Background
The authors write that "human activities have greatly accelerated emissions of both carbon dioxide and biologically reactive nitrogen to the atmosphere," and that "as nitrogen availability often limits forest productivity, it has long been expected that anthropogenic nitrogen deposition could stimulate carbon sequestration in forests." However, they note that spatially-extensive evidence for this phenomenon "has been lacking," and, therefore, they proceed to provide some.

What was done
Thomas et al., in their words, used "spatially extensive forest inventory data to discern the effect of nitrogen deposition on the growth and survival of the 24 most common tree species of the northeastern and north-central US, as well as the effect of nitrogen deposition on carbon sequestration in trees across the breadth of the northeastern US."

What was learned
It was determined, as the four researchers describe it, that "nitrogen deposition (which ranged from 3 to 11 kg ha^-1 yr^-1) enhanced the growth of eleven species and decreased the growth of three species," while it "enhanced [the] growth of all tree species with arbuscular mycorrhizal fungi associations," leading to "a 40% enhancement over pre-industrial conditions," which response "includes the direct effects of nitrogen deposition on tree growth through soil fertilization, foliar nitrogen uptake and other potential interactions between nitrogen deposition and other environmental changes, including CO2 fertilization." And to give some feeling for the significance of the size of this response, they say that it "exceeds the 23% enhancement of net primary production anticipated for the year 2050 from a doubling of atmospheric CO2 over preindustrial levels, as estimated using free-air CO2 enrichment studies," citing in this regard the work of Norby et al. (2005).

What it means
Thomas et al. conclude that "nitrogen deposition is an important mechanism contributing to carbon sequestration within these temperate forests," but they say that this phenomenon is still "unlikely to explain all of the observed terrestrial carbon sink." Nevertheless, it does go a long way towards doing so, while demonstrating the major benefits of the concomitant increases in the air's CO2 content and temperature with which it interacted over the course of the industrial revolution and its aftermath, which latter increases climate alarmists decry as two of the worst things ever to have happened to the biosphere. Clearly, however, they were not. And neither was the increase in anthropogenic nitrogen deposition. All three of these phenomena interacted with each other in such a way as to greatly increase the productivity of earth's forests, both in temperate latitudes, as demonstrated by Thomas et al., and in tropical regions, as revealed in the many materials we have archived under the heading of Trees (Types -- Tropical) in our Subject Index.

Reference
Norby, R.J., DeLucia, E.H., Gielen, B., Calfapietra, C., Giardina, C.P., King, S.J., Ledford, J., McCarthy, H.R., Moore, D.J.P., Ceulemans, R., De Angelis, P., Finzi, A.C., Karnosky, D.F., Kubiske, M.E., Lukac, M., Pregitzer, K.S., Scarasci-Mugnozza, G.E., Schlesinger, W.H. and Oren, R. 2005. Forest response to elevated CO2 is conserved across a broad range of productivity. Proceedings of the National Academy of Sciences 102: 18,052-18,056.