential for long-term C sequestration in forest ecosystems. To unravel the role of forest composition on phytolith production, soil phytolith distribution, and phytolith C sequestration in soils, we investigated community composition and examined phytoliths and PhytOC of mature leaves or needles of dominant trees and understory herbs, as well as soil profiles (50 cm depth) within Quercus , Betula , Larix and Pinus forest ecosystems of northern China. Results showed that herb layers contributed 72%, 52%, 40%, and 5% to the flux of phytolith production within Betula forest (18.0 ± 1.26 kg ha − 1 yr − 1 ), Quercus forest (28.5 ± 0.77 kg ha − 1 yr − 1 ), Larix forest (37.7 ± 1.80 kg ha − 1 yr − 1 ) and Pinus forest (16.9 ± 0.30 kg ha − 1 yr − 1 ), respectively. The distribution pattern of soil phytoliths from topsoil to subsoil could be classified into three types: significantly decreasing pattern ( Betula forest and Quercus forest), non-significantly decreasing pattern ( Larix forest), and initially increasing and then decreasing pattern ( Pinus forest). Within 0–50 cm soil depth, the PhytOC storage of Betula forest, Quercus forest, Larix forest and Pinus forest were 0.29 ± 0.02 t ha − 1 , 0.67 ± 0.03 t ha − 1 , 0.46 ± 0.03 t ha − 1 and 0.37 ± 0.02 t ha − 1 , respectively. Moreover, the soil PhytOC turnover times of these four forest types were estimated to be 537,503,363 and 560 year, respectively, which were at least 8–20 times slower than soil organic carbon contributing to climate change mitigation. Overall, our findings indicate that composition of the forest community controls the production flux of phytoliths and the distribution of soil phytoliths, and influences the biogenic silica and its coupled carbon cycles. [ABSTRACT FROM AUTHOR]

During silicate weathering, atmospheric carbon dioxide (CO 2 ) is consumed and base cations are released from silicate minerals to form carbonate and bicarbonate ions, which are finally deposited as carbonate complexes. Continental silicate weathering constitutes a stable carbon sink that is an important influence on long-term climate change, as it sequesters atmospheric carbon dioxide at a million-year time scale. Traditionally, CO 2 sequestered through silicate weathering is estimated by measuring the flux of the base cations to watersheds. However, plants also absorb considerable amounts of base cations. Plant biomass is often removed from ecosystems during harvesting. The base cations are subsequently released after decomposition of the harvested plant materials, and thereby enhance CO 2 consumption related to weathering. Here, we analyze plant biomass storage-harvest fluxes (production and removal of biomass from forests) of base cations in forests across China to quantify the relative contribution of forest trees to the terrestrial weathering-related carbon sink. Our data suggest that the potential CO 2 consumption rate for biomass-related silicate weathering (from the combined action of with afforestation/reforestation, controlled harvesting and rock powder amendment) in Chinese forests is 7.9 ± 4.1 Tg CO 2 yr − 1 . This represents ~ 34% of the chemical weathering rate in China. Globally, forests may increase CO 2 sequestration through biologically-mediated silicate weathering by ~ 32%. [ABSTRACT FROM AUTHOR]

Grasslands play an important role in the terrestrial biogeochemical carbon (C) cycle and partly mitigate climate change through C occlusion within phytoliths. Grassland degradation has a significant influence on the coupled biogeochemical cycles of C and silicon in the Inner Mongolian steppe of China, but there are few reports about the impact of grassland degradation on phytolith C sequestration in the steppe, the main grassland in northern China. Twelve sampling sites were chosen in the Xilingol League. Soil samples (0–50 cm) were collected from grasslands of four different degradation degrees to investigate the impact of grassland degradation on the soil phytolith and phytolith-occluded C (PhytOC) accumulation using a mass-balance approach. Soil phytolith storages were 12.97 ± 2.15, 15.90 ± 0.65, 14.35 ± 0.79 and 13.22 ± 1.07 t ha − 1 in non-degraded, lightly degraded, moderately degraded and seriously degraded grasslands, respectively. The corresponding storages of soil PhytOC were 0.11 ± 0.02, 0.16 ± 0.02, 0.12 ± 0.01 and 0.07 ± 0.01 t ha − 1 , respectively. The observed significant differences in soil phytoliths and PhytOC among grasslands of different degradation degrees indicate that grassland degradation influenced the phytolith and PhytOC accumulation in grassland soils. Grazing and harvesting are likely the major factors affecting soil phytolith and PhytOC storages through reducing the litterfall returning fluxes. Our preliminary findings imply that grassland restoration could be a promising way to increase long-term phytolith C sequestration through maximizing plant PhytOC production fluxes and soil PhytOC accumulation in degraded grasslands. [ABSTRACT FROM AUTHOR]

Terrestrial biogeochemical carbon (C) sequestration is coupled with the biogeochemical silicon (Si) cycle through mechanisms such as phytolith C sequestration, but the size and distribution of the phytolith C sink remain unclear. Here, we estimate phytolith C sequestration in global terrestrial biomes. We used biome data including productivity, phytolith and silica contents, and the phytolith stability factor to preliminarily determine the size and distribution of the phytolith C sink in global terrestrial biomes. Total phytolith C sequestration in global terrestrial biomes is 156.7 ± 91.6 Tg CO 2 yr − 1 . Grassland (40%), cropland (35%), and forest (20%) biomes are the dominant producers of phytolith-based carbon; geographically, the main contributors are Asia (31%), Africa (24%), and South America (17%). Practices such as bamboo afforestation/reforestation and grassland recovery for economic and ecological purposes could theoretically double the above phytolith C sink. The potential terrestrial phytolith C sequestration during 2000–2099 under such practices would be 15.7–40.5 Pg CO 2 , equivalent in magnitude to the C sequestration of oceanic diatoms in sediments and through silicate weathering. Phytolith C sequestration contributes vitally to the global C cycle, hence, it is essential to incorporate plant-soil silica cycling in biogeochemical C cycle models. [ABSTRACT FROM AUTHOR]

Carbon sequestered in phytoliths, the silicified features of plants, may accumulate in soils and sediments for several hundreds to thousands of years depending mainly on the morphology and chemical composition of phytoliths and environmental conditions. Phytolith carbon sequestration is thought to be one of the key biogeochemical carbon sequestration mechanisms. This review summarizes the current state of understanding of occurrence, turnover and carbon sequestration potential of phytoliths in terrestrial ecosystems. The accumulation rate of phytoliths in soils is controlled not only by plant litter inputs but also by geochemical stability and turnover of phytoliths. Most of the organic carbon in phytoliths is from photosynthesis of atmospheric carbon dioxide (CO 2 ) during formation of phytoliths. Phytolith carbon sequestration in terrestrial ecosystems is a promising biogeochemical carbon sequestration mechanism and may contribute to the mitigation of global climate warming. Further questions such as the turnover of phytoliths with different properties and under different environmental conditions, the relative contribution of different ecosystems to the global terrestrial phytolith carbon sequestration, and the cost and potential of each management measure to enhance phytolith carbon sequestration should be investigated. [ABSTRACT FROM AUTHOR]

