Characteristics of Methane Exchange in a Black Spruce Forest Over Permafrost in Interior Alaska

Methane (CH₄) is a strong greenhouse gas, and it is expected that the CH₄ emission will accelerate owing to the enhanced activity of microorganisms under projected warming. Most forest ecosystems are generally thought to be net sinks of CH₄ due to the dominance of CH₄ oxidization in the aerated soil. However, it is also known that forests switch to act as CH₄ sources when the soil is in an anaerobic condition (Megonigal and Guenther, 2008). Hence, to accurately estimate the CH₄ exchange in forest ecosystems, it is necessary to understand the responses of the CH₄ exchange to changes of environmental conditions.

Since fall 2002, we have observed methane flux continuously using the aerodynamic gradient technique in a black spruce forest (64°52'N, 147°51'W) over permafrost in Alaska (Ueyama et al., 2006). The black spruce forest is 120 years old. The forest floor is covered with mosses, sedges, and shrubs.

The forest generally acted as a net methane sink; the mean daily exchange rate was approximately -10 mgCH₄m-2 day-1. However some methane emission events were observed: (1) in the period after snowmelt, (2) in the period after heavy rain, and (3) at the beginning of soil surface freezing. (1) Methane emissions were observed approximately ten days after complete snowmelt. These emissions are attributable to anaerobic saturated soil condition caused by snowmelt water. The magnitude of emission ranges from 20 to 60 mgCH₄ m-2 day-1. This variability may be influenced by depth of soil thawing. (2) The heavy rain event also enhanced anaerobic soil conditions, leading to net methane emission of 0 to 20 mgCH₄ m-2 day-1. Permafrost inhibits deep percolation of soil water and thus helps to form a saturated water condition in the soil, allowing methane to be produced. (3) The emissions at the beginning of soil surface freezing ranged from 0 to 20 mgCH₄ m-2 day-1. The soil anaerobic condition may be caused by a reduced oxygen supply blocked by the frozen soil surface and also high water content in this period.

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