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Will There Be Enough Nitrogen?
Volume 15, Number 26: 27 June 2012

The progressive nitrogen limitation hypothesis suggests that low concentrations of soil nitrogen will gradually curtail the ability of the productivity-enhancing effect of rising atmospheric CO2 concentrations to maintain increased plant growth and ecosystem carbon sequestration rates as time progresses (Hungate et al., 2003; Luo et al., 2004). But is this really so?

In a study that comes to bear upon this important question, Butler et al. (2012) examined how warmer-than-current temperatures influenced various fluxes and pools of nitrogen (N) within an even-aged, mixed deciduous forest that is dominated by red oaks (Quercus rubra) in central Massachusetts (USA). There, during the summer and fall of 2001, they installed by hand (to minimize disturbance in a 30 x 30 meter area) about 5 km of heating cables that were buried at 10 cm depth and spaced about 20 cm apart, while a similar sized unaltered plot was maintained nearby as a control against which to compare the climatic and biological impacts of the cable-heated plot, where they say that "each minute, heating cables were turned on or off automatically to maintain a 5°C temperature differential between heated and control areas." So what did they learn?

"Since the start of the experiment," in the words of the thirteen researchers, they say that they "have documented a 45% average annual increase in net nitrogen mineralization [the process by which organic forms of nitrogen found in dead plant material are converted by microbes to inorganic forms that may be taken up by living plants] and a three-fold increase in nitrification [the biological process that converts nitrogen-containing organic compounds into nitrates and nitrites that can be used by living plants] such that in years 5 through 7, 25% of the nitrogen mineralized [was] then nitrified." In addition, they further state that "the increase in N availability in the warmed area has led to increases in leaf N and plant C storage relative to the control (Melillo et al., 2011) and to an increase in relative growth rates, especially for red maples," although they add that "leaf N is positively correlated to photosynthetic rate and carbon storage in many plants growing across the globe," citing Field and Mooney (1986), Reich et al. (1994, 1995, 1997) and Ollinger et al. (2008).

In discussing their findings, Butler et al. write that "the increase in N mineralization in response to warming that we documented in this study has also been observed in other studies; some in forests (Peterjohn et al., 1994; Hartley et al., 1999; Rustad et al., 2001; Melillo et al., 2002), some in grasslands (Shaw and Harte, 2001) and some in tundra (Chapin et al., 1995)." And they add that "this sustained increase in net N mineralization with warming has been accompanied by increases in net nitrification, which has also been reported by others (Hartley et al., 1999; Barnard et al., 2004; Emmett et al., 2004)."

And thus it is that the US research team concludes that "as CO2 concentrations increase and warming stimulates increases in N availability, it is possible that we may see further increases in growth rates of these species," as has, in fact, already been demonstrated to be the case in several free-air CO2 enrichment or FACE studies, including those of Finzi et al. (2006), Norby and Iversen (2006), Zak et al. (2007), McCarthy et al. (2010) and Hofmockel et al. (2011).

Sherwood, Keith and Craig Idso

Barnard, R., Barthes, L., Roux, X.L., Harmens, H., Raschi, A., Soussana, J., Winkler, B. and Leadley, P.W. 2004. Atmospheric CO2 elevation has little effect on nitrifying and denitrifying enzyme activity in four European grasslands. Global Change Biology 10: 488-497.

Butler, S.M., Melillo, J.M., Johnson, J.E., Mohan, J., Steudler, P.A., Lux, H., Burrows, E., Smith, R.M., Vario, C.L., Scott, L., Hill, T.D., Aponte, N. and Bowles, F. 2012. Soil warming alters nitrogen cycling in a New England forest: implications for ecosystem function and structure. Oecologia 168: 819-828.

Chapin, F.S., Shaver, G.R., Giblin, A.E., Nadelhoffer, K.J. and Laundre, J.A. 1995. Responses of Arctic tundra to experimental and observed changes in climate. Ecology 76: 694-711.

Emmett, B.A., Beier, C., Estiarte, M., Tietema, A., Kristensen, H.L., Williams, D., Penuelas, J., Schmidt, I. and Sowerby, A. 2004. The response of soil processes to climate change: results from manipulation studies of shrublands across an environmental gradient. Ecosystems 7: 625-637.

Field, C. and Mooney, H.A. 1986. The photosynthesis-nitrogen relationship in wild plants. In: Givnish, T.J. (Ed.). On the Economy of Plant Form and Function. Cambridge University Press, Cambridge, United Kingdom, pp. 25-55.