Global warming as a result of rapid increase in atmospheric CO emission is significantly influencing world's economy and human activities. Carbon sequestration in phytoliths is regarded as a highly stable carbon sink mechanism in terrestrial ecosystems to mitigate climate change. However, the response of plant phytolith-occluded carbon (PhytOC) to external silicon amendments remains unclear. In this study, we investigated the effects of basalt powder (BP) amendment on phytolith carbon sequestration in rice ( Oryza sativa), a high-PhytOC accumulator. The results showed that the contents of phytolith and PhytOC in rice increased with BP amendment. The PhytOC production flux in different rice plant parts varied considerably (0.005-0.041 Mg CO ha a), with the highest flux in the sheath. BP amendment can significantly enhance flux of phytolith carbon sequestration in croplands by 150 %. If the global rice cultivation of 1.55 × 10 ha had a similar flux of PhytOC production in this study, 0.61 × 10 to 1.54 × 10 Mg CO would be occluded annually within global rice phytoliths. These findings highlight that external silicon amendment such as BP amendment represents an effective potential management tool to increase long-term biogeochemical carbon sequestration in crops such as rice and may also be an efficient way to mitigate the global warming indirectly. [ABSTRACT FROM AUTHOR]

Global agricultural ecosystems, the largest biospheric sources of atmospheric carbon dioxide (CO 2 ), may turn into considerable net carbon (C) sinks through adopting management strategies advised by research. As C sequestration is usually coupled with the silicon (Si) cycle, strategic manipulation of the biogeochemical Si cycle in agricultural ecosystems offers a not yet fully explored possibility to enhance C sequestration. This review summarizes current knowledge of C sequestration coupled with the Si cycle and its management in agricultural ecosystems. Carbon sequestration is coupled with the Si cycle through many processes including dynamics of phytoliths and aggregates, and silicate weathering at different temporal and spatial scales. Cultivation of deep rooting crops, erosion mitigation with buffer strips, fertilization of Si-rich materials are some of the potential management strategies to increase both crop production and C sequestration coupled with the Si cycle. Further questions such as identifying the controlling factors of bioavailable Si pools and C sequestration, and quantifying the cost-efficiency of different management strategies to manipulate the Si cycle with the aim to enhance C sequestration should be investigated. [ABSTRACT FROM AUTHOR]

Highlights: [•] A recent found persistent carbon sink is C occluded within plant phytoliths. [•] The Chinese cropland phytolith C sink contributes 18% of world's croplands. [•] The predominant phytolith producing crop species were rice, wheat and corn. [•] The sink has doubled since 1978 due to fertilizer application and irrigation. [•] Management practices may further enhance the cropland phytolith C sink. [ABSTRACT FROM AUTHOR]

Highlights: [•] Reed PhytOC content depends on phytolith content and carbon content of phytoliths. [•] The above-ground production rate of PhytOC is about 2.01×108 gCO2 a−1. [•] Reed has a major role in sequestrating CO2 through formation of PhytOC. [ABSTRACT FROM AUTHOR]

• Status of PASi across China's paddy fields is quantified on a national scale. • Natural and anthropogenic factors control the spatial distribution of PASi content. • At least 65% of China's paddy fields are deficient in PASi. • The principal regions where PASi deficit are mainly located in southern China. • PASi content of China's paddy fields decreases with an annual rate of 0.54%. Silicon (Si) is beneficial for rice health and production by alleviating various biotic and abiotic stresses. However, the continual export of grain off-farm may result in Si deficiency for rice plants. The current levels of plant available Si (PASi) in rice paddies in China remain unclear, as do the factors that control PASi content in these soils. We conducted a nationwide sampling campaign across the paddy fields of China between 2016 and 2019, and used calcium chloride extractable Si (Si-CaCl 2) and buffered acetate extractable Si (Si-NaAc, pH = 4) to quantify PASi. We show that Si-CaCl 2 pool was mainly influenced by mean annual temperature (MAT), soil salinity, soil organic carbon (SOC), mean annual precipitation (MAP), and soil pH, suggesting both pedological and biological control mechanisms. However, the Si-NaAc pool was influenced most by soil pH, MAT and MAP, implying pedological control. Compared to data from the 1990s, the Si-NaAc content decreased by 14.1% on a national scale with an annual decline rate of 0.54%. Based on our investigation, at least 65% of China's paddy fields are deficient in PASi, which is an increase in area of ~15% over the last 20 years. The principal regions where PASi deficiency was recorded are mainly located in southern China, with the levels of Si deficiency lowering as the paddy fields are located further north. The continual off-site removal of PASi from rice grain and straw will need to be addressed through the use of Si-fertilizers, including organic amendments, to maintain a productive and sustainable rice industry in China. [ABSTRACT FROM AUTHOR]

Abstract: The coupled terrestrial biogeochemical cycles of silicon (Si) and carbon (C) that are driven by plant action play a crucial role in the regulation of atmospheric CO2. Generally, the processes involved in the coupled cycles of Si and C include plant-enhanced silicate weathering, phytolith formation and solubilization, secondary aluminosilicate accumulation, phytolith occlusion of C as well as physico-chemical protection of organic C in soils. There is increasing evidence of biological pumping of Si in terrestrial ecosystems, suggesting that complex feedbacks exist amongst the processes within the coupled Si and C cycles. Recent advances in the coupled Si and C cycles offer promising new possibilities for enhancing atmospheric CO2 sequestration. Organic mulching, rock powder amendment, cultivating Si-accumulating plants and partial plant harvesting are potential measures that may allow for long-term manipulation and biogeochemical sequestration of atmospheric CO2 in soil–plant systems. [Copyright &y& Elsevier]

• Phragmites australis is efficient in taking up silica (Si), storing the biogenic Si (BSi), and releasing Si, thus playing a crucial role in the biogeochemical cycling of Si in wetlands. • BSi can substitute lignin as a structural component, due to their similar eco-physiological functions, reduces costs associated with lignin biosynthesis. • BSi accumulation increased total biomass C storage, and BSi changed the composition and quality of litter substrates. • Competing processes of carbon cycles (i.e., biomass quantity vs quality) can be influenced by Si cycling in wetlands. Silicon (Si) is one of the most abundant elements in the Earth's crust but its role in governing the biogeochemical cycling of other elements remains poor understood. There is a paucity of information on the role of Si in wetland plants, and how this may alter wetland C production and storage. Therefore, this study investigated Si distribution, nutrient stoichiometry and lignin abundance in Phragmites australis from a wetland system in China to better understand the biogeochemical cycling and C storage. Our data show that Si content (ranging between 0.202% to 6.614%) of Phragmites australis is negatively correlated with C concentration (38.150%–47.220%). Furthermore, Si content was negatively antagonistically related to the concentration of lignin-derived phenols in the stem (66.763–120.670 mg g-1 C) and sheath (65.400–114.118 mg g-1 C), but only a weak relationship was observed in the leaf tissue (36.439–55.905 mg g-1 C), which is relevant to the photosynthesis or stabilization function of the plant tissues. These results support the notion that biogenic Si (BSi) can substitute lignin as a structural component, due to their similar eco-physiological functions, reduces costs associated with lignin biosynthesis. The accumulation of BSi increased total biomass C storage and nutrient accumulation due to greater productivity of Phragmites australis. On the other hand, BSi regulated litter composition and quality (e.g., nutrient stoichiometry and lignin) that provide a possibility for the factors affecting litter decomposition. Thus competing processes (i.e., biomass quantity vs quality) can be influenced by Si cycling in wetlands. [ABSTRACT FROM AUTHOR]

