Dryland ecosystems are increasing in geographic extent and contribute greatly to interannual variability in global carbon dynamics. Disentangling interactions among dominant primary producers, including plants and autotrophic microbes, can help partition their contributions to dryland C dynamics. We measured the delta C-13 signatures of biological soil crust cyanobacteria and dominant plant species (C(3)and C-4) across a regional scale in the southwestern USA to determine if biocrust cyanobacteria were coupled to plant productivity (using plant-derived C mixotrophically), or independent of plant activity (and therefore purely autotrophic). Cyanobacterial assemblages located next to all C(3)plants and one C(4)species had consistently more negative delta C-13 (by 2 parts per thousand) than the cyanobacteria collected from plant interspaces or adjacent to two C(4)Boutelouagrass species. The differences among cyanobacterial assemblages in delta C-13 could not be explained by cyanobacterial community composition, photosynthetic capacity, or any measured leaf or root characteristics (all slopes not different from zero). Thus, microsite differences in abiotic conditions near plants, rather than biotic interactions, remain a likely mechanism underlying the observed delta C-13 patterns to be tested experimentally.