- Abstract. Biological soil crusts (BSC) constitute a spatially prominent part of the photosynthesizing vegetation in many dryland ecosystems. This study assesses the annual net carbon deposition related to BSC growth in a Mediterranean shrubland for the years 2001–2003 using a model developed to account for the nature of hydration in the poikilohydric life trait of the BSC. Data for BSC-related net CO 2 fluxes were obtained from in-situ measurements at the International Long-term Ecological Research site Sayeret Shaked (ILTER-SSK) in the northern Negev Desert, Israel. The BSC was smooth to rugose, up to 15 mm thick and consisted mainly of mosses, cyanobacteria and cyano-lichens. In order to obtain annual estimates, BSC-related CO 2 fluxes were correlated with climate records provided by the meteorological station of the Terrestrial Ecosystem Monitoring Site network (TEMS) adjacent to SSK. The annual carbon deposition related to BSC growth was assessed from (1) an overall mean of net CO 2 flux multiplied with annual activity periods of BSC based on precipitation records, and (2) from a simple precipitation-driven activity model (PdAM). This model combines an algorithm, previously developed to model gas exchange processes in vascular plants, with an empirical module that switches the algorithm on as soon as water is available to maintain activity of poikilohydric BSC. Based on a constant BSC area index of 0.6 m 2 m −2 at ILTER-SSK, the final model suggests a large inter-annual variability in BSC-related net carbon deposition ranging from 7 to 51 kg ha −1 yr −1 .