Biochar engineering and ageing influence the spatiotemporal dynamics of soil pH in the charosphere

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The charosphere, the interface microzone between biochar and soil, plays a vital role in biochemical processes following biochar application to soil. However, the development of the charosphere over time, and the pH dynamics within and around it, remain poorly understood as biochar ages. In this study, two kinds of biochars with distinct characteristics, a pristine biochar (BCSE) and a hydroxyapatite engineered biochar (BCHA), were subjected to artificial physicochemical ageing treatment. The localized impact of the fresh and aged biochars on soil pH were quantified, and spatiotemporal changes at the microscale visualized, using the planar optode technique. Association between the biochar characteristics and their charospheres was assessed using correlation and redundancy analyses to identify controls on charosphere properties.Significant localized effects on soil pH were induced by biochar application, with pH gradients around biochar particles forming gradually over 24 h. Fresh biochars generated charospheres with radii ranging from 1.13 mm to 1.63 mm. However, ageing treatment slightly narrowed the charosphere radius to 1.08–1.12 mm. The spatiotemporal variations of pH in the charosphere were closely related to biochar characteristics. Ageing treatment resulted in large increases in the oxygen (91%–349%) and available phosphorus (670%–1094%) contents of biochar, but decreases in ash content (42%–45%), as well as pH (26%–54%) and electrical conductivity (EC) (17%–64%) values. The pore structure of biochar was altered and minerals were lost during the ageing process, so that aged biochars had much smaller specific surface area compared to the fresh biochars. Correlation and redundancy analyses revealed that the biochar EC value was the main factor determining the charosphere radius and pH within it. This study is the first to visualize and compare the charosphere derived from different fresh and aged biochars at a high resolution. The results provide new insight into the pH dynamics of the charosphere and the availability of elements as biochar ages following application to soil, which are important for understanding nutrient availability to plants and mobility of soil contaminants.