Biochar-induced charosphere microenvironment modulates soil cadmium bioavailability and wheat uptake

When biochar is mixed with soil, it creates a distinct microenvironment, charosphere, within a few millimeters of each particle. Here, physical, chemical, and biological processes are decoupled from those of the bulk soil, providing a local control point for heavy metal mobility and plant uptake. These spatiotemporal effects were examined in a Cd-contaminated soil using a novel microcolumn that allowed 2-mm-resolution sampling for 28 d after amendment with wheat-straw biochar (450 °C). Relative to the unamended bulk (> 10 mm), the charosphere (≤ 8 mm) rapidly became more alkaline (pH increasing by 0.01–0.36 units) and carbon-rich (DOC increasing by 14.7%–69.1%). The steepest changes occurred within 2 mm of the particle surface, where DTPA-extractable Cd reduced by 3.2%–14.2%. The magnitude of Cd immobilization increased with biochar rate (2.5–7.5 wt%) and declined sharply with distance. Surface functional group chemistry, DOC release, and proximity jointly governed these gradients. Cd concentrations in wheat shoots and roots decreased by 5.3%–28.3% and 2.3%–46.3%, respectively, across the 2–8 mm zone vs the 10 mm control. Thus, microscale placement of biochar, rather than bulk soil loading, dictates its capacity to limit Cd uptake, and higher application rates intensify the effect by expanding the reactive charosphere volume.