Background
The author writes that "levels of N [nitrogen] deposition and CO2 have risen in recent decades and are expected to increase further," and in the case of natural ecosystems, he says that "the suppression of diversity by increasing N availability is almost ubiquitous," while noting that "evidence of CO2 effects on species richness is scarce and shows mixed results, with positive, neutral, and negative responses seen in the few published reports." So what is most likely to happen when the two phenomena occur together?

What was done
In what Collins (2009) appropriately describes as "a rare gem in long-term ecological research," Peter B. Reich of the University of Minnesota (USA) presided over the ten-year-long BioCON study conducted at the Cedar Creek Long-Term Ecological Research site, where, as Reich describes it, "species richness was measured in 48 experimental grassland plots (each 2 m by 2 m) planted in 1997 with 16 perennial species [four species from each of four functional groups (C4 grasses, C3 grasses, legumes and non-legume forbs)] and treated since 1998 with all combinations of ambient and elevated atmospheric CO2 (ambient and +180 ppm delivered by means of a free-air CO2 enrichment technique) and ambient and enriched N (ambient and +4 g N m^-2 year^-1 delivered as ammonium nitrate in three equal doses each year)," while a number of plant physiological processes and properties were measured concomitantly throughout each growing season.

What was learned
Reich reports that at the ambient soil N concentration elevated CO2 had minimal impact on observed species richness (-2%), while at the ambient atmospheric CO2 concentration elevated N decreased species richness by fully 15% over the last seven years of the ten-year-long study. When the elevated soil N concentration was combined with the elevated atmospheric CO2 concentration, however, species richness declined by only 5%, leading Reich to conclude that "elevated CO2 reduces losses of plant diversity caused by nitrogen deposition," which finding was so important that he made it the title of his paper.

What it means
Although additional long-term experiments are needed to confirm the generality of the beneficial impact of atmospheric CO2 enrichment on ecosystem biodiversity in the face of increased N deposition, with levels of nitrogen deposition "expected to increase further," as noted by Reich, earth's many natural ecosystems could well breath a collective -- albeit tentative -- sigh of relief that the atmosphere's CO2 concentration is rising in tandem with the increasing level of nitrogen deposition that is being experienced throughout the world. Contrary to climate-alarmist claims that the ongoing rise in the air's CO2 content will lead to the extinction of many species of plants, it will likely do just the opposite.

Reference
Collins, S.L. 2009. Biodiversity under global change. Science 326: 1353-1354.