Nitrogen-induced suppression of methane uptake is alleviated by biochar in a subtropical forest soil

Forest soils serve as an essential atmospheric methane (CH₄) sink; however, exogenous nitrogen (N) inputs from intensive anthropogenic management and ambient atmospheric deposition typically suppress the uptake of CH₄ by soils. The application of biochar has been proposed as a potential strategy to mitigate this suppression, yet critical uncertainties persist regarding its dose dependency, interactions with N input levels, and underlying kinetics. This study investigated the impacts of N addition (0, 0.2 and 0.6 g N kg^-1), biochar amendment (0, 5 and 15 g C kg^-1) and their interactions on the CH₄ uptake, soil physicochemical properties, and microbial activities of forest soils. The results demonstrated that N addition decreased the CH₄ uptake by 22 % and 37 % under low (0.2 g N kg^-1) and high (0.6 g N kg^-1) applied doses, respectively, while biochar amendment increased the uptake by 24 % and 50 % under low (5 g C kg^-1) and high (15 g C kg^-1) applied doses, respectively. The stimulated CH₄ uptake intensified under higher biochar input rate and was further amplified by greater N addition level. Under high N level, greater biochar input resulted in a maximal CH₄ uptake, achieving a 110 % increase. As N levels increased, the pathways through which biochar augments the uptake of soil CH₄ became more multifaceted. Specifically, under high N level, biochar inputs reduced NH₄⁺-N, NO₃^--N, and water-soluble organic N levels, while lowering the abundance of mcrA gene. Furthermore, biochar increased the water-soluble organic C concentration and the pmoA abundance. Collectively, these changes suppressed the CH₄ production rate while stimulating the CH₄ oxidation rate, translating to a significant enhancement in CH₄ uptake. These findings emphasize the potential of biochar to enhance CH₄ sequestration in forest soils with increasing exogenous N inputs through the modulation of soil C and N pools, as well as microbial activities.