Biochar-driven shifts in the soil bacterial community structure of degraded semiarid soils

Abstract Low biochar rates (5 Mg ha −1 ) increased bacterial richness and diversity in degraded soil. Biochar origin controlled microbial assembly and niche specialization patterns. Sewage-sludge biochar favored stress-tolerant and metal-adapted bacterial taxa. Cashew bagasse biochar enhanced nitrogen cycling and chemoheterotrophic functions. Network complexity peaked at intermediate biochar doses and declined at high rates. Biochar has emerged as a nature-based solution to restore degraded drylands; however, its effects on soil microbiomes in semiarid and desertification-prone environments remain less understood. Here, we evaluated how biochar origin and application influence the structure of bacterial communities in a highly degraded Planosol from the Brazilian Caatinga biome. A controlled experiment was conducted using two biochar types, cashew bagasse-derived biochar (CB) and sewage-sludge biochar (SSB), applied at four doses (5, 10, 20, and 40 Mg ha −1 ), plus an unamended control (0 Mg ha −1 ), followed by 90 days of maize cultivation. Biochar changed bacterial community composition, with the emergence of distinct microbial assemblages depending on biochar type and dose. A single low dose (5 Mg ha −1 ) of biochar was sufficient to enhance bacterial richness and Shannon diversity, while SSB promoted the dominance of stress-tolerant taxa (e.g., Bacillus ). Functional prediction and co-occurrence networks revealed that CB promoted functions related to nitrogen cycling and chemoheterotrophy, whereas SSB favored metal-tolerant and hydrocarbon-degrading taxa. Network complexity increased with biochar addition, especially at intermediate SSB doses. Overall, CB promoted more stable ecological responses, while sewage-sludge biochar caused stronger shifts. Our results demonstrate that biochar origin and application rate are critical to microbial restoration, i.e., moderate doses stimulate diversity, ecological specialization, and soil multifunctionality, whereas higher inputs, especially of nutrient- and metal-rich biochars, may compress functional traits. Plant-derived biochars, particularly CB, demonstrated potential for restoring soil bacterial structure and resilience in degraded semiarid landscapes when applied at appropriate rates.