Leonardite-aged biochar with soil inoculation of arbuscular mycorrhizal fungi enhances soil health, microbial activity, and lettuce antioxidant-biometric traits in sandy soil

This study evaluated the potential of leonardite-aged biochar (ABC), applied alone or inoculated with arbuscular mycorrhizal fungi (AMF), to improve sandy soil health within a shorter time compared to fresh biochar (FBC). A pot experiment using lettuce as a test crop assessed changes in soil physicochemical properties, nutrient availability, microbial biomass, and cumulative respiration, enzymatic activities, and plant physiological and antioxidant responses under various treatments: control (CK), FBC, ABC, AMF, and ABC + AMF. Aging with leonardite significantly modified biochar surface chemistry, reduced alkalinity, and enhanced functional groups, resulting in improved interactions with soil physicochemical and biological processes. Among treatments, ABC + AMF at a 3% application rate produced pronounced benefits. It increased soil total organic carbon, microbial biomass carbon and nitrogen, soil respiration, and key soil enzymatic activities compared with CK, indicating enhanced microbial functioning and nutrient cycling. In contrast, FBC primarily influenced soil properties through alkalinity-driven effects, increasing soil pH, electrical conductivity, cation exchange capacity, and available phosphorus. Plant responses mirrored the improvements in soil, with ABC-3%+AMF markedly enhancing photosynthetic efficiency (NDVI, NDRE, SPAD), antioxidant capacity (TPC, TFC, CTC), and lettuce growth parameters. Multivariate analysis confirmed positive relationships between soil indicators and plant performance, demonstrating that biochar aging improves its agronomic effectiveness. Overall, the findings show that ABC, particularly inoculated with AMF, offers a practical strategy for restoring sandy soils by stimulating biological activity and nutrient availability while reducing reliance on chemical inputs. This integrated approach provides a sustainable tool for improving soil health and crop productivity in degraded sandy agroecosystems.