In dryland ecosystems, interactions between biological soil crusts and plants are important drivers of productivity. Specifically, biocrusts have been found to reflect the δ13C signatures of nearby C3 and C4 plants. We investigated this pattern across multiple sites to discover if the pattern holds across dryland ecosystems. Using the natural difference in δ13C signature between photosynthetic pathways (C3 average -28‰ and C4 average -14‰), we hypothesized that biocrusts near C3 plants would be relatively more depleted, and biocrusts near C4 plants would be relatively more enriched. We collected leaf and paired biocrust samples at multiple distances: under the plant (0cm) to the interspace (25cm) from plants in the Chihuahuan Desert and Colorado Plateau. Sites vary in temperature, precipitation, and dominant vegetation. Cyanobacterial filaments were removed from biocrust samples under a microscope using forceps and were analyzed for their δ13C isotopic signature using an isotope ratio mass spectrometer.
δ13C of cyanobacterial filaments collected near C3 plant species (Guiterreza sarothrae, Achnatherum hymenoides) were depleted by ~2% per mil under the plant in comparison to the interspace (P= 0.002, P=0.003 respectively). The δ13C signatures of cyanobacterial filaments near C3 plants is consistent with the hypothesis that carbon is shared between biocrust communities and plants, though we are careful to note that we cannot propose a specific mechanism. The δ13C of cyanobacterial filaments collected near two C4 plant species (Bouteloua eriopoda, Bouteloua gracilis) did not differ between distances (P=0.86, P=0.41), consistent with previous findings. However, cyanobacterial filaments collected near the remaining C4 grass (Pleuraphis jamesii) showed a depletion of ~3% per mil (P= 0.001) between distances. This result was unexpected as we hypothesized that all C4 plants would not differ. The unexpected results in P. jamesii suggest that these patterns may not hold true for all plant species. If the mechanisms can be shown using more detailed tracer studies and isotope source/sink analyses, natural abundance data could provide a means for quick assessment for C cycling in dryland ecosystems.