• • Phytolith-occluded carbon (PhytOC) plays an important role in terrestrial long-term carbon sequestration. • • Phytolith stores along precipitation gradient were significantly and positively correlated with MAP. • • Elevated MAP would enhance ecosystem primary productivity and consequently promote soil PhytOC storage. • • The soil phytolith dynamics in East Asian steppe should be considered in predicting global climate change. Climatic factors including mean annual precipitation (MAP) significantly influence the carbon (C) cycle in terrestrial ecosystems and Earth overall. Phytolith-occluded carbon (PhytOC) is an important C sequestration mechanism and as such plays a vital role in global long-term C sequestration. Understanding the spatial variability in the storage of soil phytoliths and PhytOC and its relationship with climate is critical for evaluating the impact of global climate change on terrestrial ecosystem functions. However, little is known about the responses of soil phytoliths and PhytOC to MAP in grassland ecosystems. This study sampled soil from 24 natural, semi-arid steppe sites along a 2,500 km transect with a precipitation gradient of 243–481 mm yr−1 in northern China. We investigated the influence of precipitation on the spatial distributions of soil phytoliths and PhytOC storage. Storage of soil phytoliths in bulk soil (0–100 cm depth) ranged from 21.3 ± 0.4 to 88.4 ± 20.3 t ha−1 along the precipitation gradient. Amounts of soil phytoliths and PhytOC storage were significantly and positively correlated with MAP. Multiple regression analysis revealed that phytolith storage in bulk soil was best predicted by MAP (R = 0.5) and soil organic carbon (SOC, R = 0.4), with these two variables accounting for about 58% of the total variation observed. Considering the forecasted increase in MAP in the Inner Mongolian steppe due to climate change, and the strong influence of MAP on the annual net primary productivity (ANPP) and related soil PhytOC input from litter decomposition in this region, we expect that ecosystem primary productivity will increase from deserts to meadow steppe and thereby promote soil PhytOC storage. These findings have important implications for understanding the dynamics of soil phytoliths, and predicting the impacts of global climate change on ecosystem functions and management practices in the East Asian steppe ecosystems. [ABSTRACT FROM AUTHOR]

• Terrestrial biogeochemical Si cycle is strongly influenced by labile Si fractions. • The contribution made by non-crystalline Si fraction to total Si was 1.51–2.31%. • Biogenic amorphous Si contributed more to the Si cycle in the broadleaf forests. • Pedogenic amorphous Si contributed more to the Si cycle in the conifer forests. • Soil organic matter, pH, and plant communities can affect the Si transformation. The terrestrial biogeochemical silicon (Si) cycle significantly contributes to maintaining the functions and sustainability of terrestrial ecosystems. Over the short term, the biogeochemical Si cycle can be strongly influenced by dissolved Si, organic bound Si, Si adsorbed to pedogenic oxides/hydroxides, and biogenic and pedogenic amorphous Si. However, quantitative studies about these relatively soluble Si fractions are rare. In this study, we quantified different Si fractions in the 0–10 cm, 10–20 cm, 20–30 cm, 30–40 cm and 40–50 cm soil layers of broadleaf forests (Betula forest and Quercus forest) and conifer forests (Larix forest and Pinus forest) in northern China using a sequential chemical extraction scheme optimized for these Si fractions. The results showed that the total Si (Si t) in the soil layers consisted of 97.7–98.5% crystalline Si (Si cry) and 1.5–2.3% non-crystalline Si (Si noncry) fractions. Within the Si noncry fraction, the proportions of dissolved Si (Si dis), organic matter bound Si (Si org), pedogenic oxides/hydroxides chemisorbed Si (Si sorb), and amorphous Si (Si amor) were 3.4–6.7%, 5.5–8.9%, 6.3–8.5%, and 77.7–84.8%, respectively. Although the Si dis fraction was the least abundant component, it is at the center of the interconversion processes among the different Si noncry fractions. The Si amor fraction was the largest component of Si noncry and was composed of 37.7–71.9% biogenic amorphous Si (Si bio-amor) and 28.1–62.3% pedogenic amorphous Si (Si ped-amor). Our study indicated that i) Si ped-amor fraction is more easily influenced by soil pH comparing to Si bio-amor fraction; ii) the Si bio-amor fraction contributes more to the biogeochemical Si cycle in broadleaf forests, whereas the Si ped-amor fraction contributes more in conifer forests; and iii) soil pH, soil organic matter, and plant community differences can influence the vertical distribution of the different Si noncry fractions and thus affect the multiple transformation processes among these Si fractions in studied forests. [ABSTRACT FROM AUTHOR]

Rainwater samples collected from Guiyang city in southwestern China between May 2009 and December 2010 were analyzed to measure their calcium and strontium isotope compositions. The results show that Ca2+ is the most abundant cation in these rainwater samples, while SO 4 2− is the most abundant anion. Most rainwater samples are alkaline, with a volume-weighted mean (VWM) pH value of ~6.2 (a 1.2-unit increase since 2008). This can be explained by the neutralization of rainwater acidity by the dissolution of atmospheric aerosols in the form of calcium-rich soil and other anthropogenic dust, which are increasing because of rapid urbanization. Both stable (δ44/40Ca and δ88/86Sr) and radiogenic (87Sr/86Sr) isotope compositions of rainwater show variations correlating with each other and with the inverse of Sr or Ca concentrations. They also correlated with the concentration ratio of (NH 4 ++K+)/(Ca2++Mg2+) ―an index reflecting the anthropogenic input relative to soil dust. Based on our results, three major reservoirs can be inferred to have contributed to rainwater chemistry in Guiyang city: (1) a carbonate aerosol source characterized by low 87Sr/86Sr (≤~0.708), δ88/86Sr (≤~0.32‰) and δ44/40Ca (~0.5‰), very low (NH 4 ++ K+)/(Ca2++Mg2+), and low 1/Ca2+ and 1/Sr2+; (2) a silicate aerosol source characterized by relatively high 87Sr/86Sr (≥0.717), δ88/86Sr (≥~0.34‰) and δ44/40Ca (~0.8‰), very low (NH 4 ++K+)/(Ca2++Mg2+), low 1/Ca2+ and 1/Sr2+; and (3) an anthropogenic component containing low 87Sr/86Sr (~0.708), δ88/86Sr (~0.30‰) and high δ44/40Ca (~0.8‰), and high (NH 4 ++K+)/(Ca2++Mg2+). The chemistry of solutes in Guiyang rainwater can be explained by (1) dissolution of carbonate and silicate aerosols, as well as the anthropogenic inputs including seasonal farming activities; (2) tightened environmental protection regulations; (3) higher precipitation volumes in 2010; and possibly (4) the increased combustion of coal for heating and electricity in winter. This study demonstrated that a combination of elemental compositions, stable Ca and Sr, and radiogenic Sr isotope compositions in rainwater can be used to trace the sources of atmospheric aerosols. • Determine the stable isotope (δ44/40Ca and δ88/86Sr) and radiogenic isotope (87Sr/86Sr) compositions of rainwater. • Investigating the sources of major ions in rainwater in a karst city. • Three major isotopic and elemental reservoirs of rainwater chemistry can be identified according to our results. [ABSTRACT FROM AUTHOR]

