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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.

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.

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.

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%.

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.

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.

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.