Species-specific nitrogenase activity in lichen-dominated biological soil crusts from the Colorado Plateau, USA

Background and aim

Biological soil crusts (biocrusts) play numerous crucial roles in drylands, which comprise over 40% of Earth’s terrestrial surface. Among these key contributions is the fixation of atmospheric nitrogen. Yet, relatively little is known about the N₂ fixation capabilities of different lichen species that are found in late successional biocrust communities across drylands globally.

Methods

In order to improve our species-specific understanding of biocrust lichen N₂ fixation, we collected biocrusts dominated by four common species of lichens – Collema spp., Gyalolechia desertorum, Psora decipiens, and Squamarina lentigera – that represent a range of lichen families and morphological types. Nitrogenase activity of the biocrust community dominated by these lichens was evaluated using the acetylene reduction assay. Additionally, biocrust community composition was assessed using the point-intercept method along transects at varied distances from exposed bedrock.

Results

As expected, Collema spp.-dominated biocrusts had the highest rates of nitrogenase activity, with rates up to seven times larger than those of the other three target species. Nitrogen concentrations and carbon:nitrogen ratios of lichen tissue differed among lichen species. However, when the composite biocrust profile was assessed (i.e., biocrust tissue, microbial cells, and mineral soil to a 2 cm depth) these among-species differences in total nitrogen disappeared. Community composition changed according to distance from exposed bedrock, with a higher diversity of lichens closer to the bedrock.

Conclusion

Multiple drivers, including climate and land use change, affect biocrust community composition and species-specific functional information, even within a group such as late successional biocrusts, could help in forecasting the potential effects of global change on N₂fixation, and consequently, soil fertility in drylands.