Biochar application alters soil properties and microbial gene profiles in a continuous cassava cropping system

Continuous cassava cropping can lead to soil degradation and disturbances in soil microbial functions, threatening the sustainability of cassava production systems. To address these challenges, this study examined the effects of biochar on soil physicochemical properties and microbial functional profiles in a continuous cassava cropping system. Two biochar application rates (0 and 3 Mg ha^-1) were applied, and metagenomic sequencing was conducted to evaluate microbial community composition and functional genes related to carbon and nitrogen cycling in rhizosphere and bulk soils. The results demonstrated that biochar was associated with higher soil pH, soil organic matter, and available nutrient content, with a stronger effect in rhizosphere soil. Biochar application altered the genetic potential of microbial communities, particularly in the rhizosphere soil. In terms of functional categories, biochar was related to higher gene abundances in "homologous recombination" and "DNA replication" categories (Kyoto Encyclopedia of Genes and Genomes (KEGG) database), as well as the "replication, recombination, and repair" category (evolutionary genealogy of genes: Non-supervised Orthologous Groups (eggNOG) database) in rhizosphere soil. Biochar also affected the abundance of carbon cycling genes, particularly in the rhizosphere. The abundance of the aerobic respiration-related gene coxA was increased, while the abundance of the anaerobic fermentation gene L-lactate dehydrogenase (LDH) was decreased. Additionally, in rhizosphere soil, biochar significantly increased the abundance of norB (denitrification), GDH2 (nitrogen mineralization), and nifD (nitrogen fixation), while decreasing the abundance of genes involved in nitrogen assimilation (gltB), assimilatory nitrate reduction (nirA), nitrogen mineralization (cynS), and nitrogen uptake (nrtA, nasF, cynA, nrtC, and nasD). Together, these results suggest that biochar application may enhance nutrient availability and reshape microbial functional potential primarily in the cassava rhizosphere, providing field evidence for biochar use in continuous cassava cropping.