Regulators of coastal wetland methane production and responses to simulated global change

Wetlands are the largest natural source of methane (CH4) to the atmosphere, but their emissions vary along salinity and productivity gradients. Global change has the potential to reshape these gradients and therefore alter future contributions of wetlands to the global CH4 budget. Our study examined CH4 production along a natural salinity gradient in coastal Alaska wetlands. In the laboratory, we incubated natural sediments to compare CH4 production rates between freshwater and intertidal wetlands, and quantified the abundances of methanogens and sulfate-reducing bacteria in these ecosystems. We also simulated sea-level rise and enhanced organic matter availability, which we predicted would have contrasting effects on coastal wetland CH4 production. Intertidal wetlands produced less CH4 than freshwater wetlands due to high sulfate availability and generally higher abundances of sulfate-reducing bacteria, whereas freshwater wetlands had significantly greater methanogen abundances. Simulated sea-level rise in freshwater sediments, however, did not reduce CH4 production, perhaps because the 14d incubation period was too short to elicit a shift in microbial communities. In contrast, increased organic matter generally enhanced CH4 production rates, but this response varied by the macrophyte species added. Our study suggests that CH4 production in coastal wetlands, and therefore their overall contribution to the global CH4 cycle, will be sensitive to increased organic matter availability and potentially sea-level rise. To better predict future wetland contributions to the global CH4 budget, future studies and modeling efforts should investigate how multiple global change mechanisms will interact to impact CH4 dynamics.

湿地是大气甲烷（CH₄）最大的自然来源，但其排放量沿盐度和生产力梯度存在差异。全球变化有可能重塑这些梯度，从而改变未来湿地对全球甲烷收支的贡献。本研究调查了阿拉斯加沿海湿地自然盐度梯度下的甲烷产生情况。在实验室中，我们培养天然沉积物以比较淡水湿地与潮间带湿地的甲烷产生速率，并量化了这些生态系统中产甲烷菌（methanogens）和硫酸盐还原菌（sulfate-reducing bacteria）的丰度。我们还模拟了海平面上升和有机质可利用性增加的情景，预测这两者会对沿海湿地甲烷产生产生相反的影响。由于硫酸盐可利用性高且硫酸盐还原菌丰度普遍较高，潮间带湿地的甲烷产生量低于淡水湿地；而淡水湿地的产甲烷菌丰度显著更高。然而，在淡水沉积物中模拟海平面上升并未降低甲烷产生量，这可能是因为14天的培养期过短，不足以引发微生物群落的变化。相反，有机质增加通常会提高甲烷产生速率，但这种响应因添加的大型植物物种而异。本研究表明，沿海湿地的甲烷产生及其对全球甲烷循环的整体贡献，将对有机质可利用性增加和潜在的海平面上升敏感。为了更好地预测未来湿地对全球甲烷收支的贡献，未来的研究和模型构建应探究多种全球变化机制如何相互作用以影响甲烷动态。