Temporal dynamics of root growth under long-term exposure to elevated CO2 in the Mojave Desert

Background/Question/Methods   Quantifying belowground carbon fluxes and stocks presents one of the greatest challenges to understanding and predicting the terrestrial carbon cycle. Deserts cover ~30% of the terrestrial biosphere and are considered “belowground dominated,” yet little is known about the impacts of rising CO2 and altered precipitation on root biomass and belowground carbon cycling in deserts. This study addresses this issue by evaluating root dynamics in an intact desert ecosystem exposed to 10 years of elevated CO2. The objectives are to (1) quantify the temporal dynamics of root standing crop, production, and mortality under long-term exposure to elevated CO2, and (2) learn how elevated CO2 interacts with variable precipitation and microsite heterogeneity to affect root dynamics. In this regard, we evaluated minirhizotron data from 168 tubes placed under two dominant shrub species (Larrea tridentata and Ambrosia dumosa) and in interspaces at the Nevada Desert FACE Facility in the Mojave Desert. Fine root production, mortality, and standing crop were visually quantified every four weeks for four years (2003-2007), resulting in 8736 observations. The data were synthesized within a Bayesian framework that incorporated a temporally dynamic model of root production, which explicitly included the effects of precipitation history, CO2 level, and species identity.

Results/Conclusions   Temporal dynamics of fine root production and, to a lesser extent, mortality, generally mirrored precipitation events. For example, production was greatest during wet spring periods but almost non-existent during extended rain-free periods. Root mortality, while also peaking during times of high soil moisture, continued to occur during dry periods. Total root standing crop differed by 2.16x across the study period, with most tubes differing from each other by factors of 2.0 to 4.5. The greatest standing crop was in the wet spring of 2005. Elevated CO2 does not appear to directly affect fine root production, mortality, or standing crop but does positively interact with cumulative precipitation over the previous 14 days. For example, root production was ~1.6x more sensitive to recent precipitation under elevated compared to ambient CO2. However, temporal variability in root production and mortality was enhanced under elevated CO2, particularly in interspace regions. Species or microsite type had large effects on root standing crop, independent of CO2 treatment, such that standing crop was 1.3x greater under Larrea compared to Ambrosia. Thus, this study lends insight into the variable nature of belowground carbon dynamics, whereby roots may play a particularly important role in desert ecosystems.