KOH-modified hydrochar with nitrogen-fixing bacteria alleviates aluminum toxicity and mitigates acidification-driven nitrogen loss in soybean rhizosphere.

Soil acidification severely constrains crop production by reducing nitrogen bioavailability. Although hydrochar shows promise for soil remediation, its application in acidified soils remains challenging due to its intrinsic acidity. Here, we developed an innovative strategy combining KOH-modified hydrochar (KOHC) with nitrogen-fixing microbial aggregates (MGs) and investigated its effects on rhizosphere acidification and nitrogen cycling in soybean. Pot experiments and multi-omics analyses revealed that 2% KOHC-MGs treatment significantly promoted the aggregation of rhizosphere nitrogen-fixing microbes and enhanced their metabolic functions. This treatment increased rhizosphere soil pH by 18.2%, reduced exchangeable Al3 + and H+ content by 80.5% and 79.6% respectively, while upregulated key nitrogen‑cycle genes (nifH, hao, nxrA, nrfA) while suppressing denitrification, resulting in 96.2% higher NH4+‑N retention than the control. Mechanistically, KOHC's porous structure and functional groups promoted microbial colonization and amino‑acid‑mediated proton consumption, while directly neutralizing acidity and immobilizing Al3+, establishing a dual abiotic-biotic loop that outperformed liming by concurrently mitigating acidification and enhancing nitrogen utilization, as reflected in improved plant growth and enzyme activities. Our findings demonstrate that integrated material-microbe strategies offer a sustainable alternative to conventional liming for restoring nitrogen dynamics and alleviating acidification in degraded agroecosystems, with implications for reducing fertilizer dependence and greenhouse gas emissions.