Biological soil crust (biocrust) communities are highly susceptible to changing temperature and precipitation, and based on field warming experiments we can expect significant reductions in cover and diversity of biocrust on the Colorado Plateau in response to climate change over the next several decades. Because these communities cover much of the plant interspaces in undisturbed areas of the Colorado Plateau, and exhibit significant rates of C and N fixation, as well as influence the ecosystem energy and water budget, reductions of biocrust may have substantial ecosystem-level impacts. Here, we use greenhouse mesocosms to evaluate the temperature sensitivity of the carbon balance of soils with different successional stages levels of biocrust. Specifically, we test the hypothesis that an imbalance of respiration (R) and net primary production (NPP) (i.e., “carbon starvation”) in response to elevated temperatures explains the loss of moss-lichen soil crust in response to warming.
Mesocosms were composed of homogenized soil above which we added either early successional stage light cyanobacterial biocrust, late successional stage moss-lichen biocrust, or a control of no biocrust. Mesocosms were maintained in a greenhouse for 83 days under ‘ambient’ or ‘warmed’(+5°C) conditions, and subjected to a pulse watering regime. Carbon dioxide fluxes were measured midday in transparent chambers and again in opaque chambers, allowing us to partition NPP and R. Fluxes were measured during wet, moist and dry phases of the watering cycle to evaluate the interactive temperature and moisture sensitivity.
Results/Conclusions
Both NPP and R were largest in the late successional biocrust, and highly sensitive to soil moisture - when soils were dry, no measurable carbon fluxes were detected. Warming stimulated soil and biocrust R strongly - R was greater in the warmed versus ambient treatment, and was greater during the warm period of late summer than the cooler period in late fall. In contrast, there was little direct effect of warming on NPP, although warming increased rates of drying, and low water content inhibited NPP. The different temperature response of R and NPP resulted in a strong negative balance of net ecosystem exchange in response to warming. Extended over multiple seasons this imbalance would result in ‘carbon starvation’, and cause desert biocrust communities to experience significant die-off. We suggest our mesocosm experiment provides a mechanism to explain the pattern of die-off in response to warming observed in a field warming experiment at our study site.