Elevated nitrogen (N) inputs to upland ecosystems substantially alter the soil methane (CH4) sink capacity. However, the magnitude and even the direction of soil CH4 uptake responses to N enrichment remain highly variable and poorly understood across different upland ecosystems. In this study, three consecutive years of in situ measurements were conducted to quantify soil CH4 uptake in cropland, shrubland, and forest ecosystems under four N addition treatments: CK, N50, N150, and N300 (0, 50, 150, and 300 kg N ha-1 year-1, respectively). Concurrent measurements of microclimatic conditions, soil physicochemical properties, and methanotroph-related characteristics were performed to elucidate the mechanisms governing ecosystem-specific responses of soil CH4 uptake to N enrichment. The results demonstrated that soil CH4 uptake exhibited distinct ecosystem-specific responses to N addition. In croplands, CH4 uptake rates were unaffected by N inputs, likely because N enrichment did not alter pmoA gene abundance, methanotroph diversity, or community composition. In shrublands, the N150 and N300 treatments increased soil CH4 uptake rates by approximately an order of magnitude relative to CK, whereas the N50 treatment produced no significant effect. The enhanced soil CH4 uptake observed under the N150 and N300 treatments was closely associated with increased pmoA gene abundance and reduced Chao1 diversity index of methanotrophic communities. In contrast, in forests, the N50 treatment significantly increased CH4 uptake rates by 61.0 %, whereas no significant response was observed under N150 or N300. The enhancement of CH4 uptake under the N50 treatment was significantly correlated with a reduction in soil water-filled pore space (WFPS), along with increases in dissolved organic carbon (DOC) concentrations and the relative abundance of Methylococcus. Overall, this study reveals divergent responses of soil CH4 uptake to N addition among different upland ecosystems and elucidates the associated mechanisms, improving our understanding of soil CH4 sink responses to elevated N inputs.
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