Nano biochar enhances organic carbon accumulation, molecular complexity, and salt leaching in saline-alkali soil columns.

Rapid population growth and intensive agricultural expansion have heightened the urgency to reclaim saline-alkali soils for crop production. Among potential soil amendments, preliminary studies show that nano biochar improves stressed soils, but its effects on organic carbon in saline-alkali soils remain unclear. This study investigated the impacts of nano biochar derived from maize-straw biochar (300 °C pyrolysis, ball-milled) on organic carbon dynamics and salt leaching in saline-alkali soils through a 120-day greenhouse column experiment. Treatments included control (CK), maize straw (S), biochar (B), nano biochar (NB), straw + biochar (SB), and straw + nano biochar (SNB), applied at equal carbon inputs. Periodic leaching with groundwater simulated field conditions, followed by analyses of soil and leachate organic matter via pyrolysis gas chromatography and fluorescence spectroscopy. Additionally, maize seedling growth was assessed over 15 days post-experiment. Results indicated NB and SNB significantly improved soil water retention, decreased soil pH, Na accumulation, and exchangeable sodium, and increased exchangeable calcium content. These treatments also elevated microbial biomass carbon and promoted formation of complex, monocyclic aromatic-rich organic carbon structures. Early-stage leaching reduced larger, hydrophobic dissolved organic matter fractions, enriching smaller phenolic and quinonic molecules. Combined straw and nano biochar application amplified microbial activity and organic carbon transformation, enhancing soil redox conditions, reducing carbon losses and salt buildup, and improving maize seedling growth. These findings highlight nano biochar's potential for remediating saline-alkali soils and optimizing organic carbon dynamics. Although production cost remains a constraint, the SNB strategy with reduced nano biochar input and renewable straw application shows strong potential for scalable saline-alkali soil remediation.