Enhancing Soil Quality and Bacteria Diversity by Increasing Soil Organic Matter and Microbial Activity Under Biochar Application

Maintaining healthy soil is fundamental to sustainable agriculture, and soil organic carbon (SOC) is a key indicator of soil health. Although the application of straw and biochar is known to enhance SOC, their specific effects on the soil health index (SHI) and the underlying microbial drivers across different soil layers—particularly in the 20–40 cm subsoil layer—remain insufficiently quantified. This study therefore aimed to evaluate the effects of straw and biochar application on the SHI and associated microbial indicators. The experiment included five treatments: conventional planting with straw removed (CK), homogeneous incorporation of mechanically crushed straws into the 0–20 cm topsoil layer (SR), biochar burial at 40 cm (SD), combined application of returning mechanically crushed straws in the topsoil and biochar burial at 40 cm (SB), and biochar incorporation into the topsoil combined with burial of entire maize plants at 40 cm (BS). Results showed that SR, SD, SB and BS all enhanced SHI values compared to CK in the topsoil, with BS exhibiting the largest increase (23.07%). In the subsoil, SB exhibited the highest SHI, representing increases of 64.52% and 18.60% compared with CK and BS, respectively. This was attributable to the greater enhancement of key subsoil parameters under SB than under BS, including microbial biomass carbon (17.52%), microbial biomass nitrogen (24.20%), leucine aminopeptidase (91.20%), and SOC (181.38%). Furthermore, surface straw application reduces fungal diversity, whereas biochar application in the subsoil significantly enhanced bacterial diversity. Biochar improved SHI both directly through SOC increase and indirectly via the increase in microbial activity and enzyme content mediated by SOC.