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Does Too Little Nitrogen and Too Much Ozone Cancel the Benefits of the Aerial Fertilization Effect of Atmospheric CO2 Enrichment?
Volume 15, Number 12: 21 March 2012

Writing in Ecology Letters, Zak et al. (2011) indicate that both an insufficient amount of soil nitrogen (N) and an overabundance of atmospheric ozone (O3) have often been claimed to either partially or totally repress the many positive effects of elevated atmospheric CO2 concentrations on plant growth and development, especially in the case of long-lived woody plants such as trees; but they state that the combined effects of elevated CO2 and O3 (eCO2 and eO3) "remain undocumented in the context of long-term, replicated field experiments." And to fill this void, they describe how they conducted such an experiment and what they learned from it.

Working at the Rhinelander (Wisconsin, USA) FACE facility, the four researchers tell how in 1997 they planted one-half of each of 12 FACE plots with various trembling aspen (Populus tremuloides) genotypes (8, 42, 216, 259, 271) of differing CO2 and O3 sensitivities, while one-quarter of each ring was planted with a single aspen genotype (226) and paper birch (Betula papyrifera), and another quarter of each ring was planted with the same single aspen genotype and sugar maple (Acer saccharum). These treatments, each of which was replicated four times, were maintained for the following twelve years at either ambient CO2 and O3 (aCO2 and aO3), aCO2 and eO3, eCO2 and aO3, or eCO2 and eO3 - where eCO2 was 560 ppm, and where eO3 was in the range of 50-60 nmol/mol - while numerous types of pertinent data were concurrently collected.

In reference to the notorious progressive nitrogen limitation hypothesis, Zak et al. (2011) say they "found no evidence of this effect after 12 years of eCO2 exposure." In fact, they report that relative to net primary production (NPP) under aCO2, there was a 26% increase in NPP over the last three years of the study, which for a more standard 300-ppm increase in atmospheric CO2 concentration equates to an approximate 42% increase in NPP, which they say "was sustained by greater root exploration of soil for growth-limiting N, as well as more rapid rates of liter decomposition and microbial N release during decay."

With respect to the concomitant stress of O3 pollution, the researchers report that "despite eO3-induced reductions in plant growth that occurred early in the experiment (i.e., after three years of exposure), eO3 had no effect on NPP during the 10th-12th years of exposure," which response, in their words, "appears to result from the compensatory growth of eO3-tolerant genotypes and species as the growth of eO3-sensitive individuals declined over time (Kubiske et al., 2007; Zak et al., 2007), thereby causing NPP to attain equivalent levels under ambient O3 and elevated O3."

In discussing various aspects of their long-term findings, Zak et al. (2011) state that "NPP in the three plant communities responded similarly to the combined eCO2 and eO3 treatment." And they say that "given the degree to which eO3 has been projected to decrease global NPP (Felzer et al., 2005), the compensatory growth of eO3-tolerant plants in our experiment should be considered in future simulations and, depending on the generality of this response, could dramatically diminish the negative effect of eO3 on NPP and carbon storage on land as well as projected increases in anthropogenic CO2 and climate warming (Stitch et al., 2007)."

Continuing in this vein, the four researchers ultimately conclude - contrary to the analysis of Norby et al. (2010) - that if forests of similar composition growing throughout northeastern North America respond in the same manner as those in their experiment (Cole et al., 2009), then enhanced forest NPP under eCO2 may be sustained for a longer duration than had previously been thought possible. In addition, they suggest that "the negative effect of eO3 may be diminished by compensatory growth of eO3-tolerant plants as they begin to dominate forest communities (Kubiske et al., 2007; Zak et al., 2007), suggesting that aspects of biodiversity like genetic diversity and species composition are important components of ecosystem response to this agent of global change."

Sherwood, Keith and Craig Idso

Cole, C.T., Anderson, J.E., Lindroth, R.L. and Waller, D.M. 2009. Rising concentrations of atmospheric CO2 have increased growth of natural stands of quaking aspen (Populous tremuloides). Global Change Biology 16: 2186-2197.

Felzer, B., Reilly, J., Melillo, J., Kicklighter, D., Sarofim, M., Wang, C., Prinn, R. and Zhuang, Q. 2005. Future effects of ozone on carbon sequestration and climate change policy using a global biogeochemical model. Climatic Change 73: 345-373.

Kubiske, M.E., Quinn, V.S., Marquart, P.E. and Karnosky, D.F. 2007. Effects of elevated atmospheric CO2 and/or O3 on intra- and inter-specific competitive ability of aspen. Plant Biology 9: 342-355.

Norby, R.J., Warren, J.M., Iversen, C.M., Medlyn, B.E. and McMurtire, R.D. 2010. CO2 enhancement of forest productivity constrained by limited nitrogen availability. Proceedings of the National Academy of Sciences USA 107: 19,368-19,373.

Zak, D.R., Holmes, W.E., Pregitzer, K.S., King, J.S., Ellsworth, D.S. and Kubiske, M.E. 2007. Belowground competition and the response of developing forest communities to atmospheric CO2 and O3. Global Change Biology 13: 2230-2238.

Zak, D.R., Pregitzer, K.S., Kubiske, M.E. and Burton, A.J. 2011. Forest productivity under elevated CO2 and O3: positive feedbacks to soil N cycling sustain decade-long net primary productivity enhancement by CO2. Ecology Letters 14: 1220-1226.