ELEVATED ATMOSPHERIC CO 2 DOES NOT CONSERVE SOIL WATER IN THE MOJAVE DESERT

Numerous studies, including those of desert plants, have shown reduced stomatal conductance under elevated atmospheric CO 2 . As a consequence, soil water has been postulated to increase. Soil water was measured for >4 yr at the Nevada Desert Free Air CO 2 Enrichment (FACE) Facility to determine if elevated atmospheric CO 2 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 CO 2 (550 μmol/mol), blower control (ambient CO 2 ), and non‐ring treatments. The treatment main effect was not significant in any analyses of variance. Although the treatment × date interaction was significant for soil water in the top 0.5 m of soil, the expected greater soil water for elevated CO 2 vs. ambient CO 2 only occurred on one sampling date. In contrast, soil water for that same depth was significantly lower under elevated CO 2 on six dates. Thus, we infer that increased water use from increased primary productivity (and therefore leaf area) under elevated CO 2 offset the decreased water use from reduced stomatal conductance, and hence soil water was not conserved under elevated CO 2 in the Mojave Desert, unlike other ecosystems.