• A paired waterlogging-control system was established to evaluate effects of sea‐level rise on BSi and BSiOC accumulations. • Sea‐level rise significantly reduced BSi and BSiOC densities, particularly in the salt marsh dominated by the Phragmites australis. • Although sea‐level rise increased SOC content, it generally decreased soil BSiOC content and its contribution to total SOC. • The depleted BSi and BSiOC stocks was attributed to increased anaerobiosis, higher salinity and pH, and higher soil erosion. The fate of soil biogenic silica (BSi) in coastal salt marshes is of global importance because of its role in providing both available silicon (Si) for the growth of plants and diatoms and in sequestering carbon (C) (blue C) in soils and sediments. However, the accumulation of BSi and BSi-occluded C (BSiOC) under different vegetation habitats in coastal ecosystems, particularly in response to relative sea‐level rise (RSLR), remains poorly understood. Here we established a paired waterlogging-control system and collected the soil samples under both waterlogged and non-waterlogged conditions from three vegetation habitats (single Phragmites australis , a mixture of P. australis and Suaeda salsa , and single S. salsa) corresponding to a gradient of distance from the coastline in three independent salt marshes. Our findings indicated that RSLR decreased average BSi content, especially in the 0–20 cm soil layer of the single P. australis and the mixed P. australis and S. salsa community. Although RSLR increased soil organic C (SOC) content, it generally decreased soil BSiOC content and its contribution to SOC. Consequently, RSLR resulted in lower soil BSi and BSiOC densities, particularly in the salt marsh dominated by the Si accumulator P. australis. Here, BSi and BSiOC densities at depths of 0–80 cm decreased by an average of 19% and 18% following waterlogging, respectively. The BSi pool was dominated by phytoliths in non-waterlogged soils, while both diatoms and phytoliths were present in soils subjected to RSLR. Soil physicochemical properties including SOC, electrical conductivity and dissolved Si were influenced by RSLR. These factors, along with erosion were suggested as key influencers for BSi and its C sequestration in coastal salt marshes. Our results provide a scientific basis for elucidating biogeochemical Si cycles and predicting changes in BSiOC sequestration to optimize its storage. [ABSTRACT FROM AUTHOR]

The publisher regrets that this article has been temporarily removed. A replacement will appear as soon as possible in which the reason for the removal of the article will be specified, or the article will be reinstated. The full Elsevier Policy on Article Withdrawal can be found at http://www.elsevier.com/locate/withdrawalpolicy . [ABSTRACT FROM AUTHOR]

Grasslands play a crucial role in the coupled biogeochemical cycles of carbon (C) and silicon (Si) because they have a large biogenic Si pool (i.e. phytoliths). In recent decades, desertification has occurred extensively in sandy grasslands due to human activities and to increased aridity as a consequence of climate change. The present study determined the contents of phytoliths and C occlusion within phytoliths (PhytOC) in sandy grassland with different vegetation coverage from eastern Inner Mongolia, China and preliminarily assessed the effects of desertification on phytoliths and PhytOC production. Our results showed that the phytolith and PhytOC contents among different plant species varied from 0.68 to 9.23% and 0.03 to 1.13‰, respectively. However, the community-weighted means of the phytolith and PhytOC contents for the total aboveground vegetation were only 1.13–3.61% and 0.09–0.35‰, respectively, and their respective production fluxes ranged from 8.94 to 47.8 kg ha −1  year −1 and from 0.06 to 0.48 kg ha −1  year −1 , respectively. As desertification progressed, the total contents of phytoliths and PhytOC in aboveground vegetation did not change significantly, whereas the production fluxes of phytoliths and PhytOC were markedly reduced. This study indicates that grassland desertification decreases the range of the total contents of phytolith and PhytOC by reducing species richness, and decreases the production fluxes of phytoliths and PhytOC by reducing aboveground biomass. Grassland restoration can theoretically enhance the production fluxes of phytoliths and PhytOC ~ five-fold. [ABSTRACT FROM AUTHOR]

Carbonaceous black shale generally contains high concentration of Cd, with weathering leading to Cd release to environment. In this study, the mobility of Cd during weathering was quantified using geochemical assessment on black shale from western Hunan, China. Results suggested that Cd was heterogeneously distributed in shale profiles with concentrations ranging from 0.16 to 109.9 (mg/kg). Cd distribution was heterogeneous resulting from the parent shale inheritance and the mobility of Cd during weathering. Black shales weathered to a moderate degree with Cd mobility characterized by both enrichment in and release from weathered shales. Cd enrichment in weathered shales resulted from the re-enrichment of Cd in secondary minerals formed during the initial stage of carbonate (and phosphorite) dissolution, and the secondary stage of sulfide oxidation. The release of Cd was caused by decomposition of the secondary Cd-bearing minerals. Cadmium was extensively released during pedogenesis, and Cd release mass flux was estimated to range from 1.26 to 9.50 (g/m2) with a mean of 6.60 g/m2. Thus, black shale weathering may lead to the releasing of large amount of Cd resulting in Cd contamination to local environments. [Display omitted] • Cd distribution in weathered shale resulted from inheritance and mobility during weathering. • Cd mobility was characterized by reenrichment in and releasing from the weathered shale. • Cd enrichment in weathered shale resulted from re-enrichment of Cd in secondary minerals. • Cd release from weathering system was caused by decomposition of secondary minerals. • Black shale weathering caused Cd contamination to local environment by offering natural Cd source. [ABSTRACT FROM AUTHOR]

This paper reports a geochemical study on the major and trace elements and Pb–Sr isotopes of a weathering profile developed in the Lower Cambrian black shales in central Hunan (China). Six weathering horizons were identified and sampled vertically throughout the profile. The chemical composition of the profile consists of variable concentrations of the major elements Fe 2 O 3 , FeO, MnO, MgO, CaO, Na 2 O, and P 2 O 5 and of less variable concentrations of SiO 2 , TiO 2 , Al 2 O 3 , and K 2 O. The chemical change caused by weathering is estimated by mass-balance calculations, and the results show that the element mobility is characterised by substantial loss of SiO 2 , FeO, CaO, K 2 O, Na 2 O, LOI, Cr, V, Ba, Cs, Rb, Sr, U, and Th, and moderate loss of Al 2 O 3 , MgO, Fe 2 O 3 , Ni, Cu, Pb, Tl, Sn, Sc, Ge and REE (Y). The high field strength elements TiO 2 , Sn, Sc, U, Ga, Ge, Zr, Hf, Nb, and Ta were immobile during weathering. The chemical changes and the Pb–Sr isotopic data suggest that four types of chemical reactions occurred: the oxidation of sulphide minerals (e.g., pyrite) and organic carbon (OS), the dissolution of less resistant clinochlore-Ia, calcite, and P-bearing minerals (DL), the dissolution of detrital albite and microcline (DA), and the transformation of clay (TC) minerals (e.g., muscovite and illite–smectite). These chemical reactions then led to two stages of geochemical processes, an early stage of chemical differentiation and a later stage of chemical homogenisation. The chemical differentiation dominated by the OS, DL, and DA reactions, led to the leaching of mobile elements (e.g., MgO, Na 2 O, K 2 O, P 2 O 5 , Sr, and REE) and the redistribution of some less mobile elements (e.g., SiO 2 and Al 2 O 3 ). In contrast, the chemical homogenisation, which was caused by TC reactions, led to the leaching of both mobile and less mobile elements from the system and ultimately transformed the weathered black shales into soil. Soils derived from black shales in South China might result from the above two geochemical processes. [ABSTRACT FROM AUTHOR]

