Net ecosystem CO2 exchange in Mojave Desert shrublands during the eighth year of exposure to elevated CO2

Arid ecosystems, which occupy about 35% of the Earth’s terrestrial surface area, are believed to be among the most responsive to elevated [CO2]. Net ecosystem CO2 exchange (NEE) was measured in the eighth year of CO2 enrichment at the Nevada Desert Free-Air CO2 Enrichment (FACE) Facility between the months of December 2003– December 2004. On most dates mean daily NEE (24 h) (lmol CO2 m 2 s ) of ecosystems exposed to elevated atmospheric CO2 were similar to those maintained at current ambient CO2 levels. However, on sampling dates following rains, mean daily NEEs of ecosystems exposed to elevated [CO2] averaged 23 to 56% lower than mean daily NEEs of ecosystems maintained at ambient [CO2]. Mean daily NEE varied seasonally across both CO2 treatments, increasing from about 0.1 lmol CO2 m 2 s 1 in December to a maximum of 0.5–0.6 lmol CO2 m 2 s 1 in early spring. Maximum NEE in ecosystems exposed to elevated CO2 occurred 1 month earlier than it did in ecosystems exposed to ambient CO2, with declines in both treatments to lowest seasonal levels by early October (0.09 0.03 lmol CO2 m 2 s ), but then increasing to near peak levels in late October (0.36 0.08 lmol CO2 m 2 s ), November (0.28 0.03 lmol CO2 m 2 s ), and December (0.54 0.06 lmol CO2 m 2 s ). Seasonal patterns of mean daily NEE primarily resulted from larger seasonal fluctuations in rates of daytime net ecosystem CO2 uptake which were closely tied to plant community phenology and precipitation. Photosynthesis in the autotrophic crust community (lichens, mosses, and free-living cyanobacteria) following rains were probably responsible for the high NEEs observed in January, February, and late October 2004 when vascular plant photosynthesis was low. Both CO2 treatments were net CO2 sinks in 2004, but exposure to elevated CO2 reduced CO2 sink strength by 30% (positive net ecosystem productivity 5 127 17 g C m 2 yr 1 ambient CO2 and 90 11 g C m 2 yr 1 elevated CO2, P 5 0.011). This level of net C uptake rivals or exceeds levels observed in some forested and grassland ecosystems. Thus, the decrease in C sequestration seen in our study under elevated CO2 – along with the extensive coverage of arid and semi-arid ecosystems globally – points to a significant drop in global C sequestration potential in the next several decades because of responses of heretofore overlooked dryland ecosystems.