What was done
Working in Europe, the authors analyzed carbon sink dynamics that are driven by urbanization-induced changes in land use, climate, atmospheric CO2 concentration and nitrogen deposition, using the BIOME-BGC terrestrial ecosystem model to calculate responses of the local biosphere to these several "urban changes" applied individually and together. Examples of the four classes of urban-induced environmental changes are (1) for land use, pristine soils and their associated vegetation being replaced by buildings, roads, parking lots, etc., (2) for climate, increases in temperature due to the urban heat island phenomenon, together with associated changes in local precipitation, (3) for atmospheric CO2 concentration, increases in the air's CO2 content from vehicle exhaust and other combustion processes that produce the urban CO2 dome phenomenon, and (4), for nitrogen deposition, many of the same processes that simultaneously release nitrogen to the air.

What was learned
Trusilova and Churkina report finding that "fertilization effects from the elevated CO2 and atmospheric nitrogen deposition made the strongest positive contributions to the carbon uptake (0.023 PgC per year and 0.039 PgC per year, respectively), whereas the impervious urban land and local urban meteorological conditions resulted in a reduction of carbon uptake (-0.005 PgC per year and -0.007 PgC per year, respectively)." The synergetic effect of the four urbanization-induced changes, however, "was an increase of the carbon sequestration in Europe of 0.058 PgC per year," a result that is greater than the algebraic sum of the four individual components of the carbon sequestration equation. The reason for the amplification of the two positive responses (which manifests itself when the two phenomena occur in unison) derives from the fact that "atmospheric CO2 and soil nitrogen availability co-limit [the] productivity of land ecosystems," as the two researchers describe it, so that if more nitrogen becomes available to plants, the aerial fertilization effect of the elevated CO2 concentration is generally magnified somewhat. Another reason for the dominance of the CO2- and nitrogen-induced phenomena is that "the land use and urban climate changes affect rather small land areas while the urban CO2 and nitrogen pollution spread over larger areas."

What it means
When all is said and done, it is clear -- although perhaps not intuitively so -- that the urbanization process actually leads to an amplification of carbon sequestration in the vegetation and soils of urban ecosystems, which in the words of Trusilova and Churkina has "led to a net increase of [the] carbon sink in Europe."