Accumulation of microplastics (MPs) and cadmium (Cd) are ubiquitous in paddy soil. However, the combined effects of MPs and Cd on physiochemical and microbial mechanisms in soils and the attendant implications for greenhouse gas (GHG) emissions, remain largely unknown. Here, we evaluated the influence of polylactic acid (PLA) and polyethylene (PE) MPs on GHG emissions from Cd-contaminated paddy soil using a microcosm experiment under waterlogged and drained conditions. The results showed that PLA significantly increased CH 4 and N 2 O emission fluxes and hence the global warming potential (GWP) of waterlogged soil. Soils treated with MPs+Cd showed significantly reduced GWP compared to those treated only with MPs suggesting that, irrespective of attendant consequences, Cd could alleviate N 2 O emissions in the presence of MPs. Conversely, the presence of MPs in Cd-contaminated soils tended to alleviate the bioavailability of Cd. Based on a structural equation model analysis, both the MPs-derived dissolved organic matter and the soil bioavailable Cd affected indirectly on soil GHG emissions through their direct influencing on microbial abundance (e.g., Firmicutes , Nitrospirota bacteria). These findings provide new insights into the assessment of GHG emissions and soil/cereal security in response to MPs and Cd coexistence that behaved antagonistically with respect to adverse ecological effects in paddy systems. [Display omitted] • MP enrichment in paddy soil enhance GHG emissions but inhibit Cd bioavailability. • MPs lead to increase of dissolved organic matter and Firmicutes bacteria abundance. • PLA have high potential to adsorb and fix soil Cd2+ due to abundant oxygenous groups. • 90-day incubation makes PLA decomposing and loading more soil particles than PE. [ABSTRACT FROM AUTHOR]

Coastal wetlands host large and dynamic reservoirs of organic carbon (C) and are also biogeochemical hotspots for a wide range of Fe (hydr-)oxides with different chemical reactivities, properties, and functions. The cycling of these iron (hydr-)oxides is closely coupled to that of organic C, which in turn strongly influences the magnitude and dynamics of organic C turnover in these ecosystems. This review synthesizes and summarizes current knowledge of distribution, turnover, and controls of Fe (hydr-)oxides, as well as their ecological roles and impacts on organic C turnover in coastal wetland ecosystems globally. Regional hydro-geochemical processes and anthropogenic activities in the uplands as well as soil texture exert a first-order control on the abundance and distribution of Fe (hydr-)oxides in coastal wetland soils, while the activities of plant roots and macro-organisms act as important biological drivers for the formation, transformation, and turnover of Fe (hydr-)oxides as well as associated organic C in both rhizosphere/burrows and bulk soils. The reported rates of dissimilatory Fe reduction (DFeR) are correlated with incubation temperature and the sizes of reactive Fe(III) phases. However, the contributions of DFeR to total anaerobic carbon oxidation were found to be correlated only with the size of reactive Fe(III) pools, meaning that all the identified processes contributing to the accumulation and formation of Fe hydroxides could increase the importance of the DFeR-dominated respiratory pathway and suppress sulfate reduction and methanogenesis. Additionally, Fe plaques dominated by amorphous Fe hydroxides are formed and cycled in close interaction with the activities of wetland plant roots, and likely provide several important ecological functions and contribute to maintaining high levels of plant productivity in coastal wetlands under different environmental stresses. The features and findings presented in this review not only contribute to an improved understanding of the biogeochemical cycle and ecological roles of Fe (hydr-)oxides in coastal wetlands, but also provide a basis for future studies on some highlighted key research areas. Such future studies will further increase our ability to understand and predict how the size, stability, and turnover of Fe (hydr-)oxides and organic C in coastal wetlands will respond to and affect global climate change. [ABSTRACT FROM AUTHOR]

• ~98% of the total C from black shales was recalcitrant C fraction. • The δ13C value was higher in SOC compared to the ROC fraction. • The ROC accounted for ~60% of the SOC pool in terrestrial ecosystems. • Paddy soils have a higher potential for SOC storage than forest soils. • LOC, Semi-LOC, and ROC content are significantly correlated with SOC content. Black shales are characterized by a high content of organic carbon (C). Few studies have focused on the influence of land use on soil organic C (SOC) fractions from soils derived from black shale (black shale soils). The objective of this study was to elucidate the influence of land use on SOC fractions in black shale soils combining chemical determination and stable C isotope analysis techniques. Herein, we determined labile organic C (LOC), semi-labile organic C (Semi-LOC), and recalcitrant organic C (ROC) fractions in various depths of soils in paddy fields (0–70 cm) and forests (0–120 cm) from black shale distribution region in Hunan province, China, and then investigated δ13C values of these soils. Results showed that the contents of LOC, Semi-LOC, and ROC in paddy soils (1.63–7.35 g kg−1, 0.35–1.21 g kg−1, and 3.75–14.8 g kg−1, respectively) and forest soils (0.73–4.94 g kg−1, 0.12–0.89 g kg−1, and 1.44–8.96 g kg–1, respectively) are significantly decreased with increasing depth. The contribution made by LOC to SOC in paddy soils was significantly lower than that in forest soils, while the contribution made by ROC to SOC was significantly higher in paddy soils than that in forest soils. In these two land uses, the δ13C values were higher in SOC compared to the ROC fraction, while the δ13C values were close in the ROC fraction below 20 cm soil depth. Our study indicated that i) new C is mainly limited to the surface soil layer (0–10 cm) in forests, while it can be leached along the soil profiles in paddy fields; ii) the estimated ROC pool is ~900 Pg within the 0–100 cm soil layer in terrestrial ecosystems, which should better represent the ability of soil C sequestration. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe p concentration and Fe complexing index decreased after plant invasion. • Fe-OC concentration decreased by 35.9% in average after plant invasion. • FeRB abundance significantly increased by 78.6% after plant invasion. • Plant invasion increased soil water content and decreased bulk density. • Fe-OC was positively correlated with SOC, Fe p , and Fe(III). Wetland ecosystems have dual functions as both carbon (C) sources and C sinks, playing a vital role in the world's C budget. Wetlands are prone to suffer from plant invasions, and for example, Spartina alterniflora , a well-known invasive species, has rapidly expanded in coastal areas of Asia since 1979 and altered the C cycles in coastal wetland ecosystems. Fe(III) oxides are critical in the OC storage by formation of Fe-bound organic carbon (Fe-OC). But the impact of the invasion by S. alterniflora in coastal wetlands on the formation and stabilization of Fe-OC is poorly known. Herein, we assessed soil Fe species contents, Fe-OC pool, and the microbial communities associated with these processes in seven wetlands dominated by both native ( Kandelia obovate , Avicennia marina and Phragmites australis) and invasive (S. alterniflora) plants. Our results showed that organical complexed Fe (Fe p) concentration and Fe complexing index under S. alterniflora community were lower than that under native communities. Soil Fe-OC concentration and molar OC:Fe ratio decreased (35.9% and 29.3%, respectively) after S. alterniflora invasion in coastal wetlands, while the abundance of FeRB increased by 78.6%. The invasion of S. alterniflora increased soil water content while decreased bulk density, and these changes could have important effects on Fe species and FeRB abundance. Moreover, Fe-OC was positive correlated with soil organic carbon (SOC), Fe p , and Fe(III), indicating that SOC and Fe(III) concentrations directly affect the formation of Fe-OC. FeRB reacted on the combination of Fe (hydr-) oxides and SOC to indirectly affect Fe-OC through dissimilatory Fe reduction, which further determined the stabilization and mineralization of SOC. Taken together, the invasion of S. alterniflora hindered the generation and accumulation of soil Fe-OC pool weakening the stabilization of SOC in coastal wetlands. Therefore, the conservation and restoration of mangroves and other coastal wetlands provide a promising strategy for SOC sequestration and climate change mitigation. [ABSTRACT FROM AUTHOR]

