Soil-atmosphere greenhouse gases (CO<sub>2</sub>, CH<sub>4</sub> and N<sub>2</sub>O) exchange in evergreen oak woodland in southern Portugal

Abstract

  A 10–20% decrease in annual precipitation is predicted in the Mediterranean basin, and in particular to the Iberian Peninsula, with foreseen effects on the exchange of soil-atmosphere greenhouse gases (GHGs; CO<sub>2</sub>, CH<sub>4</sub>, and N<sub>2</sub>O). To simulate this scenario, we setup an experimental design in the particularly dry period of 2008–2009 using rainfall exclusion and irrigation, to obtain plots receiving 110% (538 mm), 100% (493 mm) and 74% (365 mm) of the natural precipitation. Soil CO<sub>2</sub> fluxes showed a strong increase from summer to autumn as a consequence of increasing soil heterotrophic respiration that resulted from rewetting. Fluxes of N<sub>2</sub>O were negligible. According to our data, soil was a permanent CH<sub>4</sub> sink independent of the soil water content (in the range between 6–26% WFPS – water-filled pore space) and of soil temperature (in the range of 7–28°C), supporting the concept that seasonally dry ecosystems (Mediterranean) may represent a significant sink of atmospheric CH<sub>4</sub>. The study provides evidence that the 26% decrease or 10% increase in the ambient rainfall from annual precipitation of ca 500 mm did not significantly affect soil functionality and had a limited impact on soil-atmosphere net GHGs exchange in evergreen oak woodlands in southern Portugal.