PS 41-136 - Drought manipulation favors biocrust abundance but decreases plant available soil nitrogen in the Mojave Desert

Abstract: Drylands contribute over 50% of the interannual variability in global carbon flux. Soil surface microbial communities in drylands, known as biocrusts, drive dryland nutrient exchanges, including nitrogen fixation and carbon fluxes, via photosynthesis and respiration. While biocrusts are adapted to harsh environmental conditions such as chronic drought, these dryland specialists are not invincible to rapid environmental change. Indeed, plant production is most sensitive to drought in drylands. However, research suggests that biocrusts dominated by a drought tolerant cyanobacteria, Microcoleus vaginatus, can resist drought better than biocrusts dominated by co-dominant cyanobacteria in the Choleofaciculaceae family.

In this study, we ask what are the effects of manipulated drought on biocrust communities and their biogeochemical contributions within the driest North American desert, the Mojave Desert? Because one continental survey showed that Mojave Desert sites contained a greater ratio of cyanobacteria in the Choleofaciculaceae compared to Microcoleus vaginatus, we hypothesize that biocrusts will show a decline in abundance under imposed drought. Furthermore, we hypothesize that available soil nitrogen will be lower in drier conditions potentially due to lower microbial activity. We selected randomly 7 control and 7 drought-manipulation plots, each 2.5x2.5 m in area at 2 sites (East and West) distanced ~450 m apart (N= 28). To impose a 1-100 year drought, we constructed rainout shelters that block 66% of precipitation year-round. We monitored biocrust photoautotrophic and nitrogen-fixing cyanobacteria abundance via chlorophyll a and scytonemin pigments and changes in total plant available soil nitrogen using PRS® probes.

Surprisingly, biocrust chlorophyll a and scytonemin concentrations were not impacted by drought treatment (P= 0.31; P= 0.13), though plant available soil nitrogen decreased (P< 0.01). However, chlorophyll a and scytonemin concentration in biocrusts varied by site (P< 0.0001; P< 0.001), seasonality (P= 0.01; P= 0.04), and their interaction (P< 0.001), while scytonemin varied by the interaction of treatment and site (P< 0.01). Our results show that biocrusts are resistant to drought, even in the driest desert of North America. However, while biocrusts may maintain biomass and evade drought through dormancy, energetically costly processes such as nitrogen fixation may be surrendered, which could result in generally lower plant available soil nitrogen in a dryer future. As drylands continue to expand in area and experience hotter, dryer conditions, it is increasingly important to understand drivers of drought resistance and the consequences of these changes on biocrusts and their ecosystem services.