Nitrogen Deposition Boosts U.S. Tree Growth

Thomas et al. (2010) 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. Therefore, in their words, the authors 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."

Based on their analysis, it was determined 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).

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 the IPCC opines are 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 these forests.

Additional 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.