What Could Explain δ¹³C Signatures in Biocrust Cyanobacteria of Drylands?

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 δ¹³C signatures of biological soil crust cyanobacteria and dominant plant species (C₃ and C₄) 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 C₃ plants and one C₄ species had consistently more negative δ¹³C (by 2‰) than the cyanobacteria collected from plant interspaces or adjacent to two C₄ Bouteloua grass species. The differences among cyanobacterial assemblages in δ¹³C 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 δ¹³C patterns to be tested experimentally.