Biocrust carbon isotope signature was depleted under a C-3 forb compared to interspace

Aims Plants and biological soil crusts (biocrusts) are the key producers in drylands, but biocrusts seldom show net CO2 uptake. I hypothesized that biocrusts could augment CO2 fixation by incorporating plant-derived carbon. Methods I collected biocrusts located at the base of Gutierrezia sarothrae (C-3 forb), Bouteloua gracilis (C-4 grass), and from bare interspaces between plants, and from a mesocosm experiment with live B. gracilis or dead B. gracilis roots. To trace carbon sources, I determined C-13 values of the biocrust community, isolated cyanobacteria and lichen, and plant leaves because the photosynthetic pathway distinguishes the tissue C-13 values. Results Biocrust communities and washed cyanobacteria and cyanolichen in G. sarothrae microsites were depleted by similar to 2 parts per thousand relative to other locations. Biocrust delta C-13 did not differ between the interspace and live or dead B. gracilis. Conclusions Potential mechanisms for the trend in biocrust delta C-13 adjacent to C-3 plants include differences in microsite conditions, biocrust communities, use of respired CO2 in the soil matrix for photosynthesis, or mixotrophic use of plant photosynthates. Further investigation of this observation may improve understanding of the degree to which the activities of dryland primary producers are coupled.