Biochar mitigates the suppressive effects of nitrogen deposition on soil methane uptake in a subtropical forest

Subtropical forests play a key role in sequestering atmospheric methane (CH₄), but rising nitrogen (N) deposition often suppresses soil CH₄ uptake. Biochar amendment has emerged as a promising approach to counteract this inhibitory effect; however, its effectiveness and underlying mechanisms remain unclear in subtropical forest soils. In a three-year field study, we evaluated the effects of N deposition (60 kg N ha⁻¹ yr⁻¹) and biochar amendment (10 t ha⁻¹), individually and combined, on soil CH₄ uptake, physicochemical properties, and microbial activities. Results showed that N deposition reduced annual CH₄ uptake by 17–23 %, while biochar alone increased this value by 11–25 %. When applied under N deposition, biochar led to a 26–42 % increase in annual CH₄ uptake. This enhancement was linked to increased water-soluble organic carbon level and elevated abundance of pmoA gene that encodes particulate methane monooxygenase, along with reduced concentrations of water-soluble organic nitrogen, NH₄⁺-N and NO₃⁻-N, and decreased abundance of mcrA gene that encodes methyl-coenzyme M reductase. These changes enhanced CH₄ oxidation rates and lowered CH₄ production rates, resulting in higher soil CH₄ uptake. The study highlights the capacity of biochar to strengthen soil CH₄ absorption within subtropical forests under N deposition by modulating carbon and nitrogen fractions and microbial functions. Applied at scale, biochar amendment in Moso bamboo forests across China could increase annual soil CH₄ uptake by approximately 6478 tons, offering a promising strategy for climate change mitigation.