Synergistic effects of biochar and nitrogen fertilizer enhance soil carbon emissions and microbial diversity in acidic orchard soils

To explore the synergistic effects of biochar and nitrogen fertilizer on soil carbon emission and microbial diversity in acidic orchards were studied. A 300-day pot experiment was conducted, including control (CK), nitrogen fertilizer (N), 1% biochar (B1), 3% biochar (B3), nitrogen fertilizer with 1% biochar (NB1), and nitrogen fertilizer with 3% biochar application (NB3). After biochar and nitrogen fertilizer treatments, soil pH increased from 4.73 to 6.75 unit, soil organic carbon (SOC), mineral-associated organic carbon (MAOC) and particulate organic carbon (POC) contents increased 17.88%-41.14%, 31.95%-73.44% and 15.50%-48.90%, respectively, Dissolved organic carbon (DOC) content decreased by 33.56%-55.35%. The release of CO₂-C increased by 0.73%-232.43%, with the synergistic effect of NB3 being the most significant. NB1 and B1 reduced VOCs-C release, while NB3 and B3 increased VOCs-C release. B1 and B3 significantly enhanced the abundance of Bradyrhizobium, while decreasing the abundance of Streptomyces and Streptacidiphilus, NB3 exhibited opposite trends. Compared with CK, B1 and B3 increased the abundance of acyl-CoA dehydrogenase (acdA), and NB1 and NB3 reduced the abundance of β-galactosidase (β-gaL) and glucosidase (GA). Correlation analysis showed that the release of CO₂-C was significantly positively correlated with MAOC and negatively correlated with DOC, while VOCs-C was significantly negatively correlated with DOC. This synergistic effect of biochar and nitrogen fertilizer has positive implications for improving soil health and represents a viable strategy for sustainable agricultural practices.