Low-release biochar for improving coastal reclaimed Saline–Alkali soil: Short-Term physicochemical responses and exploratory microbial transcriptional associations under controlled pot conditions

Coastal land reclamation is an important approach to expanding useable land, yet reclaimed soils are often constrained by low fertility, weak structural stability, and saline-alkaline stress. Considering traditional amendments may introduce environmental risks in sensitive coastal settings, this study evaluated the short-term responses of coastal reclaimed saline-alkali soil (Hengsha Island, Shanghai) under controlled pot conditions to a novel low-release biochar (BC-SR). Ten days after application, conventional biochar (BC-O) increased soil salinity to 2.5 times the initial level and raised pH by 0.22, whereas BC-SR exerted negligible disturbance on salinity and pH. Soil organic carbon (SOC) exhibited a net 13% reduction under BC-O during incubation, whereas BC-SR induced a 24% net rise in SOC alongside improved macroaggregate formation and microbial biomass. This effect of BC-SR further elevated the net SOC increment to 105% in the plant-containing treatments, whereas SOC decreased by 41% in the plant-only control. Exploratory metatranscriptomic analysis indicated that the treatment combining plant and BC-SR was associated with higher transcript abundances of genes related to the Calvin–Benson–Bassham cycle and other carbon fixation pathways. Overall, these findings provide short-term evidence that low-release biochar could reduce release-related risk signals while increasing soil organic carbon and improving fertility properties in reclaimed coastal saline–alkali soils under controlled pot conditions.