Biochar, which contains abundant nutrients like phosphorus (P), is considered a potential source of nutrients for plant use after its application in soil. However, the localized distribution of P in the vicinity of biochar (that is, the charosphere) is poorly understood. In this study, the biochars derived from pinewood (PWB), maize straw (MSB), and chicken manure (CMB) were granulated and investigated for their short-term charosphere effect, using an integrated imaging technique in situ. The results showed that biochars significantly elevated the soil pH and P concentration around their granules, leading to the development of a charosphere over several days. Both pH and P were distributed in a gradient in the charosphere: the values were highest where the biochars were located and declined with increasing distance from the biochar. A highly positive correlation (p < 0.01) was found between the pH, electric conductivity (EC), ash content of the biochar, and the scope of the pH gradient in the charosphere. Moreover, the scope of the P gradient was positively related to the pH and EC values of the biochar as well (p < 0.05). The charosphere effect was prominent around the CMB and MSB granules because of their high ash content, an especially soluble component. The P diffusion in the charosphere was inferred to be influenced by the soil pH as well as the endogenous P species in the biochars. These findings provide new insight into the localized effect of biochars in soil, which is conducive to understanding the nutrient availability to plants after biochar application.

Both cadmium (Cd) contamination and phosphorus deficiency pose major constraints to sustainable crop production. This study loaded highly efficient Cd-tolerant phosphate-solubilizing bacteria (CdTPSB) onto calcium- and phosphorus-rich pig bone biochar (PBB) to construct a functional composite (MPBB), which was subsequently applied in a Brassica napus L. pot experiment. The results revealed that the CdTPSB possessed strong capabilities in phosphate solubilization (383.67–520.19 mg L⁻¹), Cd immobilization (64.67–66.67%), IAA production (113.24–114.27 μg mL⁻¹), and siderophore activity (65.02–89.08%). Batch experiments further showed that MPBB’s efficiencies in phosphate solubilization and Cd immobilization were significantly enhanced, which was putatively attributable to improved CdTPSB colonization and viability as well as to the activation of PBB functional groups. In the pot experiment, relative to the control treatment, MPBB significantly decreased the soil diethylenetriaminepentaacetic acid-extractable Cd content by 50.47% while increasing the soil labile phosphorus content by 3.32 times (p ≤ 0.05). This was correlated with the enrichment of Cd-immobilizing and phosphate-solubilizing bacteria, the upregulation of phosphate-solubilizing functional genes (pqqC and phoD), and enhanced soil enzyme activity. Consequently, Cd content in the shoots of Brassica napus L. was further decreased by 74.00%, which was correlated with increased polysaccharide content and enhanced Cd retention in root cell walls. Overall, this study highlighted MPBB’s potential as an eco-friendly Cd remediation agent and phosphate fertilizer to support the sustainable production of Brassica napus L.