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In the context of an ongoing monitoring study of the Cabo de Gata-Nijar Natural Park (SE of Spain), we explored the use of soil respiration as an indicator of ecosystem functioning reflecting changes in ecological processes in semiarid environments. With this purpose, we measured soil CO(2) efflux in six different and representative ecosystems of the Natural Park, with different land uses (forest and agricultural sites) and under different soil covers (under plant and bare soil) in two distinctive periods of the year: summer (dry period) and spring (growing season). We also measured the main soil properties and environmental variables. Soil CO(2) efflux ranged from 0.40 mu mol m(-2) s(-1) in the dry period to 1.93 mu mol m(-2) s(-1) in the growing season. Soil CO(2) efflux showed a large spatial variability, with different behaviour between the measured periods. Whereas in the dry period differences among ecosystems were larger (CVs 75-80%) than within them (CVs 40-55%), in the growing season the CVs were smaller (40-50%) and no differences were observed between or within ecosystem. The factors controlling soil CO(2) efflux also differed in the two measurement occasions. Whereas in the dry period soil CO(2) efflux was mainly the result of transport processes in the soil and therefore related to local factors (OC content, CN ratio, clay, rock outcrop, etc.) assigned to ecosystem conditions, in the growing season soil CO(2) efflux was dominated by soil CO(2) production and thus related only to organic carbon content and plant cover. In the growing season environmental variables explained ca. 10% of the variation in soil CO(2) efflux. In order to capture these different processes in different times of the year, i.e., diffusion versus production processes we calculated a new index, normalised seasonal difference in soil respiration (SDSR), which is proposed as a good indicator of the state and functioning of the ecosystem. (C) 2011 Elsevier Ltd. All rights reserved.