Carbon (C) present in lake sediments is an important global sink for CO 2 ; 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. Unlabelled Image • Climate and vegetation are important factors controlling the CAR. • Three sub-regions have different temporal trends and factors influencing the CAR. • Reduced future C storage is projected for lakes in the East Asian monsoonal margin. • Lower C storage is likely to be driven by the loss of vegetation and coverage. [ABSTRACT FROM AUTHOR]

Concentrations of heavy metals Ba, Sc, V, Cr, Mn, Co, Ni, Th, U, Cu, Pb, Zn, and Cd in sediments from the lower reaches of the Xiangjiang River were analyzed using inductively coupled plasma mass spectrometry. The results suggest that there are two metal distribution patterns in these sediments: (1) Ba, Sc, V, Cr, Mn, Th, and U are relatively homogeneously distributed and (2) Cd, Pb, Zn Cu, Co and Ni are heterogeneously distributed. The heterogeneously distributed metals are significantly enriched in these sediments and, thereby, contribute to contamination. Assessments of heavy metal contamination using the Geoaccumulation index, Pollution load index, and potential ecological risk index suggest that the levels of contamination from Cd and Mn are extremely high and moderately high, respectively, in all the sediments from the lower river. In comparison, the levels of contamination by Cu, Zn, and Pb varied spatially, decreasing progressively downriver. The level of contamination by Pb, Zn, and Cu in sediments from the Zhuzhou reach is extremely high, and is moderate to significant high for the Xiangtan, Changsha, and Xiangyin reaches. The ecological potential risks posed heavy metals are ranked, in descending order, as Cd > Pb > Cu > Zn > Cr > Ni > Co > Mn for sediments from the Zhuzhou reach and Cd > Pb > Cu > Ni > Cr > Co > Zn > Mn for sediments from the Xiangtan, Changsha, and Xiangyin reaches. Principal component analysis and enrichment factor calculations suggest that Ba, Sc, V, Cr, Th, and U mostly originate from natural processes. While, Cd, Pb, Zn, Cu, Co, Ni, and Mn are derived from both natural processes and anthropogenic activities. Therefore, strategies for environmental protection in this watershed should focus on contamination by Cd, Pb, Zn, and Cu, with Cd requiring particular attention. Unlabelled Image • There are two distributional patterns of heavy metals in the river sediments. • The sediments are contaminated by heavy metals Cd, Pb, Zn, Cu, and Mn. • Metals Cd, Pb, Cu, and Mn in the sediments are potential ecological risk. • The degree of contamination and risk decreases progressively downriver. • Cd, Pb, Zn, Cu, Mn, Co, and Ni are derived from anthropogenic and natural sources. [ABSTRACT FROM AUTHOR]

• The alpine meadow ecosystem in northeast Tibetan Plateau was a CO 2 sink. • The flux exchanges of the ecosystem were energy-limited. • The CO 2 fluxes of the ecosystem were mainly controlled by soil temperature. • Warming will reduce the CO 2 sink because of a larger increase in R e than in GPP. The alpine meadow in the Tibetan Plateau (TP) is a fragile ecosystem and very sensitive to climate change and human activities. However, the flux (energy, water, and carbon dioxide) exchanges (FEs) and their controls in this ecosystem are not well understood. Using two years (2015–2016) of observations from three eddy covariance towers and different grazing management types, we quantified the FEs of an alpine meadow ecosystem in the northeastern TP. A boosted regression trees (BRT)-based method was used to determine the environmental controls of FEs and clarify the factors causing the differences in FEs among the sites. The mean annual latent heat flux (LE), sensible heat flux (H), and evapotranspiration (ET) of the sites were 36.3 W m−2, 23.2 W m−2, and 458.2 mm, respectively. The mean annual gross primary productivity (GPP) and ecosystem respiration (R e) of the sites were 504.8 and 317.2 g C m−2, respectively. The mean annual net ecosystem CO 2 exchange (NEE) of the sites was –187.6 g C m–2, indicating that the ecosystem was a CO 2 sink, not accounting for grazing losses. The BRT-based analysis showed that, (i) The LE (ET) and H were mostly determined by soil temperature (ST) and downward shortwave radiation (DR), while the NEE, GPP, and R e were primarily correlated with ST. As a result, the FEs of the ecosystem were energy-limited; (ii) The high spatial heterogeneity in multiple environmental variables jointly determined the differences in FEs of the ecosystem. The differences in LE (ET) and H among the sites resulted from multiple environmental variables, including DR, ST, precipitation, and SM; and differences in NEE, GPP, and R e were mainly caused by precipitation, SM, and RH. The study highlights that future warming will enhance GPP and R e of the ecosystem, while reducing the CO 2 sink because of the larger increases in R e than in GPP. [ABSTRACT FROM AUTHOR]