Finzi, A.C., Moore, D.J.P., DeLucia, E.H., Lichter, J., Hofmockel, K.S., Jackson, R.B., Kim, H.-S., Matamala, R., McCarthy, H.R., Oren, R., Pippen, J.S. and Schlesinger, W.H. 2006. Progressive nitrogen limitation of ecosystem processes under elevated CO2 in a warm-temperate forest. Ecology 87: 15-25.

Hartley, A.E., Neill, C., Melillo, J.M., Crabtree, R. and Bowles, F.P. 1999. Plant performance and soil nitrogen mineralization in response to simulated climate change in subarctic dwarf shrub heath. Oikos 86: 331-343.

Hofmockel, K.S., Gallet-Budynek, A., McCarthy, H.R., Currie, W.S., Jackson, R.B. and Finzi, A. 2011. Sources of increased N uptake in forest trees growing under elevated CO2: results of a large-scale 15N study. Global Change Biology 17: 3338-3350.

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.

McCarthy, H,.R., Oren, R., Johnsen, K.H., Gallet-Budynek, A., Pritchard, S.G., Cook, C.W., LaDeau, S.L., Jackson, R.B. and Finzi, A.C. 2010. Re-assessment of plant carbon dynamics at the Duke free-air CO2 enrichment site: interactions of atmospheric [CO2] with nitrogen and water availability over stand development. New Phytologist 185: 514-528.

Melillo, J.M., Butler, S., Johnson, J., Mohan, J., Steudler, P., Lux, H., Burrows, E., Bowles, F., Smith, R., Scott, L., Vario, C., Hill, T., Burton, A., Zhou, Y.-M. and Tang, J. 2011. Soil warming, carbon-nitrogen interactions, and forest carbon budgets. Proceedings of the National Academy of Sciences USA 108: 9508-9512.

Melillo, J.M., Steudler, P.A., Aber, J.D., Newkirk, K., Lux, H., Bowles, F.P., Catricala, C., Magill, A., Ahrens, T. and Morrisseau, S. 2002. Soil warming and carbon-cycle feedbacks to the climate system. Science 298: 2173-2176.

Norby, R.J. and Iversen, C.M. 2006. Nitrogen uptake, distribution, turnover, and efficiency of use in a CO2-enriched sweetgum forest. Ecology 87: 5-14.

Ollinger, S.V., Richardson, A.D., Martin, M.E., Hollinger, D.Y., Frolking, S.E., Reich, P.B., Plourde, L.C., Katul, G.G., Munger, J.W., Oren, R., Smith, M.-L., Paw, U.K.T., Bolstad, P.V., Cook, B.D., Day, M.C., Martin, T.A., Monson, R.K. and Schid, H.P. 2008.Canopy nitrogen, carbon assimilation, and albedo in temperate and boreal forests: functional relations and potential climate feedbacks. Proceedings of the National Academy of Sciences USA 105: 19,336-19,341.

Peterjohn, W.T., Melillo, J.M., Steudler, P.A., Newkirk, K.M., Bowles, F.P. and Aber, J.D. 1994. The response of trace gas fluxes and N availability to elevated soil temperatures. Ecological Applications 4: 617-625.

Reich, P.B., Kloeppel, B.D., Ellsworth, D.S. and Walters, M.B. 1995. Different photosynthesis-nitrogen relations in deciduous hardwood and evergreen coniferous tree species. Oecologia 104: 24-30.

Reich, P.B., Walters, M.B. and Ellsworth, D.S. 1997. From tropics to tundra: global convergence in plant functioning. Proceedings of the National Academy of Sciences USA 94: 13,730-13,734.

Reich, P.B., Walters, M.B., Ellsworth, D.S. and Uhl, C. 1994. Photosynthesis-nitrogen relations in Amazonian tree species. Oecologia 97: 62-72.

Rustad, L., Campbell, J.L., Marion, G.M., Norby, R.J., Mitchell, M.J., Hartley, A.E., Norby, R.J., Mitchell, M.J., Hartley, A.E., Cornelissen, J.H.C. and Gurevitch, J. 2001. A meta-analysis of the response of soil respiration, net nitrogen mineralization, and aboveground plant growth to experimental ecosystem warming. Oecologia 126: 543-562.

Shaw, R.M. and Harte, J. 2001. Response of nitrogen cycling to simulated climate change: Differential responses along a subalpine ecotone. Global Change Biology 7: 193-210.

Zak, D.R., Holmes, W.E. and Pregitzer, K.S. 2007. Atmospheric CO2 and O3 alter the flow of N-15 in developing forest ecosystems. Ecology 88: 2630-2639.