Temperature and moisture response of biological soil crust C flux and predictions for contributions to total ecosystem carbon exchange

Background/Question/Methods

Understanding organismal contributions to carbon balance can enable more accurate forecasts on future carbon emissions scenarios. In arid lands, biomass and production is low due to water constraints. However, biological soil crust communities (cyanobacteria, mosses, and lichens) occupy the spaces between plant canopies, increasing the potential land surface area for photosynthesis. Crusts may respond to different abiotic conditions than vascular plants (e. g., small moisture pulses, fewer temperature constraints), and thus may contribute to ecosystem-level carbon cycling. We compared photosynthesis and respiration rates of soil crusts across an elevation gradient representing the major ecosystem types in central New Mexico: desert grassland, shrubland, juniper savanna, and piñon-juniper woodland. Each site had an eddy covariance flux tower recording ecosystem carbon exchange. We hypothesized that sites would differ in the type and amount of crust cover and that crust types would differ in their responses to temperature and moisture, resulting in site-level variability in the total carbon contributions of the crusts. We recorded carbon flux for each sample at three temperatures and three moisture levels in a response surface design. We used the response surface to predict the contribution of crusts at each site to the total yearly carbon flux.

Results/Conclusions

Piñon-juniper woodlands had many mature, dark crusts, juniper-savanna and shrublands had high cover of light crust with scattered patches of mature crust, and grassland had low crust development. Crusts from the piñon-juniper woodland had the highest net photosynthesis rates, juniper-savanna and shrubland crusts were intermediate, and grassland crusts had the lowest rates. As predicted, crust types differed in their responses to temperature and moisture, as indicated by a significant interaction (P<0.05) among site, temperature, and moisture response for both net photosynthesis and respiration. Piñon-juniper and juniper-savanna crusts showed positive net photosynthesis responses to temperature, and piñon-juniper had a temperature-by-moisture interaction (P<0.05), with less response to temperature at low moisture levels. Shrubland crusts showed a hump-shaped response to moisture (P<0.05), suggesting inactivity at low moisture, highest net photosynthesis levels at intermediate moisture, and reduced CO₂ exchange with excess water. Grassland crusts showed no significant response to temperature or moisture. In all sites, crusts occupied <5% of total ground cover, and thus the contribution to ecosystem exchange was low. Results suggest that although the total biomass and rates of crust exchange are low, because they occupy otherwise sparse areas, they are important for carbon cycling in arid lands at a global scale.