Cadmium-induced disruptions in soil nitrogen cycling: Unraveling microbial impacts, functional gene dynamics, and...

Cadmium (Cd) contamination in soils poses a serious risk to agricultural production and ecological health. It enters agricultural soils as a result of human activities such as phosphate fertilizer use, industrial discharge, and natural weathering of Cd-enriched parent substrates. After reaching the soil, it interferes with nitrogen (N) cycling by reducing microbial biomass, enzyme activity, and the expression of functional genes (amoA, nirk/nirS, nosZ), affecting N transformations, which ultimately control soil N availability and loss. Moreover, Cd stress downregulates nosZ gene expression, which limits complete reduction of N₂O to N₂, thus increasing N₂O emissions. To reduce Cd stress, biochar (BC) has been recognized as a potential strategy due to its multidimensional role in both Cd immobilization and alleviating Cd-induced disturbances in the soil N cycle. BC, through different mechanisms like ion exchange, precipitation, and surface complexation, decreases Cd bioavailability and nurtures microbial habitats. Furthermore, BC promotes the restoration of N-cycling microbial populations and functional genes, which enhances N retention, lowers N₂O emissions, and increases plant-accessible N. The current review summarizes the recent research findings on how Cd toxicity affects N cycling processes and its associated microbes and genes and further emphasizes the dual role of BC in immobilizing Cd and restoring N cycling for sustainable soil remediation, greenhouse gas mitigation, and crop production. Moreover, the review also highlights the critical knowledge gaps, particularly regarding the effect of Cd on gross N transformations using ¹⁵N tracing, that need to be addressed in future studies.