Biogeochemical function and microbial diversity in Chihuahuan Desert biocrusts

Mikaela Hoellrich

Biological soil crusts (biocrusts) are living soil aggregates hosting diverse communities of lichens, bryophytes, cyanobacteria, and other microorganisms in the uppermost millimeters of dryland soils. Biocrusts array along a gradient of structural complexity distinguished by the dominant photoautotroph. Consequently, each biocrust type can host a unique microbial community with different biogeochemical processes. Additionally, site specific conditions influence the microorganisms present in a given location. However, no studies to date have analyzed the interactions between diversity, soil conditions, and biocrust carbon fixation. This project aims to 1) assess the carbon fixation capacity of biocrust types under varying incubation times; 2) understand microbial community composition as well as biomass differences within these biocrust types; and 3) identify how geographic location and soil properties explain differences in carbon fixation and microbial diversity. Five biocrust types were collected from four locations in the Chihuahuan Desert. Carbon fixation rates were quantified using a LI-6400XT portable photosynthesis system. Measurements were taken after biocrust wetting and light incubation at different time periods (30min, 2hr, 6hr, 12hr, 24hr). Microbial biomass was assessed for each biocrust type using phospholipid fatty acid assays. Microbial community composition was examined using 16S rRNA gene Illumina sequencing for bacteria and archaea. Carbon fixation tended to increase and respiration decrease over time. Thicker crusts had greater carbon fixation and respiration rates. Net carbon fixation rates differed between sites. Moss crusts recorded the lowest net fixation rates of all biocrusts. Alpha diversity did not change with increasing structural complexity across all sites, but beta diversity showed microbial communities shaped by parent material and biocrust type. This study provides insights into how microbial diversity and biogeochemical cycling can vary across sites within a desert biome. It also provides answers to the questions of how biocrust carbon fixation is affected by time, thickness, community composition, geography, and soil conditions.