Background
The authors write that "the ability of plants to acquire and assimilate nitrogen (N) is an important determinant of plant growth," and they say that many steps in this process "are affected under stressful environmental conditions such as drought and/or salinity."

What was done
To explore this complex subject in some detail, Perez-Lopez et al. grew well watered and fertilized barley (Hordeum vulgare L. cv. Iranis) plants from seed to maturity within a pair of controlled-environment chambers that were maintained at either ambient (350 ppm) or elevated (700 pm) atmospheric CO2 concentrations in 2.5-L pots containing a 3:1 mix of perlite and vermiculite, during which time they measured a number of important plant properties and processes.

What was learned
The six Spanish scientists report that "under ambient CO2 conditions, 80, 160 and 240 mM NaCl reduced the total plant biomass by 12%, 30% and 44%, respectively," while "growth at elevated CO2 levels led to 24%, 20% and 33% higher total biomass than under ambient CO2 levels for 80, 160 and 240 mM NaCl, respectively," demonstrating that "the relative stimulation of total plant biomass in response to elevated CO2 levels was higher in salt-stressed plants than in non-stressed ones." They attribute this phenomenon to the fact that "under mild salt stress, N metabolism was better maintained under elevated CO2 levels than under ambient CO2 levels, since nitrogen uptake rate, nitrogen translocation rate, photosynthetic nitrogen use efficiency, and nitrate reductase activity were less affected," all of which impacts resulted in "higher growth."

What it means
"As a consequence of all these results," in the words of Perez-Lopez et al., "barley plants subjected to elevated CO2 levels will likely overcome mild saline conditions because of their capacity to maintain efficiency in N metabolism," which, of course, is good news for humanity.