CO2 efflux from a Mediterranean semi-arid forest soil. I. Seasonality and effects of stoniness

We studied the seasonality of total soil CO2 efflux and labeled C-CO2 relea sed from C-14 labeled straw incubated in the H horizon of a semi-arid Medit erranean forest soil. Field measurements were carried out over 520 days in a series of reconstructed soil profiles with and without a gravel layer bel ow the H horizon. We monitored soil climate and related this to soil CO2 ef flux. Seasonal variations in soil CO2 efflux in a semiarid Mediterranean fo rest were mainly related to changes in soil temperature. In spite of drough t, high respiration rates were observed in mid summer. High soil CO2 efflux in hot and dry episodes was attributed to increases in soil biological act ivity. The minimum soil CO2 efflux occurred in late summer also under dry c onditions, probably related to a decrease in soil biological activity in de ep horizons. Biological activity in organic layers was limited by water pot ential (Psi) in summer and by temperature in winter. Rewetting a dry soil r esulted in large increases in soil CO2 efflux only at high temperatures. Th ese large increases represented a significant contribution to the decomposi tion of organic matter in the uppermost horizons. Soil biological activity in the uppermost horizons was more sensitive to changes in soil Psi and hen ce to summer rainstorms than the bulk soil microbial activity. The presence of a layer of gravel improved both moisture and temperature conditions for the decomposition of organic matter. As a result, soil CO2 efflux increase d in soils containing rock fragments. These effects were especially large f or the organic layers.