Effects of biochar-based amendments on enzyme activity, microbial biomass, and bacterial communities in mildly cadmium-contaminated paddy soils.

Mild cadmium (Cd) contamination seriously restricts the safe utilization of paddy soils in southern China. Biochar materials can immobilize soil Cd, yet a single biochar often exhibits limited comprehensive improvement effects. At present, field studies systematically revealing how composite biochar amendments regulate soil enzyme activity, microbial biomass, and bacterial communities remain insufficient. This study aims to clarify the responses of soil enzyme activity, microbial biomass, and bacterial community structure to compound biochar amendments under rice-rape rotation field conditions. We hypothesize that composite amendments consisting of biochar, rape organic fertilizer (ROF), calcium magnesium phosphate fertilizer (CMPF), and fulvic acid (MFA) can optimize soil microhabitat, reduce Cd bioavailability, activate nutrient cycling-related enzymes, and stabilize soil bacterial community diversity. Three different types of biochar, maize straw biochar, bamboo biochar, and coconut shell activated carbon, were mixed with ROF, CMPF, and MFA according to certain proportions, respectively. The results revealed that biochar-based amendments significantly increased urease activity by 19.95%-42.09% (p < 0.05); however, acid phosphatase activity was reduced. Additionally, sucrase, nitrate reductase, and cellulase activity did not change significantly compared to the control (CK). Biochar-based amendments significantly increased soil microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) by 30.10%-64.16 and 12.07%-123.68% (p < 0.05), respectively. Maize straw and bamboo biochar amendments significantly increased biomass phosphorus (MBP), but coconut shell activated carbon amendments decreased MBP. Both Chao 1 and Shannon indices of bacterial communities were higher in biochar-based amendments than in CK. Proteobacteria, Chloroflexi, Acidobacteriota, and Actinobacteriota dominated across all treatments. Relative abundances of Proteobacteria and Actinobacteriota slightly increased, whereas Acidobacteriota decreased. Chloroflexi was enriched by maize straw and bamboo biochar amendments but suppressed by coconut shell activated carbon. At the genus level, biochar-based amendments significantly increased the relative abundance of Thiobacillus, with minor influences on other genera. In conclusion, the application of composite biochar-based amendments improves paddy soil quality and promotes the richness and diversity of soil bacterial communities. This study supplements the microecological mechanism of biochar-based amendments technology, offering theoretical support and technical reference for the safe utilization of mildly Cd-polluted paddy soils in Southwest China.