Anthropogenic Inputs of Nitrogen to the Environment

In a Geochemistry Perspective in the 16 December 2011 issue of Science, Elser (2011) writes that "in early agricultural periods, humans returned animal and human wastes to the fields, thereby adding nitrogen (and phosphorus) and enhancing crop productivity." After acquiring the ability to produce reactive nitrogen (Nr) from inert nitrogen (N₂) in the atmosphere, however, man used this knowledge to create artificial nitrogen-rich fertilizers, which greatly increased the amount of reactive N residing in various biospheric nitrogen pools. And with the widespread release of oxidized nitrogen (NO_x) to the atmosphere by the ever more numerous and powerful internal combustion engines that accompanied the industrialization of the world, Elser says that rates of nitrogen inputs to the biosphere are "thought to have at least doubled since preindustrial times," citing Vitousek et al. (1997), with the result that Holtgrieve et al. (2011) have identified a coherent signature of anthropogenic nitrogen deposition throughout remote areas of the Northern Hemisphere that began about 1895, and that could, in their words, "have important effects on primary producers and food webs."

So is this something about which we should be concerned? Or is it actually a positive thing?

Elser says the evidence cited above adds weight to speculation that the biosphere is heading toward a widespread condition of chronic phosphorus limitation, as suggested by Peñuelas et al. (2011). It should be noted, however, that greater inputs of reactive nitrogen to the biosphere more importantly act to thwart the negative implications of the progressive nitrogen limitation hypothesis, whereby historically less-than-adequate amounts of nitrogen in the environment have been speculated to greatly reduce - if not totally counter - the aerial fertilization effect of the CO₂ emitted to the air from the burning of coal, gas and oil. And this phenomenon appears to have more than compensated for any new degree of phosphorous limitation to plant productivity that may have occurred up to this point in time.

Elser writes, for example, that "Hietz et al. (2011) gathered data in tropical rainforests of Panama and Thailand," and he says the results indicate that "increasing levels of nitrogen deposition in the tropics have alleviated forest nitrogen limitation." And as a result, this phenomenon has allowed the aerial fertilization effect of the ongoing rise in the air's CO₂ content to significantly stimulate tropical forest growth nearly everywhere on Earth.

All things considered, therefore, it would appear that the extra anthropogenic-produced nitrogen deposited to the surface of the Earth, and the extra anthropogenic-produced CO₂ deposited in the planet's atmosphere - each as a result of the industrialization of our species - are both helping to significantly enhance the growth and promote the vitality of Earth's life-supporting vegetation ... as the Greening of the Earth continues.

Additional References
Hietz, P., Turner, B.L., Wanek, W., Richter, A., Nock, C.A. and Wright, S.J. 2011. Long-term change in the nitrogen cycle of tropical forests. Science 334: 664-666.

Holtgrieve, G.W., Schindler, D.E., Hobbs, W.O, Leavitt, P.R., Ward, E.J., Bunting, L., Chen, G., Finney, B.P., Gregory-Eaves, I., Holmgren, S., Lisac, M.J., Lisi, P.J., Nydick, K., Rogers, L.A., Saros, J.E., Selbie, D.T., Shapley, M.D., Walsh, P.B. and Wolfe, A.P. 2011. A coherent signature of anthropogenic nitrogen deposition to remote watersheds of the Northern Hemisphere. Science 334: 1545-1548.

Peñuelas, J., Sardans, J., Rivas-Ubach, A. and Janssens, I.A. 2012. The human-induced imbalance between C, N and P in Earth's life system. Global Change Biology 18: 3-6.

Vitousek, P.M., Aber, J.D., Howarth, R.W., Likens, G.E., Matson, P.A., Schindler, D.W., Schlesinger, W.H. and Tilman, D.G.. 1997. Human alteration of the global nitrogen cycle: Sources and consequences. Ecological Applications 7: 737-.