• A sequential chemical extraction method was used to assay soil Si fractionations along different slope positions at the margin of a coastal wetland. • The Na 2 CO 3 -Si fraction accounted for 82%–90% of the labile Si in soil and sediment, mainly resulting from phytoliths and diatoms. • The storage of labile Si in sediment was significantly lower than that in soil. • With the influence of margin erosion under sea level rise, coastal wetland ecosystems will be an important source of Si to the estuary. Silicon (Si) and its biogeochemical cycling play an important role in maintaining the functions of terrestrial and marine ecosystems. However, the effects of sea level rise on the biogeochemical cycling of Si in coastal wetlands remain poorly understood. To explore this impact on biogeochemical Si cycling, we sampled a gradient from sediment to soil without the impact of tidal inundation in the Beidagang Wetland Nature Reserve, and then assayed non-crystalline Si (labile Si), including mobile Si (CaCl 2 -Si), adsorbed Si (Acetic-Si), Si bound to soil organic matter (H 2 O 2 -Si), Si occluded in pedogenic oxides/hydroxide (Oxalate-Si), and amorphous Si (Na 2 CO 3 -Si) fractions. Analytical results showed that the content of CaCl 2 -Si ranged from 13.0 to 53.3 mg kg−1 and the content of Acetic-Si ranged from 32.3 to 80.9 mg kg−1, both of which were lower in sediments compared to soils. The content of H 2 O 2 -Si (84.1–160.1 mg kg−1) and Oxalate-Si (306.6–655.1 mg kg−1) in the soil profiles showed non-significant variations along the sampling slope. The Na 2 CO 3 -Si fraction accounted for 82%–90% of labile Si in soil and sediment, mainly being contributed from phytoliths or diatoms. Diatoms were only detected in sediment profiles. The storage of labile Si in sediment was significantly (p = 0.0009) lower than the storage in soil, suggesting that the coastal wetland ecosystems are an important source of Si to the estuary. With future sea level rise and increased margin erosion, the inter-transformation processes among different Si fractions would likely be weakened to increase dissolved Si for marine diatoms. [ABSTRACT FROM AUTHOR]

[Display omitted] • Plant invasion strongly altered microbial residue C (MRC) accumulation. • The proportion of microbial residues in SOC increased after plant invasion. • Factors affecting MRC retention differed in different wetlands. • Fungal and bacterial residues responses were divergent in marsh and mangrove. • Fungal residues play a more predominant role in C sequestration in mangrove. Coastal wetlands play a critical role in global carbon (C) cycling while they are frequently challenged by exotic plant invasion. Microbial residues are increasingly recognized as the key constituent of stable soil organic C (SOC) but their responses to plant invasion in coastal wetlands remain largely unknown. Here, we investigated the effect of Spartina alterniflora invasion on microbial residues and their contribution to SOC in coastal wetland ecosystems with distinct native species (Phragmites australi s (marsh), Kandelia candel or Avicennia marina). S. alterniflora invasion differentially altered microbial residue C accumulation in three coastal wetlands. Overall, the invasion of S. alterniflora significantly increased microbial residue C accumulation (17.8%) in marsh with native P. australis , while it showed negative or minor impacts on mangroves with K. candel and A. marine communities, despite nuanced site-specific differences. The divergent responses of microbial residues could be related to changes in edaphic conditions and plant inputs along with plant invasion process. Invasion of mangrove and marsh communities by S. alterniflora triggered divergent fungal and bacterial residue responses, leading to contrasting fungal to bacterial C ratios in invaded marsh and mangrove wetlands. The proportion of microbial residues in SOC pool was enhanced to some extent, with an increase by 14.4%, 33.0% and 1.9% in P. australis , K. candel and A. marine wetlands, respectively, despite inconsistent variations in total SOC with S. alterniflora invasion. This has an implication for an elevated preservation of microbial residue C relative to the other organic C fractions in the invaded coastal wetlands. These findings highlight the importance of anabolic microbial C sequestration process for the maintenance of wetland soil C, which may impact C cycling and balance in coastal wetland ecosystem subjected to S. alterniflora invasion. Our work provides new insights to capture the properties and mechanisms of microbial control over C sequestration as well as SOC source configuration in coastal wetlands undergoing S. alterniflora invasion. [ABSTRACT FROM AUTHOR]

Abstract Abundance and allocation of organic matter, microbial community and their extracellular enzyme activities in soil aggregates had been increasingly concerned for mediating biogeochemical cycle and biodiversity provided by soil. In this study, the variations of soil organic carbon (SOC) accumulation and extracellular enzyme activities in aggregate size fractions of a rice paddy with long-term fertilization treatments were analyzed. These treatments included no fertilizer application (NF) as a control, chemical fertilizer only (CF), chemical fertilizer combined with swine manure (CFM), and with straw residue return (CFS). Hereby, size fractions of soil aggregates respectively of >2000 μm (large macroaggregates), 250–2000 μm (macroaggregates), 53–250 μm (microaggregates), 2–53 μm (fine aggregates), and <2 μm (fine clay) were obtained from undisturbed soil cores following water-dispersion and subsequent wet-sieving using a low energy separation protocol. Of these aggregate fractions, SOC and total nitrogen (N) contents were determined with wet digestion and soil enzyme activities were analyzed with substrate-induction incubation assays. Contents of SOC and total N were both highest in macroaggregates, followed by large macroaggregates and microaggregates, but depleted in fine microaggregates. Activities of carbon-degrading enzymes and N -acetyl-glucosaminidase were markedly enriched in soil aggregate fractions in size >53 μm. Whereas, activity of phenol oxidase was generally concentrated in soil aggregate fractions in size between 2 and 2000 μm while that of peroxidase in size between 53 and 2000 μm. Activity of β -glucosidase and cellobiohydrolase was both significantly elevated under CFS and that of peroxidase under CF, CFM and CFS treatments, but phenol oxidase activity was suppressed under CFS treatment. The enzyme activities of β -xylosidase, β -glucosidase, cellobiohydrolase and N -acetyl-glucosaminidase varied greatly (CVs up to 50%), rather with size fractions than with treatments, while those of α-Glucosidase and phenol oxidase varied moderately (CVs up to 35%), rather with treatments than with size fractions. However, those of acid phosphatase and peroxidase changed slightly (CVs up to 20%), regardless of both size fractions and fertilization treatments. Thus, abundance and enrichment of all the soil enzyme activities analyzed depended not on SOC although the overall extracellular enzyme activity was generally well correlated to SOC abundance, among aggregate size fractions. Only activities of carbon-degrading enzymes were greatly enriched in macroaggregates, which were seen reduced under combined organo/mineral fertilization. This study highlighted macroaggregates as biochemically active micro- hotspots and lower activities of carbon-degrading enzymes with SOC accumulation under long-term rational fertilization in the rice paddy. Graphical abstract Unlabelled Image Highlights • Water stable aggregates separated from a rice paddy under fertilization treatment • Enzyme activities changed inconsistently with C and N levels among size fractions. • Aggregate enzyme activities varied rather with size classes than with fertilization. • Much increased carbon degrading enzyme activities in macro-aggregates • Macroaggregates as biochemical hotpots, sensitive to fertilization practice [ABSTRACT FROM AUTHOR]

Abstract 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 Ca 2 UO 2 (CO 3) 3 0(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. Graphical abstract Unlabelled Image Highlights • Granite plutons may be a source of U and Th to the environment. • Sources and controls of U Th in 4 nemo-boreal catchments were evaluated. • Granite pluton and its erosional products were significant sources of U and Th. • Organic matter and alkalinity were the key drivers for U and Th export. • The export of U and Th may be enhanced by future climate change. [ABSTRACT FROM AUTHOR]

