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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]

• Soil quality in the karst trough valley was mostly restricted by microbial biomass. • Soil quality of the anti-dip slope was better than that of the dip slope. • SQI-NL model is suitable for soil quality evaluation on erosional slopes. Typical bedrock strata dips and land use types can significantly influence the soil quality in karst trough valleys. However, data on the influence of bedrock strata dips on the assessment of soil quality based on the soil quality index in karst trough valleys are limited. Thus, the aim of this study was to investigate the impact of different land use types and bedrock strata dips on soil quality and to evaluate the suitability of soil quality assessment methods for karst trough valleys. Five land-use types were selected as research objects for each dip and anti-dip slope, and a total data set (TDS) consisting of soil physico-chemical properties and soil microbial property indices was collected. The minimum data set (MDS) was constructed using principal component analysis (PCA) and norm values, and two evaluation methods, i.e., linear (SQI-L) and non-linear (SQI-NL) were used to assess the soil quality in the study area. The results showed that the order of weight of the MDS was urease activity > protease activity > silt > soil bulk density > total phosphorus. The overall soil quality obtained from the SQI-L and SQI-NL models exhibited similar results for the dip slope; however, there were differences in the details of the anti-dip slope. The soil quality of the anti-dip slope was better than that of the dip slope (P < 0.05) and the soil quality indices of the forest and grassland were significantly higher than those of the other land use types (P < 0.05). Furthermore, the SQI-NL model can accurately replace the SQI-L model for soil quality evaluation based on its high variation interval and variable coefficients. Overall, our results suggest that the SQI-NL model is suitable for soil quality evaluation on dip and anti-dip slopes, and returning farmland to forest is an effective land-use change in the karst trough valleys. [ABSTRACT FROM AUTHOR]