Biochar and drilosphere soil synergistically form a low-cadmium, high-nutrient remediation hotspot: Insights from microbial functional gene and enzymatic stoichiometry analyses.

The drilosphere is a biogeochemical hotspot, but its role in biochar-assisted remediation of cadmium (Cd)-contaminated soil remains unclear. Comparisons were made between fresh (RS) and aged rice straw (ARS) biochar treatments at different Cd contamination levels to assess their effects on the drilosphere microenvironment. The results showed that ARS biochar was more effective in forming a low‑Cd, high‑nutrient hotspot, reducing total Cd in burrow wall soil (BWS) by 13.42% and increased TOC, TN, and TP by 17.89%, 76.16%, and 132.36% (P < 0.05). Enzyme activity followed BWS > lightly disturbed soil > undisturbed soil. Under low Cd, ARS biochar decreased microbial C limitation by 5.21% but increased N limitation by 8.81% (P < 0.05); no effect at high Cd. RS biochar increased bacterial α-diversity (Chao1) (2.41%, P < 0.05), decreased archaeal (14.00%) and fungal (2.16%) diversity, enriched Myxococcota, Gemmatimonadota, Acidobacteriota (72.64%-216.67%), and reduced Firmicutes (47.83%). ARS biochar decreased drilosphere pH and altered eco-enzymatic stoichiometry, making archaeal and fungal α-diversity significantly correlated with C-N-P enzyme activities. It enriched Acidobacteriota (116.30%) and increased Firmicutes, Actinobacteriota, Planctomycetota (60.08%-75.52%), showing a distinct pattern (Acidobacteriota, Firmicutes) contrasting with RS biochar (Myxococcota, Gemmatimonadota). Under low Cd, both RS and ARS biochar promoted C fixation (AA/GT genes, 3.98%-5.75%) and inhibited C decomposition (PL/GH genes, 4.00%-11.37%); compared to RS biochar, ARS biochar significantly enhanced denitrification (4.47%) and nitrification (14.41%), inhibited anammox (45.17%), and upregulated P metabolism pathways (2.54%-6.73%). Under high Cd, both RS and ARS biochar shifted to promoting C decomposition and inhibiting C fixation; ARS biochar maintained N fixation capacity (19.8%) and enhanced organic P hydrolysis (9.99%) (P < 0.05), indicating that ARS biochar retains key nutrient cycling potential under Cd stress. Thus, biochar aging status is crucial for synergistic drilosphere remediation, offering a strategy for simultaneous Cd immobilization and soil fertility improvement.