Abstract 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 ± 0.88 mg g−1, 5.36 ± 0.39 mg g−1, 3.81 ± 0.37 mg g−1 and 3.60 ± 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%. Graphical abstract Unlabelled Image Highlights • Grasslands play a crucial role in the Si cycle. • The content and storage of noncrystalline Si decrease with grassland degradation. • Grassland degradation can cause the decreased soil Si bioavailability. • Degradation can significantly affect the Si cycle in global grasslands. [ABSTRACT FROM AUTHOR]

Farmland top soils and soil profiles situated in the karst area of Guilin, Guangxi Zhuang Autonomous Region, southern China, reveal different degrees of heavy metal pollution, both in respect to the lateral as well as the vertical dimension. Pb isotope ratios clearly identify that heavy metal contributions to the soil represent the legacy of former Pb-Zn mining and smelting in the area. Depending upon soil properties, differences in the intensity of the vertical penetration of heavy metal pollution are discernible. Top soil coverage by local farmers provides little remediation. Consequently, hazardous conditions for the regional ecology, for agricultural usage and ultimately for human health remain in place. Based on chemical and isotopic results obtained, more effective remediation strategies need to be developed. [ABSTRACT FROM AUTHOR]

In recent years, biochar has been extensively studied as a sorbent for immobilizing contaminants and minimizing their bioavailability in soils. Few studies have been conducted to evaluate the interactions between biochar and compost in soils and their impact on degradation of organic contaminants. In the present study, soils with high organic carbon content (HOC) and low organic carbon content (LOC) were spiked with 100 mg·kg − 1 di-(2-ethylhexyl) phthalate (DEHP) amended with biochar derived from dead pigs, bamboo, and composted sheep manure. The soils were thereafter incubated for 112 days at 25 °C and periodically sampled for monitoring DEHP concentrations. Degradation of DEHP was described by a logistic model. Results showed that the initial degradation rates were slow, but accelerated after 14 days of incubation. The DEHP degradation rates were higher in the HOC soils than in the LOC soils over the incubation period. The half-lives of DEHP were shorter in the LOC soils treated with pig biochar, and bamboo/pig biochar plus compost than in the untreated soil. However, there was no significant difference in the half-lives of DEHP in the HOC control and treated soils. The differential effects of soil amendments on DEHP degradation between LOC and HOC soils could be explained by the properties of the organic amendments, soil pH and the organic carbon contents of the soils. [ABSTRACT FROM AUTHOR]

Biochar effect on the potential mobility of dibutyl phthalate (DBP), cadmium (Cd), and lead (Pb) in co-contaminated soils is not well investigated. A laboratory leaching study was conducted to evaluate the effect of biochars derived from bamboo (BB) and pig (PB) on the leachability of DBP, Cd, and Pb through soil columns packed with two soils with low or high organic carbon content (LOC; 0.35% C: HOC; 2.24% C) and spiked with DBP, Cd, and Pb. Application of PB to the LOC soil significantly ( P  < 0.05) reduced the leaching loss by up to 88% for DBP, 38% for Cd, and 71% for Pb, whereas its impact was insignificant in the HOC soil. The higher efficacy of PB in reducing the leaching of DBP, Cd, and Pb in the LOC soil than that of BB might be related to PB's higher specific surface area, surface alkalinity, pH, and mineral contents compared to those of BB. Co-contamination of Cd and Pb enhanced leaching of DBP in the LOC soil treated with PB, possibly by competition for the sorption sites. Leaching of DBP, Cd, and Pb were significantly ( P  < 0.05) higher in the LOC soil than in the HOC soil. This study revealed that the effectiveness of biochars was dependent on the soil organic carbon content. Application of PB to the LOC soil was effective in reducing the leaching risk of DBP, Cd, and Pb. [ABSTRACT FROM AUTHOR]

This study investigated interfacial reactions between aqueous Ce(III) and two synthetic nanosized Mn (hydr-)oxides (manganite: γ-MnOOH, and vernadite: δ-MnO 2 ) in the absence and presence of Nordic Lake fulvic acid (NLFA) at circumneutral pH by batch experiments and cryogenic X-ray photoelectron spectroscopy (XPS). The surfaces of manganite and vernadite were negatively charged (XPS-derived loadings of (Na + K)/Cl > 1) and loaded with 0.42–4.33 Ce ions nm −2 . Manganite stabilized Ce-oxidation states almost identical to those for vernadite (approximately 75% Ce(IV) and 25% Ce(III)), providing the first experimental evidence that also a Mn(III) phase (manganite) can act as an important scavenger for Ce(IV) and thus, contribute to the decoupling of Ce from its neighboring rare earth elements and the development of Ce anomaly. In contrast, when exposed to Ce(III)-NLFA complexes, the oxidation of Ce by these two Mn (hydr-)oxides was strongly suppressed, suggesting that the formation of Ce(III) complexes with fulvic acid can stabilize Ce(III) even in the presence of oxidative Mn-oxide surfaces. The experiments also showed that Ce(III) complexed with excess NLFA was nearly completely removed, pointing to a strong preferential sorption of Ce(III)-complexed NLFA over free NLFA. This finding suggests that the Ce(III)-NLFA complexes were most likely sorbed by their cation side, i.e. Ce(III) bridging between oxide groups on the Mn (hydr-)oxides and negatively-charged functional groups in NLFA. Hence, Ce(III) was in direct contact with the oxidative manganite and vernadite but despite that not oxidized. An implication is that in organic-rich environments there may be an absence of Ce(IV) and Ce anomaly despite otherwise favorable conditions for Ce(III) oxidation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Comparing Si-rich and Si-poor biochar proofs crucial roles of Si in Cd remediation. • Modification of Si-poor-precursor derived biochar demonstrably improves Cd removal. • A close relationship between dissolved-Si in biochar with Cd removal is observed. • Chemical as opposed to physical adsorption dominates Cd removal by Si-rich biochar. Silicon (Si) is the main inorganic component in biochar, but its key role in pollutant removal and the corresponding mechanisms are still unclear. In this study, rice husk and wood biochars were modified with calcium silicate (CaSiO 3) by ball milling method and used to investigate the removal efficiency of cadmium (Cd) from water and the removal mechanisms underlying the role of dissolved Si in biochar. The modification of biochar resulted in significant decrease in specific surface area, but increase in oxygen-containing functional groups (i.e., Si-O, Si-C). The Cd(II) removal efficiency was improved significantly by the modified wood biochar (M Si WB) from 23 % to 57 %, but only slightly (albeit from a higher baseline level) by the modified rice husk biochar (M Si HB) from 95 % to 98 %. It's the first time to quantitatively verify a significant (p < 0.05) positive relationship between the dissolved Si in biochar and Cd(II) removal capacity, proving the crucial role of Si in Cd(II) removal via complexation with Si-O-Si groups or cation exchange with silicate minerals. Quantitative analysis of elements and particle size distribution of biochars before and after adsorption showed that the cation exchange of Ca2+ and physical attachment of Cd compound precipitation dominated Cd(II) removal by wood-derived biochar, while the carbon-Si coupled structure of rice husk derived biochar plays an important role in the chemical adsorption of Cd(II). This study provides an important reference for understanding the role of Si in biochar to improve Cd remediation. [ABSTRACT FROM AUTHOR]