Intensive agriculture has intensified soil acidification in southern China, threatening crop productivity and ecosystem sustainability. Biochar can neutralize acidity, improve pH buffering, and enhance nutrient retention and microbial habitat in acidic soils. Accordingly, we produced biochars from pruned eucalyptus (ABC), camphora (ZBC), and guava (FBC) branches via pyrolysis at 500 °C. The three biochars were characterized by elemental analysis, Fourier Transform Infrared Spectroscopy (FTIR), and SEM (Scanning Electron Microscopy), and their effects on soil properties, enzyme activities, and bacterial communities were evaluated through a 56-day incubation experiment in an acidified, continuously cropped soil. Physicochemical characterization revealed that ZBC and FBC possessed more oxygen-containing functional groups and greater potential for pH buffering and nutrient release, whereas ABC exhibited higher aromaticity and structural stability. Biochar significantly increased soil pH by 0.62–1.42 units and improved nutrient availability and carbon pools (p < 0.05). Additionally, 4% ZBC increased urease and sucrase activities by 21.54% and 79.34%, respectively, while 2% FBC increased cellulase activity by 25.99%. High-throughput sequencing identified Acidobacteria and Proteobacteria as the dominant phyla; ZBC and FBC at 0.5% and 2% significantly increased Shannon and Chao1 indices. Redundancy analysis indicated that available potassium, pH, soil organic carbon, urease, sucrase, and cellulase were the primary drivers of bacterial community variation and positively associated with carbon-cycling phyla. These findings demonstrate that feedstock-specific biochar properties critically regulate soil biogeochemical processes, offering a sustainable strategy to remediate acidified soils and valorize agroforestry residues.

Soil calcium is a vital component in plant growth and soil health. Earthworm activities impact metal distribution and speciation a lot by changing soil pH. Nevertheless, little is known about how ecological earthworm species, particularly in Terra Rossa soil, affect soil Ca speciation distribution. This research examined the effects of the activities of four different earthworm species (epigeic species Eisenia fetida (noted as EF), endogeic species Amynthas robustus (noted as AR) and Pontoscolex corethrurus (noted as PC), anecic species Amynthas aspergillum (noted as AA)) on Ca speciations (water-soluble (Ca_Ws), exchangeable (Ca_Ex), acid-soluble bound (Ca_Ac), organic-bound (Ca_Or), and residual (Ca_Re)), soil pH, the release contents of exchangeable cations (Ca, Mg, K, and Na), total calcium (Ca_Total) contents, total nitrogen (TN) contents, soil organic carbon (SOC) concentrations, cation-exchange capacity (CEC), and NIRS spectral characteristics in Terra Rossa soil for 40 days under lab conditions. In contrast to control soil, 108.3%, 158.3%, 91.7%, and 125.0% of Ca_Ws contents in casts and 116.6%, 108.3%, 58.3% and 91.6% of Ca_Ws in uningested soil increased significantly with the inoculation of EF, PC, AR, and AA, respectively. In addition, compared with control, for casts, the contents of exchangeable Ca, Mg, K, CEC, and available-K were significantly increased in the presence of EF, PC, AR, and AA, respectively. In the casts of EF, PC, AR, and AA, soil pH values declined by 0.72, 0.80, 0.45, and 0.60 units relative to control soil, while they decreased by 0.65, 0.84, 0.34, and 0.59 units in uningested soil. The soil inoculated with PC had higher soil pH values and Ca_Ws contents than those with the other three earthworm species. Principal component analysis revealed significant differences in soil pH, Ca speciation, NIR spectra, and exchangeable base cation release between casts and uningested soil in treatments with EF, PC, AR, and AA inoculation. These findings expand, for the first time, to the ecological functions of earthworm species, especially for PC, demonstrating a capacity to alter soil Ca speciation, decrease soil pH, affect the exchangeable base cations’ release, and participate in and regulate the geochemical circulation processes in limestone regions.