What was done
Experimental plots differing in plant diversity were established in a nutrient-poor calcareous grassland in northwestern Switzerland by removing selected species from some of the plots.  Although plots contained 31 (which was the naturally occurring background number of species per square meter), 12 or 5 species, the proportion of plant functional types in each plot remained unchanged (55% graminoids, 15% legumes, and 30% non-legume forbs).  In addition, plots were fumigated with air containing either 360 or 600 ppm CO2 for four years to determine the effects of elevated CO2 across a biodiversity gradient in this grassland community.  In this paper, the authors report on relationships found to exist among species diversity, elevated CO2 and soil nitrate concentrations.

What was learned
As plant community diversity decreased at ambient CO2, soil nitrate concentrations increased.  Elevated CO2, however, acted to reduce soil nitrate concentrations at all levels of plant diversity.  Moreover, nitrification, a biological process that yields nitrate, increased with decreasing species diversity at ambient CO2; while at elevated CO2, rates of nitrification were 25% lower than those observed at ambient CO2, regardless of community diversity.

What it means
As the air's CO2 content continues to rise, it is likely that plants will experience significant increases in their rates of photosynthesis.  Hence, it seems assured that plants in a CO2-enriched world will have the energy-producing resources they require to extract the greater amounts of nitrate from the soil that they need to enhance their production of biomass; and with lower soil nitrate concentrations, there is less risk of nitrate pollution of groundwater.

The importance of this simple finding cannot be understated, for both the quantity and quality of water that will be needed by future inhabitants of the globe has been predicted by many organizations to decrease dramatically as time progresses.  This CO2-enhanced natural purification process may thus help to maintain the relative purity of freshwater drinking supplies that might otherwise be compromised by higher levels of nitrate pollution.