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Will Less-Than-Adequate Nitrogen Keep Earth's Ecosystems from Positively Responding to Increases in Atmospheric CO2?
Holmes, W.E., Zak, D.R., Pregitzer, K.S. and King, J.S.  2003.  Soil nitrogen transformations under Populus tremuloides, Betula papyrifera and Acer saccharum following 3 years exposure to elevated CO2 and O3Global Change Biology 9: 1743-1750.

With respect to the ongoing FACTS-II FACE experiment described by Karnosky et al., (1999) and Dickson et al. (2000), Holmes et al. note that "elevated CO2 [an extra 200 ppm] has increased the production of leaf and fine root litter (King et al., 2001; Lindroth et al., 2001), which has resulted in greater rates of microbial respiration and increased activities of extracellular enzymes involved with the degradation of plant and fungal cell wall components (i.e. cellobiohydrolase and N-acetylglucosamidase; Larson et al., 2002; Phillips et al., 2002)."  This greater metabolic capability to degrade plant and fungal litter under elevated CO2 suggests, in the words of Holmes et al., "that microbial communities have a greater biosynthetic demand for N, which could potentially decrease N availability to plants and increase the movement of N into soil organic matter."

This hypothesis is similar to the claim of Hungate et al. (2003) - which we evaluated in our Editorial of 10 Dec 2003 and found wanting - wherein the latter group of authors contends that "when CO2 enrichment increases soil C:N, decomposing microorganisms require more nitrogen."  They further state that this latter effect "can reduce nitrogen mineralization," which they say is "the main source of nitrogen for plants."  Hence, they conclude, as described in the press release that accompanied the publication of their paper, that in a high-CO2 world of the future "the availability of nitrogen, in forms usable by plants, will probably be too low for large increases in carbon storage," which they claim will ultimately lead to more rapid global warming.

What was done
The study of Holmes et al. provides data that can be used to evaluate this hypothesis.  Specifically, the Michigan scientists determined gross rates of N mineralization and nitrification, as per Davidson et al. (1992) and Hart et al. (1994), while tracing inorganic 15N from the soil solution into microbial biomass and soil organic matter in a number of 30-m-diameter FACE plots that three years earlier had been planted to trembling aspen (Populus tremuloides Michx.), paper birch (Betula papyrifera Marsh.), and sugar maple (Acer saccharum Marsh.).  Also, they measured microbial biomass using the chloroform fumigation-extraction technique of Horwath and Paul (1994).

What was learned
The scientists determined that elevated CO2 did not alter gross N transformation rates and that microbial biomass was not affected by it either.  This result, they say, "is consistent with previous studies of gross N transformation rates beneath aspen exposed to elevated CO2 in open-top chambers for a similar length of time (Zak et al., 2000)."  Then, they further emphasize this fact by explicitly stating they "found no significant increases in microbial N or soil organic N under elevated CO2."

What it means
Although these findings pretty much torpedo the claims of Hungate et al., Holmes et al. exercise extreme caution in their conclusions.  With respect to the predicted changes for which they searched in vain, they say "it could take several years for such changes to become detectable."  Nevertheless, for the present, their results provide no support for the strongly-framed contentions of Hungate et al., tending, in fact, to refute them.

Davidson, E.A., Hart, S.C. and Firestone, M.K.  1992.  Internal cycling of nitrate in soils of a mature coniferous forest.  Ecology 73: 1148-1156.

Dickson, R.E., Lewin, K.F., Isebrands, J.G., et al.  2000.  Forest atmosphere carbon transfer storage-II (FACTS II) - The aspen free-air CO2 and O3 enrichment (FACE) project in an overview.  General Technical Report, NC-214, USDA Forest Service, North Central Experiment Station.

Hart, S.C., Stark, J.M., Davidson, E.A. et al.  1994.  Nitrogen mineralization, immobilization, and nitrification.  In: Weaver, R.W., Angle, S., Bottomley, P., Bezdicek, D., Smith, S., Tabatabai, A. and Wollum, A. (Eds.), Methods of Soil Analysis Part 2.  Microbiological and Biochemical Properties.  Soil Science Society of America, Segoe, WI, USA, pp. 985-1018.

Horwath, W.R. and Paul, E.A.  1994.  Microbial biomass.  In: Weaver, R.W., Angle, S., Bottomley, P., Bezdicek, D., Smith, S., Tabatabai, A. and Wollum, A. (Eds.), Methods of Soil Analysis Part 2.  Microbiological and Biochemical Properties.  Soil Science Society of America, Segoe, WI, USA, pp. 753-773.

Hungate, B.A., Dukes, J.S., Shaw, M.R., Luo, Y. and Field, C.B.  2003.  Nitrogen and climate change.  Science 302: 1512-1513.

Karnosky, D.F., Mankovska, B., Percy, K. et al.  1999.  Effects of tropospheric O3 on trembling aspen and interaction with CO2: results from an O3-gradient and a FACE experiment.  Water, Air, and Soil Pollution 116: 311-322.

King, J.S., Pregitzer, K.S., Zak, D.R. et al.  2001.  Fine root biomass and fluxes of soil carbon in young stands of paper birch and trembling aspen is affected by elevated CO2 and tropospheric O3Oecologia 128: 237-250.

Larson, J., Zak, D.R. and Sinsabaugh, R.I.  2002.  Extracellular enzyme activity and metabolism of root-derived substrates beneath temperate trees growing under elevated CO2 and O3Soil Science Society of America Journal 66: 1848-1856.

Lindroth, R.L., Kopper, J.B., Parsons, W.F.J. et al.  2001.  Consequences of elevated carbon dioxide and ozone for foliar chemical composition and dynamics in trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera).  Environmental Pollution 115: 395-404.

Phillips, R.L., Zak, D.R. and Holmes, W.E.  2002.  Microbial community composition and function beneath temperate trees exposed to elevated atmospheric carbon dioxide and ozone.  Oecologia 131: 236-244.

Zak, D.R., Pregitzer, K.S., Curtis, P.S. et al.  2000.  Atmospheric CO2, soil-N availability, and allocation of biomass and nitrogen by Populus tremuloidesEcological Applications 10: 34-46.

Reviewed 25 February 2004