CO2 efflux from a Mediterranean semi-arid forest soil. II. Effects of soilfauna and surface stoniness

Many forest soils in the Mediterranean basin are shallow and contain high a mounts of gravel in the organic layers. Recent studies on soil organic matt er accumulation have shown high amounts of organic matter occurring mainly in soils with high levels of stoniness at the soil surface. The gravel laye r may affect the microclimatic conditions of the soil surface and probably the distribution and activity of soil fauna. In order to quantify the combined effects soil fauna (epigeic macrofauna an d earthworms) and stoniness on the release of soil CO2, we performed a thre e factor field experiment by using a series of reconstructed soil profiles. Factors 1 and 2 consisted of the exclusion/presence of soil epigeic macrof auna and earthworms, and factor 3 of the presence/absence of a gravel layer intermingled with the H horizon. We incubated C-14 straw in the H horizon and carried out three 40 mm rainfall simulations. Soil respiration primarily depended on the season. The effects of soil faun a were generally small and did not coincide with periods of high faunal act ivity. The largest effects of both earthworms and soil epigeic fauna were f ound after wetting the soil in summer. The effects of the earthworms were c oncentrated in the mineral soil while the effects of the epigeic fauna were concentrated in the H horizon and mainly arose towards the end of the expe riment. This suggests that the effects of epigeic fauna may have been under estimated due to the length of the experiment. The gravel layer increased t he effect of fauna probably by creating more favorable microclimatic condit ions. The accumulation of organic matter in soils with high levels of stoni ness cannot be explained by the effect of gravel on soil microclimate nor b y its effect on the activity of soil fauna.