Biochar is commonly used to improve acidic soil and reduce nitrogen loss. However, the impact of biochar on soil nitrogen retention, especially at varying pH levels, is not fully understood. Soil samples were obtained from an acidic red soil citrus orchard. The soil pH was adjusted using CaO, with five levels (4.0, 5.1, 5.8, 6.6, and 7.2), and two biochar doses (0% and 1%) were applied. The study used ¹⁵N-Tracer and Ntrace to investigate biochar’s influence on soil nitrogen retention at different pH levels. The results showed that soil amendment with biochar improved gross mineralization rates (TM) and gross NH₄⁺ immobilization rates (TI), except at pH 4.0 for TI. Biochar enhanced heterotrophic nitrification (O_Nrec) within pH 4.0–7.4, with a threshold for autotrophic nitrification (O_NH4) at pH 6.4. The findings revealed biochar’s positive effect on soil nitrogen retention within pH 4.5–6.4. Biochar had a greater impact on TI than TM and inhibited O_NH4, potentially enhancing nitrogen retention in this pH range. These results highlight the significance of considering biochar incorporation for improving nitrogen use efficiency and reducing NO₃⁻-N loss in subtropical pomelo orchards.

The aim of this study was to explore the effect of biochar on N₂O emissions in soils with different pH levels. Soils with five pH levels (4.0, 5.1, 5.8, 6.6, and 7.2) were incubated in two conditions, with 0% biochar (CK) and 1% biochar (BC), for 23 days. N₂O emissions were measured at nine time points, and soil chemical properties, AOA-amoA, AOB-amoA, nirK, nirS, and nosZ, were analyzed. Partial least squares path modelling (PLS-PM) was used to assess the effect of nitrification and denitrification pathways on potential N₂O emissions. The results showed that biochar reduced N₂O emissions in highly acidic soil (pH 4.0) but increased emissions in soils with pH values ranging from 5.1 to 7.2. In highly acidic soils, decreased N₂O emission was associated with increased soil pH (p < 0.05) and decreased dissolved organic carbon content (p < 0.05), leading to higher nosZ gene abundance (p < 0.05). Meanwhile, in acidic to neutral soils, biochar application increased soil pH (6.6–11.7%), dissolved organic nitrogen (5.9–29.5%), dissolved organic carbon (8.6–41.0%), stimulated AOB-amoA, nirK, nirS gene abundance (p < 0.05), and thus increased N₂O emissions. The results verified the influence of nitrification and denitrification genes on N₂O production in soils with different pH values. In conclusion, biochar had different effects on N₂O emissions based on soil pH, highlighting the need to consider pH when using biochar to mitigate N₂O emissions in subtropical citrus orchards.

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.