2) Dr. Nina A. Kamennaya

In drylands, biocrusts that harbor microscopic cyanobacteria and algae cover soil surface between the perennial keystone plants. When the increasing aridity causes plant mortality, the biocrusts remain to maintain a nutrient balance and structural stability of the soil. To test this assumption, we studied biocrusts in the protected long-term environmental research site where some areas were shrubbed whereas other areas lost shrubs after a drought episode a decade earlier. During three years, we compared the CO₂ and N₂ fixation, algal and cyanobacterial biovolume and taxonomic diversity in biocrusts in the two areas. We found that algae dominated the biocrusts in the shrubless area, whereas taxonomically more diverse algae and cyanobacteria co-dominated the biocrusts in the shrubbed area. Higher CO₂ and N₂ fixation in the biocrusts in the shrubbed area resulted in higher soil content of organic matter in drier winters. During the wettest winter, however, algal biomass and the total CO₂ fixation in the biocrusts in the shrubless area dramatically increased and algae contributed ~50% of the fixed CO₂. Hence, in the shrubless area, total CO₂ fixation was more dependent on seasonal precipitation. Though, even during the wet winter, biocrusts in the shrubless area did not accumulate organic matter. Finally, the clay soil content in the shrubless area was lower than in that in the shrubbed area. Consequently, the algae-dominated biocrusts in the shrubless area were mechanically less stable. Therefore, in areas which lost perennial keystone plants, the natural development of biocrusts was insufficient to counter the impact of the increasing aridity.