Effects of Biochar Addition and Nitrogen Deposition on Forest Soil CO2 Emissions and CH4 Uptake in a Temperate Mixed Conifer–Broadleaf Forest: An Incubation Study

In this study, pristine biochar (BC1) and magnesium-modified biochar (BC2) were prepared from corn straw. Different nitrogen deposition intensities (0, 8, 30, and 50 kg N/ (ha·yr) ) were simulated by adding NH4NO3 solution. A laboratory incubation experiment was conducted to investigate the effects of biochar addition and N deposition on CO2 emissions, CH4 uptake, and microbial community structure in soils from a temperate mixed conifer–broadleaf forest. The results showed that BC1 significantly increased cumulative CO2 emissions (p < 0. 05), while no significant difference was observed between BC2 and the control. N deposition had no significant effect on CO2 emissions. Biochar addition significantly promoted cumulative CH4 uptake (p < 0. 05), with BC2 exhibiting a stronger promoting effect than BC1. In contrast, N deposition significantly inhibited CH4 uptake (p < 0. 05) in a dose-dependent manner. Spearman’s correlation analysis revealed that cumulative CO2 emissions were significantly or highly significantly negatively correlated with the relative abundances of Elusimicrobiota, Actinomycetota, Chloroflexota, Planctomycetota, Acidibacter, Bacillus, Paenibacillus, Acidothermus, and Mycobacterium, and significantly positively correlated with Bacteroidota, Bdellovibrionota, Pseudomonadota, Devosia, and Mesorhizobium. Cumulative CH4 uptake was highly significantly positively correlated with the relative abundance of Bacteroidota and significantly negatively correlated with Chloroflexota, CandidatusEremiobacterota, and Mycobacterium. These findings demonstrate that N deposition has no significant impact on soil CO2 emissions but significantly inhibits CH4 uptake, while magnesium-modified corn straw biochar promotes CH4 uptake without substantially increasing CO2 emissions, highlighting its promising application potential.