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Aims: Silicon (Si) is an essential element for siliceous organisms, including macrophytes, phytoplankton, and diatoms. Coastal wetlands are critical for bridging the river-estuary-ocean continuum to drive the biogeochemical Si cycles. However, it remains unclear about the contents and distribution patterns of bioavailable Si in soils under various scenarios, and their environmental controls in coastal wetlands. Methods: We conducted a nationwide sampling campaign across ca. 5000 km of coastal wetlands, covering temperate, subtropical, and tropical climates in China, and quantified plant available Si (ASi) using calcium chloride extractable Si (Si-CaCl2). Results: S. alterniflora invasion did not significantly influence ASi content. In contrast, ASi content in the subtropical zone was higher than in the both temperate (medium) and tropical zones (lowest). ASi content was significantly positively correlated with nutrients (i.e., soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP)), soil water content (SWC), clay and silt contents, but negatively with soil bulk density (BD) and sand content. ASi content, in detail, increased with increasing pH (pH < 7) but decreased with increasing pH (pH > 7), showing a quadratic function relationship. Conclusions: ASi in coastal wetlands was predominately directly influenced by pH, particle size, and nutrients of coastal soil, while vegetation compositions and plant-derived lignin (Λ8) inputs illustrated a minor effect on ASi patterns. Mean annual temperature (MAT) and precipitation (MAP) indirectly regulated ASi content via affecting soil geochemistry and nutrients distribution. Taken together, ASi distribution are mostly controlled by primary pedogenesis and specific weathering processes in China's coastal wetlands. [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]

Rivers link terrestrial ecosystems and marine ecosystems, and they transport large amounts of substances into oceans each year, including several forms of silicon (Si), carbon (C), and other nutrients. However, river damming affects the water flow and biogeochemical cycles of Si, C, and other nutrients through biogeochemical interacting processes. In this review, we first summarize the current understanding of the effects of river damming on the processes of biogeochemical Si cycle, especially the source, composition, and recycling process of biogenic silica (BSi). Then, we introduce dam impacts on the cycles of C and some other nutrients. Dissolved silicon in rivers is mainly released from phytolith dissolution and silicate weathering. BSi in suspended matter or sediments in most rivers mainly consists of phytoliths and mainly originates from soil erosion. However, diatom growth and deposition in many reservoirs formed by river interception may significantly increase the contribution of diatom Si to total BSi, and thus significantly influence the biogeochemical Si, C, and nutrient cycles. Yet the turnover of phytoliths and diatoms in different rivers formed by river damming is still poorly quantified. Thus, they should be further investigated to enhance our understanding about the effects of river damming on global biogeochemical Si, C and nutrient cycles. [ABSTRACT FROM AUTHOR]

Aims: The natural abundances of stable carbon (C) and nitrogen (N) isotopes (δ13C and δ15N) are extensively used to indicate the C and N biogeochemical cycles at large spatial scales. However, the spatial patterns of δ13C and δ15N in plant-soil systems of grasslands in northern China and their main driving factors across regional climatic gradient are still not well understood. Methods: We measured plant and soil δ13C and δ15N compositions as well as their associated environmental factors across 2000 km climatic gradient (-0.2 to 9 °C; 152 to 502 mm) in grasslands of northern China. Results: The soil δ13C and δ15N values in surface were lower than those in bottom for temperate typical steppe but had no significant differences for temperate meadow steppe and temperate desert steppe. Soil δ13C values declined with increasing soil organic carbon (SOC) but increased as mean annual temperature (MAT). These changes were attributed to the microbial decomposition rate. The δ15N values in soil and plant were negatively correlated with MAT and mean annual precipitation (MAP), which were mainly related to the low soil organic matter mineralization rate and the shift of dominant species from C4 to C3. Conclusions: Our results indicate the spatial patterns and different influencing factors on δ13C and δ15N values along the climatic gradient in grasslands of northern China. The findings will provide scientific references for future research on the C and N biogeochemical cycles of temperate grasslands. [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]

• 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]

• 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]

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]

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]

• 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]