Increased soil salinization slows biochar aging and limits microbial colonization

Abstract Biochar is widely recognized as a green and sustainable amendment for saline-alkali soils. However, the aging process can alter its properties and compromise its long-term effectiveness in improving soil quality. Despite its significance, limited information is available on biochar aging in saline soils. This study investigated the mechanisms by which soil salinization influences biochar aging and microbial colonization characteristics. Biochar samples collected from agricultural fields with different salinity levels were subjected to repeated wet–dry cycles simulating approximately 8 years of natural aging and analyzed for changes in chemical and microbial properties. The results revealed that increased soil salinization slowed the aging process of biochar. Biochar aged in high-salinity soils retained significantly higher total carbon, aromaticity, and surface C–C/C = C carbon contents than those in low-salinity soils, whereas its total oxygen content, degree of oxidation, and surface C–O content were markedly lower. In the final cycle, the O/C ratio of biochar aged in high-salinity soil was 9.82% lower than that in low-salinity soil. Soil salinity also suppressed microbial community activity and diversity in the biochar, particularly fungi, likely contributing to the retardation of biochar aging. In addition, the barrier formed by soil mineral salts on the biochar surface further slowed the aging process. Additionally, temporal analysis revealed that over time, the oxidability of biochar increased due to the loss of labile carbon and the mineralization of organic matter, whereas the total carbon content decreased by approximately 20%. These findings enhance our understanding of biochar aging in saline soils and provide insights into its long-term environmental performance as a sustainable amendment. Graphical Abstract