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 (BC_SE) and a hydroxyapatite engineered biochar (BC_HA), 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.

炭层是生物炭和土壤之间的界面微区，在将生物炭应用于土壤后，在生化过程中起着至关重要的作用。然而，随着生物炭年龄的增长，炭层随着时间的发展以及其内部和周围的pH动力学仍知之甚少。在这项研究中，两种具有独特特征的生物炭，即原始生物炭（BC _SE）和羟基磷灰石工程生物炭（BC _HA）。），进行了人工理化老化处理。使用平面光电技术，量化了新鲜和老化生物炭对土壤pH的局部影响，并在微观尺度上显示了时空变化。使用相关性和冗余分析来评估生物炭特性及其控制层之间的关联，以鉴定生物炭特性。

通过施用生物炭可引起对土壤pH的显着局部影响，生物炭颗粒周围的pH梯度会在24小时内逐渐形成。新鲜的生物炭产生了半径范围为1.13毫米至1.63毫米的炭层。但是，时效处理将大气层的半径稍微缩小到1.08-1.12 mm。炭层中pH的时空变化与生物炭特征密切相关。老化处理导致生物炭中的氧气（91％–349％）和有效磷（670％–1094％）含量大量增加，但灰分含量（42％–45％）以及pH值（26％）降低–54％）和电导率（EC）（17％–64％）值。在老化过程中，生物炭的孔结构发生了变化，矿物质损失了，因此与新鲜的生物炭相比，老化的生物炭的比表面积要小得多。相关性和冗余度分析表明，生物炭EC值是决定其中炭层半径和pH值的主要因素。这项研究是第一个以高分辨率可视化和比较源自不同新鲜和老化生物炭的炭层的研究。结果提供了对大气层pH动力学的新见解，以及施用到土壤后随着生物炭老化的元素的可用性，这对于理解植物的养分利用率和土壤污染物的迁移性很重要。