Relationships between biological soil crusts, bacterial diversity and abundance, and ecosystem functioning: Insights from a semi-arid Mediterranean environment

Questions:To what degree do biological soil crusts (BSCs), which are regulators of the soil surface boundary, influence associated microbial communities? Are these associations important to ecosystem functioning in a Mediterranean semi-arid environment? Location: Gypsum outcrops near Belmonte del Tajo, Central Spain. Methods: We sampled a total of 45 (50cm × 50cm) plots, where we estimated the cover of every lichen and BSC-forming lichen species. We also collected soil samples to estimate bacterial species richness and abundance, and to assess different surrogates of ecosystem functioning. We used path analysis to evaluate the relationships between the richness/abundance of above- and below-ground species and ecosystem functioning. Results: We found that the greatest direct effect upon the ecosystem function matrix was that of the biological soil crust (BSC) richness matrix. A few bacterial species were sensitive to the lichen community, with a disproportionate effect of Collema crispum and Toninia sedifolia compared to their low abundance and frequency. The lichens Fulgensia subbracteata and Toninia spp. also had negative effects on bacteria, while Diploschistes diacapsis consistently affected sensitive bacteria, sometimes positively. Despite these results, very few of the BSC effects on ecosystem function could be ascribed to changes within the bacterial community. Conclusion: Our results suggest the primary importance of the richness of BSC-forming lichens as drivers of small-scale changes in ecosystem functioning. This study provides valuable insights on semi-arid ecosystems where plant cover is spatially discontinuous and ecosystem function in plant interspaces is regulated largely by BSCs.