Despite important advances in the understanding of biological soil crusts and their key role in ecosystem processes in arid and semi-arid environments, little is known about those factors driving the small-scale patterns of abundance and distribution of crust-forming lichens and mosses. We used constrained ordination techniques (RDAs) to test the hypothesis that the spatial patterning of lichens and mosses is related to surface and subsurface soil variables in two semi-arid gypsum environments of Spain. Our results show that the abundance of mosses and lichens forming biological soil crusts was related to a limited set of variables (cover of bare soil and litter, soil respiration, potassium content and aggregate stability). Moreover, they provide some insights into the importance of these variables as drivers of biological soil-crust composition and abundance in semi-arid gypsum environments.