Episodic High CH4 Emission Events can Damage the Potential of Soils to Act as CH4 Sink: Evidence from 17 Years of CO2 Enrichment in a Temperate Grassland Ecosystem

Abstract

With a 100-year global warming potential of 25 times of that of CO2, methane (CH4) is the second-most important greenhouse gas of dominantly biological origin. The stimulating effect from elevated CO2 on CH4 emissions from anaerobic ecosystems is well known1. However, in aerobic grassland ecosystems where the ground water table does not approach the surface, ascending CH4 produced at greater depths can be oxidized by methanotrophic bacteria in the top soil, so that the soil acts as a net methane sink.At the Giessen-FACE site, greenhouse gas emissions have been continuously measured 1-3 times per week with large (1m diameter) closed chambers in triplicate per plot, since 1998 under elevated (eCO2, +20%) and ambient (aCO2) conditions (n=3 plots), i.e. through more than 17 years. During the entire time series of gas flux measurements, periods of point-source CH4 emissions were repeatedly observed during late-summer or autumn. The emission events were spatially very heterogeneous and usually continued over weeks occurring repeatedly through the 17 years of study. We suggest that within the chambers the peaks were micro-site specific events while the surrounding soil concomitantly consumed CH4. Three mechanisms are discussed as possible explanations for the episodic CH4 peaks: 1) soil cracks, forming during the late summer period, may allow the ascendance of CH4 from deeper saturated layers; 2) the seasonal occurrence of Scarabaeidae larvae which are known point-source CH4 emitters2,and 3) seasonally enhanced fungal decomposition activity by saprotrophic fungi, aerobically emitting CH4 while decomposing biomass.We evaluate three different approaches, based on different mechanistic explanations for the CH4 emission events, to assess the overall effect of long-term CO2 enrichment on the CH4 exchange in the temperate grassland ecosystem.