Elevated atmospheric CO2 does not increase root biomass or carbon stocks in the Mojave Desert

Background/Question/Methods

Increased atmospheric CO₂ increases net assimilation rates of almost all plant species, and this increased assimilation response persists over long time periods even under field conditions.  Increased assimilation under elevated CO₂ is expected to increase plant growth rates, and indeed increased aboveground production has been observed in many Free Air CO₂ Enrichment (FACE) experiments, including the Nevada Desert FACE Facility in the Mojave Desert of the southwestern US.  Similarly, increased availability of photoassimilates is expected to increase root production, especially where soil resources such as soil water or nutrients may constrain plant responses to elevated CO₂.  The purpose of this study was to determine if 10 years of elevated CO₂ treatment (513 ppm) increased root biomass or root carbon stocks in this arid ecosystem relative to ambient CO₂ (375 ppm).  At the end of the Desert FACE experiment, we conducted extensive harvests of both aboveground and belowground biomass.  Here we report on harvests of root systems to 1 m soil depth for the 5 major species at the site plus interspace microsite locations as well as harvests of root systems to 1 m soil depth for 0.5 x 8.0 m trenches to characterize the plant community.

Results/Conclusions

Biomass of coarse (1-2 mm diameter) and woody roots averaged over 3 trenches per plot and 3 plots per treatment were not significantly different among the 3 treatments (plots with the FACE apparatus and fumigated with elevated CO₂ air, plots with the FACE apparatus and fumigated with ambient CO₂ air, and plots without the FACE apparatus).  Similarly, coarse and woody root biomasses were not significantly different between elevated and ambient CO₂ treatments for all 5 species and the interspace microsite locations, although differences occurred among species and the interspace.  Root:shoot ratios and root mass ratios also were not significantly different among treatments. Fine roots (≤ 1 mm diameter) were measured only for 2 species, and fine root biomass also was not significantly affected by elevated CO₂.  Because carbon content as a percent of root biomass was not significantly different among treatments for fine, coarse, and woody roots, we conclude that root carbon stocks also were not significantly affected by elevated CO₂.  Thus unlike other ecosystems, especially forested ecosystems, elevated CO₂ does not increase root biomass or root carbon stocks in the Mojave Desert ecosystem.