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.

Farmland soil is an important source of the greenhouse gas nitrous oxide (N₂O), and soil nitrification and denitrification are key processes affecting N₂O production. In this study, the acidic soil of a pomelo orchard was used to investigate the effects of a bamboo-biochar-based fertilizer (BB) on soil N₂O emissions and nitrifier and denitrifier communities. In this study, five treatments, namely, CK (no urea and BB), N (0.72 g·kg⁻¹ urea), 5BB+N (0.72 g·kg⁻¹ urea plus 5 g·kg⁻¹ BB), 10BB+N (0.72 g·kg⁻¹ urea plus 10 g·kg⁻¹ BB), and 20BB+N (0.72 g·kg⁻¹ urea plus 20 g·kg⁻¹ BB) were applied to the acidic soil of a pomelo orchard. The nitrification (AOA-amoA, AOB-amoA) and denitrification (nirS, nirK, nosZ) gene copy numbers were analyzed by qPCR, and their community diversities were determined by Illumina MiSeq sequencing. The results showed that N treatment significantly promoted soil N₂O emissions compared with CK, while all BB+N treatments significantly inhibited soil N₂O emissions compared with N treatment. BB fertilizer promoted soil nitrification, alleviated the adverse effects from N fertilizer inputs on the AOA-amoA gene copy numbers and community diversity, and restored the AOA-amoA diversity to the initial level. BB had a strong effect on Crenarchaeota (AOA-amoA) and Nitrosospira (AOB-amoA). BB significantly promoted the denitrification gene copy numbers; increased nirS and nirK community diversity; particularly affected the relative abundance of denitrifiers such as Nonomuraea (nirS), Proteobacteria (nirK), and Rhodanobacter (nosZ); and, finally, reduced N₂O emissions.