CARBON-DIOXIDE EFFLUX AND PCO(2) IN SOILS OF 3 QUERCUS-ILEX MONTANE FORESTS

Soil CO2 efflux and pCO(2) in the soil atmosphere were measured during one year at three montane sites of Mediterranean sclerophyllous fores ts in NE Spain. Two sites were located in the upper and lower slopes o f a small catchment in the Prades mountains (mean precipitation 550 mm year(-1)), and a third site was located on a lower slope in the Monts eny mountains (mean precipitation 900 mm year(-1)). The three sites we re similar in bedrock and vegetation, but differed in soil characteris tics and water availability. Seasonal variation of CO2 efflux and soil pCO(2) were affected by soil temperature and, to a lesser extent, by soil moisture. Annual mean soil CO2 efflux (considered as soil respira tion) was similar at Montseny and at the comparably located site at Pr ades (83 +/- 18 S.E. vs. 75 +/- 9 mg CO2 m(-2) hour(-1), respectively) , and was highest at the Prades upper slope site (122 +/- 22 mg CO2 m( -2) hour(-1)). Despite those relatively similar CO2 effluxes, mean soi l pCO(2) was much higher at both Prades sites than at Montseny. Soil p CO(2) always increased with depth at Prades while maxima pCO(2) at Mon tseny were often at 20-30 cm depth. A model based on gas diffusion the ory was able to explain why soil pCO(2) was much higher at Prades than at Montseny, and to reproduce the shape of the vertical profile of pC O(2) at the Prades soils. Nevertheless, the model failed to simulate t he soil pCO(2) maximum found at 20-30 cm depth at the Montseny site. M odel simulations using a time-variable CO2 production rate suggested t hat pCO(2) maxima at intermediate depth could be the result of a trans ient situation instead of an equilibrium one.