A field study was conducted on olive trees growing at two locations in Tuscany, Italy, to determine seasonal changes in soil respiration rates (Rs) under different conditions of soil moisture. The two orchards differed in cultivar, age, soil texture and soil management. The soil was managed by either periodic disking or permanent grass cover. Soil water availability was modified using drip irrigation. Soil respiration rates were measured using a closed-chamber system connected to an infrared gas analyzer at different positions around six trees in one orchard (three irrigated and three non-irrigated) and three trees in the other orchard. Seasonal courses of Rs were similar in both orchards, with maximum values reached either in mid-spring or autumn over the two years of study. Low Rs occurred in the summer under rain-fed conditions. In both orchards irrigation resulted in dramatic increases of Rs. Soil respiration rates of the orchard with a grass cover were higher than those of the periodically disked soil and, on an annual basis, we estimated 11.79 and 7.84 t ha-1 of carbon emissions out of the soil in the two orchards, respectively. Carbon emissions due to soil respiration of irrigated trees were 7% higher than those of rain-fed ones.