What was done
Working with 43 perennial plant species associated with four separate studies of both laboratory- and field-grown plants that ranged in age from one month to 25 years, and that spanned five orders of magnitude in size, the authors assembled a massive data set derived from ~500 coupled measurements of whole-plant dry mass, nitrogen (N) content and respiration rate.  In addition, and indicative of the great variety of circumstances they investigated, they report that the plants they studied came from a wide range of functional groups and "were grown under a heterogeneous set of environmental conditions that included experimentally controlled contrasts involving temperature, light, N supply and atmospheric CO2 concentration."

What was learned
Looking for any fundamental or universal relationship that might possibly exist among total and above-ground plant mass, total and above-ground plant nitrogen content, and total and above-ground plant respiration, Reich et al. determined, in their words, that "the only relationship common across all data in our compilation is that relating respiration per plant and N per plant."  More specifically, they say that "the data for all plants from all studies, including field and laboratory, are described by a single, common relationship between total respiration and total plant N content or above-ground respiration and above-ground N content," such that when plant N content rises, plant respiration rises.

What it means
Since atmospheric CO2 enrichment typically leads to decreases in plant N concentrations per unit biomass under natural field conditions, plant respiration rates per unit biomass in a CO2-enriched world of the future likely will be lower than they are today, which finding bodes well for the sequestration of carbon in plant tissues.  Not only will carbon storage in vegetative tissues be enhanced by the near-universal stimulation of photosynthesis that is produced by the ongoing rise in the air's CO2 content, it will additionally be enhanced by the near-universal reduction in plant respiration that results from the lower plant N concentrations produced by the rising atmospheric CO2 concentration.  Consequently, as the air's CO2 content continues to rise, we can expect ever more carbon to be transferred from the atmosphere to the biosphere, where it will first be used to sustain more plant life and then more animal life (as a result of the greater abundance of plant life, if mankind does not foolishly destroy it).  In addition, this phenomenon will slow the rate of increase in whatever impetus for global warming is provided by the air's rising CO2 concentration.