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Deserts are considered ’below-ground dominated’, yet little is known about the impact of rising CO2 in combination with natural weather cycles on long-term dynamics of root biomass. This study quantifies the temporal dynamics of fine-root production, loss and standing crop in an intact desert ecosystem exposed to 10yr of elevated CO2. We used monthly minirhizotron observations from 4yr (20032007) for two dominant shrub species and along community transects at the Nevada Desert free-air CO2 enrichment Facility. Data were synthesized within a Bayesian framework that included effects of CO2 concentration, cover type, phenological period, antecedent soil water and biological inertia (i.e. the influence of prior root production and loss). Elevated CO2 treatment interacted with antecedent soil moisture and had significantly greater effects on fine-root dynamics during certain phenological periods. With respect to biological inertia, plants under elevated CO2 tended to initiate fine-root growth sooner and sustain growth longer, with the net effect of increasing the magnitude of production and mortality cycles. Elevated CO2 interacts with past environmental (e.g. antecedent soil water) and biological (e.g. biological inertia) factors to affect fine-root dynamics, and such interactions are expected to be important for predicting future soil carbon pools.