Biocrusts play a key role in supporting nitrogen (N) cycling and bioavailability in drylands. However, the taxon-specific contribution of biocrust constituents to soil N fixation and bioavailability is far less understood. We measured the composition of soil-associated biocrusts and soil pigment content in a semiarid shrubland from central Spain, and evaluated the potential regulatory capacity of biocrusts, and their forming constituents (mosses, lichens, and cyanobacteria) over the N cycle (storage, bioavailability, and processing). We found that key biocrust species contributed to explain N fixation potential and availability. In particular, the cyanolichen Collema was positively correlated with N fixation potential, as was total cyanobacterial abundance, and, in particular, the morphotypic genera Microcoleus, and Leptolyngbya. Also, the cyanobacterial morphotype Microcoleus was associated with ammonium accumulation and negatively with ammonification potential. Total biocrust cover and the cover of the moss Tortella squarrosa were negatively correlated with soil N content in this relatively low-fertility site, and positively with N fixation potential. Our results underline the taxon-specific contributions of biocrust components, such as Collema, which emerged as a key predictor of N bioavailability. We provide a novel empirical demonstration of how different biocrust components play distinct roles in N cycling processes at the local scale, by promoting or inhibiting N fixation and/or storing organic and inorganic N, thus contributing to the efficient cycling of this critical element in the typically N-poor Mediterranean ecosystems of central Spain. Keywords: Biocrusts; Biological nitrogen fixation; Cyanolichens; Mediterranean shrublands; N cycle.