Volume 13, Number 22: 2 June 2010
Earth's land plants grow best when supplied with optimum levels of soil nitrogen; and when this reactive nitrogen (Nr) is present in soils in concentrations that are less than optimal, terrestrial vegetation grows less vigorously and removes less CO2 from the atmosphere. As a result, climate alarmists contend that less-than-optimal soil nitrogen concentrations will sooner or later lead to a significant reduction in the strength of the growth stimulation that is provided by the aerial fertilization effect of the ongoing rise in the air's CO2 content, due to their claim that the limited amount of Nr in the soil simply cannot supply enough of this essential nutrient to maintain the increase in growth that could otherwise be supported by the atmosphere's rising CO2 concentration, which string of suppositions has come to be known as the progressive nitrogen limitation hypothesis (Hungate et al., 2003; Luo et al., 2004). And the scientists that adhere to this point of view thus contend there is too little reactive nitrogen in the terrestrial biosphere.
At the other end of the Nr spectrum are scientists such as Fujimaki et al. (2009), who contend that "anthropogenic addition of reactive nitrogen to the biosphere" - "through production of fertilizers, cultivation of N-fixing plants, and utilization of fossil fuels" - "is increasing globally and some terrestrial ecosystems are suffering from a state of excess Nr." They contend, for example, that "excess Nr has a harmful impact on vegetation cover and species diversity," due to "increasing competitive abilities for a small number of nitrophilic species, increase of herbivory, decreases in mutualistic fungi, increases in pathogenic fungi, and enhanced invasibility by exotic species that are highly competitive under conditions of high soil nitrate." And the scientists that adhere to this point of view thus contend there is too much reactive nitrogen in the terrestrial biosphere.
Both sides of this debate feel strongly about their positions; and both see undesirable consequences in the not too distant future, if certain aspects of the way modern societies operate are not radically reformed. But is there a middle way? ... a way whereby the two extreme phenomena might come together - on their own, or naturally - to produce a truly harmonious outcome that is actually to be preferred above what adherents to each of the two opposed philosophies separately seek? We believe there is.
Let each phenomenon proceed unimpaired by overt human intervention. Let the air's CO2 content rise in accordance with the natural (unforced) course of technological development; and let the production of reactive nitrogen likewise proceed. In so doing, we should see the latter phenomenon provide what is needed (more reactive nitrogen) to enable the world's terrestrial vegetation to capture and sequester more of the carbon supplied to the air by anthropogenic CO2 emissions, much of which will be needed to enhance the productivity of the world's cropped and naturally-vegetated land, enabling us to feed the still-growing human population of the planet without the taking of what yet remains of the world's land and freshwater resources that provide the habitat and food that sustains the dwindling fragments of what we could call "wild nature."
Evidence that the outcome of such a laissez faire approach to the two issues is indeed reasonable is provided by the fact that even Fujimaki et al. admit that "ecosystem net primary production seems to be under N limitation," and that "in global trends to date, plant growth itself tends to be stimulated with increase of N deposition," which leads them to conclude that "if N deposition continues at the present rates in the next few decades, ecosystem net primary production would be released from the limitation of N availability."
Policy wonks seem always in a hurry to "fix" things, when letting nature take its course is often the best policy of all.
Sherwood, Keith and Craig Idso
References
Fujimaki, R., Sakai, A. and Kanedo, N. 2009. Ecological risks in anthropogenic disturbance of nitrogen cycles in natural terrestrial ecosystems. Ecological Research 24: 955-964.
Hungate, B.A., Dukes, J.S., Shaw, M.R., Luo, Y. and Field, C.B. 2003. Nitrogen and climate change. Science 302: 1512-1513.
Luo, Y., Su, B., Currie, W.S., Dukes, J.S., Finzi, A., Hartwig, U., Hungate, B., McMurtrie, R.E., Oren, R., Parton, W.J., Pataki, D.E., Shaw, M.R., Zak, D.R. and Field, C.B. 2004. Progressive nitrogen limitation of ecosystem responses to rising atmospheric carbon dioxide. BioScience 54: 731-739.