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Background and aims. Although Si is recognized as a beneficial element for crops, the determination of plant-available silicon (PAS) in soils has become challenging. A correlation between pH and PAS indicators has been reported in the literature due to the higher degree of weathering of acidic soil. We tested this hypothesis in the cultivated rice fields of Karnataka State, India, which exhibit gradients of pH and climate. Method. Two hundred surface soil samples were collected from rice fields representing nine of the ten agro-climatic zones (ACZs) defined for Karnataka. We analyzed the Si extracted by calcium chloride (Si CC) and the Si extracted by acetate-acetic acid (Si AA) as PAS indicators. The samples were analyzed for particle size distribution, pH, electrical conductivity and cation exchange capacity. Forty subsamples were selected for mineralogical and chemical analysis. Results. PCA of the 200 samples showed that PAS indicators and pH were positively correlated. Si AA was also positively correlated with electric conductivity (EC), CEC, and the silt fraction and negatively correlated with the sand fraction. A separation of the data arbitrarily made at pH 7.5 showed that below 7.5, the correlation between PAS indicators and pH was better than considering the whole 200 samples. The distribution of Si AA with pH matched the curve of adsorbed Si given in the literature. Soils characterized by low pH and high contents of sand, SiO 2 , Zr and Hf, showed a higher degree of weathering. They were preferentially located along the coast, where the rainfall is the most abundant. The depletion of PAS indicators was also in good agreement with the predominance of kaolinite instead of smectite, which characterizes a higher degree of desilication. Conclusion. We demonstrated that the correlation between pH and PAS indicators is explained by natural Si depletion (weathering intensity) and the effect of Si adsorption. We suggest that at pH values up to 7.5, pH can be used as a proxy for PAS in similar types of pedo-climatic conditions.

Biochar is known to decrease the soil acidity and in turn enhance the plant growth by increasing soil fertility. Major objective of the present work was to understand the effect of biochar treatment on alleviation of soil aluminium (Al) toxicity and its role in enhancing plant growth parameters. Soil incubation study was conducted to understand the effect of biochar (Eucalyptus wood, bamboo, and rice husk) on soil pH, soluble and exchangeable Al in soil with and without Al addition. Another independent pot experiment with rice crop (Oryza sativa L. var. Anagha) was carried out for 120 days to examine the effect of biochars on soil properties and growth parameters of rice plants. Wood biochar application to soil at 20 t ha −1 was found to be highly consistent in decreasing soil acidity and reducing soluble and exchangeable Al under both studies. We conclude that wood biochar at higher dose performed better in reducing soluble and exchangeable Al in comparison to other biochars indicating its higher ameliorating capacity. However, rice husk biochar was effective under Al untreated soil, indicating the role of Si-rich biochars in enhancing plant growth. Soil acidity in many parts of the world poses a significant challenge to crop productivity. Around 30% of the total land area in the world and more than 50% of the world's potentially arable area fall under acidic soils 1-3. Approximately 30% of arable land in India are acidic in nature, resulting in low crop yield 3. Acid soils have low pH values (< 5.5 or 6) and are usually associated with the severe aluminium (Al) toxicity to plants. Aluminium is mainly in the form of insoluble silicate or oxide in neutral soils (pH = 6.5-7.5). However, low soil pH (< 5) leads to the solubilization of Al, primarily to the phytotoxic form of Al 3+ in soil solution 4,5. Aluminium toxicity has been reported to cause direct inhibition of root elongation and further interfere with uptake of plant nutrients 6,7. Aluminium toxicity can therefore be considered a primary limiting factor in acid soils for plant growth and development. Consequently, focusing on worldwide acid soil remediation is crucial to enhance crop yield and thus alleviating world hunger. Liming has been the prominent approach for amending acid soil. However, biochar application as soil amendment has been receiving lot of attention, for many reasons such as neutralizing acidity in soil, creating a carbon (C) sink to mitigate global warming, increasing soil water holding capacity, reducing greenhouse gas emissions and stabilizing mobile heavy metals, pesticides and other organic pollutants in soil 8-12. Biochar is carbon rich material obtained by pyrolysis of biomass with little or no oxygen 13. Pyrolysis of plant biomass normally results highly alkaline biochar 13-15. However, alkalinity varies with respect to feedstock properties used for the biochar production. Greater the alkalinity of biochar, greater is the reduction in acidity 16. Addition of biochar to nutrient poor soil has been reported to enhance nutrient avail...

Plants’ ability to take up silicon from the soil, accumulate it within their tissues and then reincorporate it into the soil through litter creates an intricate network of feedback mechanisms in ecosystems. Here, we provide a concise review of silicon’s roles in soil chemistry and physics and in plant physiology and ecology, focusing on the processes that form these feedback mechanisms. Through this review and analysis, we demonstrate how this feedback network drives ecosystem processes and affects ecosystem functioning. Consequently, we show that Si uptake and accumulation by plants is involved in several ecosystem services like soil appropriation, biomass supply, and carbon sequestration. Considering the demand for food of an increasing global population and the challenges of climate change, a detailed understanding of the underlying processes of these ecosystem services is of prime importance. Silicon and its role in ecosystem functioning and services thus should be the main focus of future research.

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