Elevated atmospheric CO2 does not conserve soil water in the Mojave Desert

Numerous studies, including those of desert plants, have shown reduced stomatal conductance under elevated atmospheric CO2. As a consequence, soil water has been postulated to increase. Soil water was measured for >4 yr at the Nevada Desert Free Air CO, Enrichment (FACE) Facility to determine if elevated atmospheric CO, conserves soil water for a desert scrub community in the Mojave Desert. We measured soil water in the top 0.2 and 0.5 m of soil with time domain reflectometry and to 1.85 m with a neutron probe for the three treatments at Desert FACE: elevated CO2 (550 mumol/mol), blower control (ambient CO2), and non-ring treatments. The treatment main effect was not significant in any analyses of variance. Although the treatment x date interaction was significant for soil water in the top 0.5 m of soil, the expected greater soil water for elevated CO2 vs. ambient CO2 only occurred on one sampling date. In contrast, soil water for that same depth was significantly lower under elevated CO2 on six dates. Thus, we infer that increased water use from increased primary productivity (and therefore leaf area) under elevated CO2 offset the decreased water use from reduced stomatal conductance, and hence soil water was not conserved under elevated CO2 in the Mojave Desert, unlike other ecosystems.