Biocrust influences on soil fertility vary among community states and are negatively impacted by warming temperatures

Background/Question/Methods

Biological soil crusts (biocrusts) are communities composed of cyanobacteria, lichens, and/or bryophytes, and numerous heterotrophic microbes. Biocrusts are widespread in drylands, where they are considered important for biogeochemical cycling and soil fertility due to roles in enhancing soil stability, fixing CO₂ under conditions when vascular plant activity and cover are low, and serving as a dominant source of ‘new’ nitrogen (N) via N₂fixation by various community members. While these and other functions are known to vary qualitatively among biocrust community states, the influence of community structure on biocrust functional roles remains poorly quantified. Using a mix of greenhouse and field studies with biocrusts and soils from the Colorado Plateau, we explored the influence of early- (cyanobacteria dominated) vs. late-successional (moss and lichen dominated) biocrust community states, and their interactions with warming temperatures, on soil biogeochemical properties.

Results/Conclusions

In field settings, we found soils from under early-successional biocrusts had lower concentrations of phosphorus, organic and inorganic C and N, and microbial biomass than soils from under late-successional biocrusts. A companion experiment revealed that the chemical content of leachate from early successional biocrusts also had lower inputs of organic C and N compared with late-successional biocrusts. Greenhouse mesocosms established from a common, homogenized soil and then either left bare (no biocrust cover), or covered with early- vs. late-successional biocrusts and maintained for 3 months at ambient or warmed (+ 5°C) temperatures revealed potentially negative influences of early-successional biocrusts on important components of soil fertility. Specifically, compared to bare and late-successional mesocoms, soils in early-successional mesocosms had lower concentrations of organic and inorganic N, higher C:N ratios, and reduced microbial biomass. Regardless of cover type, warming led to lower concentrations of total N, greater concentrations of NO₃^- (which is more easily lost from this system), higher C:N ratios, and reduced microbial biomass compared to mesocosms at ambient temperatures. Collectively, our results support a larger influence of late-successional biocrusts on the maintenance of soil fertility and suggest the potential for negative impacts of warming temperatures on soil fertility regardless of biocrust community state. These findings combine with previous experimental studies that have revealed rapid climate change-induced mortality of various late-successional community members to raise concern that potential shifts in biocrust community structure toward early-successional states could reduce dryland soil health and overall ecosystem functioning.