Environmental and biological controls on CH4 exchange over an evergreen Mediterranean forest

Abstract

Methane (CH4) fluxes in a Mediterranean holm oak (Quercus ilex L.) forest were measured over 19 months, using eddy covariance technique and soil chambers measurements, complemented by microbiological characterization of CH4-cycling microorganisms along a vertical soil profile.CH4 budget at the site was close to the neutrality, although periodical switching of the ecosystem from sink to source was observed at daily and seasonal scale. Forest soil represented a net sink for atmospheric CH4 throughout the whole study period (mean uptake rate of 1.64×10−3μmolm−2s−1).Different environmental parameters influenced CH4 fluxes at different time scales.At half-hour timescale, CH4 fluxes followed a half-sinusoid curve with high emission recorded during the central hour of day (mean emission rate of 1.05×10−2μmolm−2s−1). Stomatal conductance, solar radiation (global and UV) and latent heat flux were the main drivers controlling ecosystem CH4 emissions, suggesting the occurrence of a plant-mediated transport through xylem as a way of CH4 emissions from canopy and supporting the hypothesis of UV-induced production of CH4 from leaves at the site.At daily scale, CH4 fluxes were strongly connected to temperature and precipitation which promote CH4-oxidizing microorganisms activity during the cold season and methanogens activity during the dry season.This study is the first long-term CH4 eddy covariance measurement above a Mediterranean forest and demonstrates that, at present, the ecosystem is a small sink of CH4 when considering both the soil and vegetation processes together, and this sink capacity is strictly connected to the water availability. Future changes in temperature and precipitation patterns may increase CH4 emissions, turning the ecosystem to a source of CH4.