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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]

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