Methane Emissions from an Alpine Wetland on the Tibetan Plateau: Magnitude, Pattern and Their Responses to Water Table Lowering

Abstract:

The alpine wetlands on the Tibetan Plateau have been subject to rapid climate changes and intensifying human activities, resulting in a lower water table. The changes may affect CH₄ emissions by influencing biological processes of production or consumption. However, the magnitude, pattern of CH₄ emissions on annual timescale and their responses to water table lowering in alpine wetlands remain poorly understood, because of technical limitations and the harsh environments. We conducted the first in situ year-round observation using Eddy Covariance method and a mesocosm experiment simulating changes in water table depth (+3 and -20 cm soil depth relative to the surface) using Manual Static Chamber method to measure the CH₄ fluxes in an alpine wetland. We found that the annual CH₄ emissions were 26.4 and 33.8 g CH₄ m^-2 in 2012 and 2013, respectively, and the non-growing season CH₄ emissions accounted for 43.2-46.1% of the annual emissions from in situ observation, highlighting an indispensable contribution that was often overlooked by previous studies. Meantime, a two-peak seasonal variation in CH₄ fluxes was observed, with a small peak in the spring thawing period and a large one in the peak growing season. Mesocosm experiment showed that water table lowering largely reduced CH₄ emissions by 52.7-64.9%, but did not affect seasonal dynamics in the growing seasons from 2011 to 2013. Our findings suggested that the CH₄ emissions from the alpine wetlands on the Tibetan plateau cannot be ignored during non-growing season, and the wetlands may be a weaker CH₄ source in the growing season considering water table lowering associated with artificial drainage and climate warming in this region.