Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA, USA.
Department of Earth and Environmental Sciences, California State University, East Bay, Hayward, CA, USA.
Glob Chang Biol. 2018 Sep;24(9):4107-4121. doi: 10.1111/gcb.14124. Epub 2018 Apr 10.
Wetlands are the largest source of methane (CH ) globally, yet our understanding of how process-level controls scale to ecosystem fluxes remains limited. It is particularly uncertain how variable soil properties influence ecosystem CH emissions on annual time scales. We measured ecosystem carbon dioxide (CO ) and CH fluxes by eddy covariance from two wetlands recently restored on peat and alluvium soils within the Sacramento-San Joaquin Delta of California. Annual CH fluxes from the alluvium wetland were significantly lower than the peat site for multiple years following restoration, but these differences were not explained by variation in dominant climate drivers or productivity across wetlands. Soil iron (Fe) concentrations were significantly higher in alluvium soils, and alluvium CH fluxes were decoupled from plant processes compared with the peat site, as expected when Fe reduction inhibits CH production in the rhizosphere. Soil carbon content and CO uptake rates did not vary across wetlands and, thus, could also be ruled out as drivers of initial CH flux differences. Differences in wetland CH fluxes across soil types were transient; alluvium wetland fluxes were similar to peat wetland fluxes 3 years after restoration. Changing alluvium CH emissions with time could not be explained by an empirical model based on dominant CH flux biophysical drivers, suggesting that other factors, not measured by our eddy covariance towers, were responsible for these changes. Recently accreted alluvium soils were less acidic and contained more reduced Fe compared with the pre-restoration parent soils, suggesting that CH emissions increased as conditions became more favorable to methanogenesis within wetland sediments. This study suggests that alluvium soil properties, likely Fe content, are capable of inhibiting ecosystem-scale wetland CH flux, but these effects appear to be transient without continued input of alluvium to wetland sediments.
湿地是全球最大的甲烷（CH ）源，但我们对过程水平控制如何扩展到生态系统通量的理解仍然有限。特别不确定土壤特性的变化如何影响年时间尺度上生态系统 CH 排放。我们通过涡度相关法从加利福尼亚州萨克拉门托-圣华金三角洲的泥炭和冲积土上最近恢复的两个湿地测量了生态系统二氧化碳（CO ）和 CH 通量。在恢复后的多年里，冲积湿地的 CH 通量明显低于泥炭湿地，但这些差异不能用湿地之间主导气候驱动因素或生产力的变化来解释。冲积土中的土壤铁（Fe）浓度明显较高，与泥炭湿地相比，冲积湿地的 CH 通量与植物过程解耦，这与根际 Fe 还原抑制 CH 产生的预期一致。土壤碳含量和 CO 吸收速率在湿地之间没有差异，因此也可以排除它们是初始 CH 通量差异的驱动因素。湿地土壤类型之间的 CH 通量差异是暂时的；恢复 3 年后，冲积湿地的通量与泥炭湿地的通量相似。时间变化的湿地 CH 排放不能用基于主要 CH 通量生物物理驱动因素的经验模型来解释，这表明除了我们的涡度相关塔没有测量到的其他因素外，还有其他因素导致了这些变化。最近堆积的冲积土的酸度低于恢复前的母土，并且含有更多的还原态 Fe，这表明随着湿地沉积物中甲烷生成条件变得更加有利，CH 排放增加。本研究表明，冲积土的土壤特性，可能是 Fe 含量，能够抑制生态系统尺度的湿地 CH 通量，但这些影响似乎是暂时的，除非有更多的冲积物输入湿地沉积物。
