- Biological soil crusts (BSC) contribute significantly to the soil surface cover in many dryland ecosystems. A mixed type of BSC, which consists of cyanobacteria, mosses and cyanolichens, constitutes more than 60% of ground cover in the semiarid grass-shrub steppe at Sayeret Shaked in the northern Negev Desert, Israel. This study aimed at parameterizing the carbon sink capacity of well-developed BSC in undisturbed steppe systems. Mobile enclosures on permanent soil borne collars were used to investigate BSC-related CO 2 fluxes in situ and with natural moisture supply during 10 two-day field campaigns within seven months from fall 2001 to summer 2002. Highest BSC-related CO 2 deposition between –11.31 and –17.56 mmol m −2 per 15 h was found with BSC activated from rain and dew during the peak of the winter rain season. Net CO 2 deposition by BSC was calculated to compensate 120%, –26%, and less than 3% of the concurrent soil CO 2 efflux from November–January, February–May and November–May, respectively. Thus, BSC effectively compensated soil CO 2 effluxes when CO 2 uptake by vascular vegetation was probably at its low point. Nighttime respiratory emission reduced daily BSC-related CO 2 deposition within the period November–January by 11–123% and on average by 27%. The analysis of CO 2 fluxes and water inputs from the various sources showed that the bulk of BSC-related CO 2 deposition occurs during periods with frequent rain events and subsequent condensation from water accumulated in the upper soil layers. Significant BSC activity on days without detectable atmospheric water supply emphasized the importance of high soil moisture contents as additional water source for soil-dwelling BSC, whereas activity upon dew formation at low soil water contents was not of major importance for BSC-related CO 2 deposition. However, dew may still be important in attaining a pre-activated status during the transition from a long "summer" anabiosis towards the first winter rain.