Background
The authors write that "arid and semiarid ecosystems are significant components of the terrestrial carbon budget," as they (1) cover "47% of the terrestrial surface (Lal, 2004)," (2) represent "the fifth largest pool of soil organic carbon (Jobbagy and Jackson, 2000)," and (3) exhibit "large increases in net primary productivity (NPP) in response to small changes in water availability (Smith et al., 1997)."

What was done
Beginning their work at the Nevada Desert Free-Air CO2 Enrichment (FACE) Facility (NDFF), where they say "no significant differences in soil carbon (C) and nitrogen (N) were observed between CO2 treatments [ambient (~367 ppm) and 513 ppm] in 1999," the eleven researchers describe how above- and below-ground biomass and soils to a depth of 1 m were harvested by plant-cover type after ten years of continuous treatment.

What was learned
Evans et al. report that at the end of the ten-year study, (1) "soils were the dominant pool of C and N and [their] contents were significantly greater under elevated CO2 across all cover types," that (2) "total ecosystem organic C under elevated CO2 was 1,170 gC/m² ... compared with 1,030 gC/m² under ambient conditions," which represents an approximate 13.5% increase in carbon sequestration for an approximate 145-ppm increase in atmospheric CO2 concentration, and that (3) "differences were owing solely to soil organic C." And so they thereby provided us with what they say is "the first direct measure of long-term enhancements to NEP [Net Ecosystem Productivity] stimulated by elevated CO2."

On another note, the eleven researchers also say that "the progressive N-limitation hypothesis predicts increased N limitations to NPP as ecosystems accumulate C," but they report that "this has not yet been observed at the NDFF," where they say that "increased C inputs accelerate rates of soil N transformations, thus increasing N mineralization and inorganic N availability," citing Billings et al. (2004) and Jin et al. (2011).

What it means
In the final sentence of their paper's abstract, Evans et al. state that their results "provide direct evidence that CO2 fertilization substantially increases ecosystem C storage and that arid ecosystems are significant, previously unrecognized, sinks for atmospheric CO2 that must be accounted for in efforts to constrain terrestrial and global C cycles."

References
Billings, S.A., Schaeffer, S.M. and Evans, R.D. 2004. Soil microbial activity and N availability with elevated CO2 in Mojave Desert soils. Global Biogeochemical Cycles 18: 10.1029/2003GB002137.

Jin, V.L., Schaeffer, S.M., Ziegler, S.E. and Evans, R.D. 2011. Soil water availability and microsite mediate fungal and bacterial phospholipid fatty acid biomarker abundances in Mojave Desert soils exposed to elevated atmospheric CO2. Journal of Geophysical Research: Biogeosciences 116: 10.1029/2010JG001564.

Jobbagy, E.G. and Jackson, R.B. 2000. The vertical distribution of soil organic carbon and its relation to climate and vegetation. Ecological Applications 10: 423-436.

Lal, R. 2004. Carbon sequestration in dryland ecosystems. Environmental Management 33: 528-544.

Smith, S.D., Monson, R.K. and Anderson, J.E. 1997. Physiological Ecology of North American Desert Plants. Springer, New York, New York, USA.