Multi-scale temporal variation in CH4 and CO2 exchange and associated biophysical controls from two wetlands in Northeast China

Abstract

Quantifying net CO2 exchange (NEE) and CH4 emissions of northern wetlands and their responses to biophysical controls are central to understanding the role of wetlands in global climate change. This study presents two growing seasons of observations of CO2 and CH4 fluxes by eddy covariance technique from a permafrost peatland and an inundated marsh in northeast China. Wavelet analysis was used to explore the temporal multi-scale variations of NEE and CH4 fluxes and their biophysical controls. NEE showed prominent variations at the diel and the seasonal scales. However, most CH4 variance was distributed at time scales longer than one day. At the permafrost peatland wavelet analysis revealed strong spectral coherency and alternating coherence between soil temperature and CH4 fluxes during the spring and summer seasons and between thaw depth and CH4 fluxes in the late growing season, with the suggestion of an as yet unknown coherence at longer timescales than could be resolved with just growing season records. At the marsh, soil temperature and CH4 flux showed strong spectral coherency throughout the growing seasons and gross ecosystem production (GEP) additionally imposed its influence on CH4 mainly during the most active part of the growing season.The growing season average CH4 emission from the peatland (0.7 g CCH4 m−2) was quite small and was negligible compared to its growing season NEE (−74.2 g CCO2). In contrast, the growing season CH4 emission from the marsh (average 31.1 g CCH4 m−2) was considerably higher and accounted for about 16.2 % of its NEE (−191.7 g CCO2 m−2). Although the two wetlands were net carbon sinks during the growing seasons and the permafrost peatland had a negative radiative forcing (i.e., cooling effect), the high CH4 emissions from the marsh resulted in a net positive radiative forcing (i.e., warming effect). To understand the function of northern wetlands in the global carbon cycle, continuous field observations of these greenhouse gases are vital.