Learn how plants respond to higher atmospheric CO2 concentrations

How does rising atmospheric CO2 affect marine organisms?

Click to locate material archived on our website by topic


Nitrogen Cycling in a CO2-Enriched Sweetgum Plantation
Reference
Johnson, D.W., Cheng, W., Joslin, J.D., Norby, R.J., Edwards, N.T. and Todd Jr., D.E.  2004.  Effects of elevated CO2 on nutrient cycling in a sweetgum plantation.  Biogeochemistry 69: 379-403.

Background
Johnson et al. introduce their intriguing paper by stating that "the potential response of terrestrial ecosystems to elevated CO2 may be constrained by the availability and cycling of nutrients," noting that Strain (1985) had speculated some two decades earlier that CO2-induced reductions in the concentrations of nitrogen (N) in litterfall and root detritus could lead to "reduced decomposition and N mineralization."  This concept was recently reworked by Hungate et al. (2003), who claim that "when CO2 enrichment increases soil C:N, decomposing microorganisms require more nitrogen," and that "this effect can reduce nitrogen mineralization," which they say is "the main source of nitrogen for plants."  Having played a role in some of the research associated with these claims, the Carnegie Institution issued a press release (see our Editorial of 10 Dec 2003) quoting one of its scientists as stating that "we should not count on carbon storage by land ecosystems to make a massive contribution to slowing climate change," due to the supposition that a current apparent insufficiency of soil nitrogen will prevent a significant CO2-induced increase in net primary production together with the subsequently-augmented storage of carbon in woody biomass and soil organic matter that normally goes hand-in-hand with it.

What was done
In a study that investigates several of these interrelated phenomena, Johnson et al. analyzed nutrient cycling in a stand of sweetgum (Liquidambar styraciflua L.) trees at the Oak Ridge National Environmental Research Park's forest FACE facility in Roane County, Tennessee, USA.

What was learned
Although both foliar and litter N concentrations were significantly lower in the CO2-enriched FACE plots, as anticipated twenty years ago by Strain, this deficiency was offset by the CO2-induced increase in total biomass production (22% over the last four years of the study, as per Norby et al., 2004), such that total foliage N content and total litter-fall N flux were not significantly different between the ambient and CO2-enriched treatments, which observations are at odds with the implications of the contentions of Strain and, ultimately, those of Hungate et al. as well.

What it means
Johnson et al. write that "the observation that N uptake increased with elevated CO2 [while litter-fall N flux remained unaltered] implies that either the soil N pool was tapped or that N2 fixation increased in order to meet the increased demand," noting that this conclusion, in their words, "is similar to findings in other studies of elevated CO2 (Johnson et al., 1997; 2002; 2003; Finzi et al., 2002)."  So where did the unanticipated CO2-prompted supply of extra N originate?

Johnson et al. suggest that the yearly "added 17-18 kg N ha-1 for increased N uptake with elevated CO2 could have been obtained from the mineralizeable N pool."  However, they say that "if the soil N pool was indeed tapped for the additional N needed for uptake, it would imply that roots somehow out-competed microbes for available N in the soil," and that "this implication runs contrary to previously-held views of soil N cycling (Paul and Clark, 1989), but is implicit in other studies of elevated CO2 where N uptake is increased with no apparent source other than the soil (e.g., Johnson et al., 1997, 2002; Finzi et al., 2002)."  In addition, they note that microbial activity and microbial N uptake were also enhanced with elevated CO2.

Another possibility is that the source of the extra N was the atmosphere.  However, Johnson et al. report that the extra CO2-induced uptake of nitrogen observed in these stands (16-18 kg ha-1 year-1) was more than the atmospheric deposition that was measured with some degree of sophistication at a nearby Pinus taeda stand (10 kg ha-1 year-1) by Johnson and Lindberg (1992).  Hence, "as in other studies," they conclude that "the exact source of and mechanisms facilitating the additional N for uptake by both roots and microbes remains unclear."  It is, nevertheless, a fact that atmospheric CO2 enrichment somehow has been bringing forth the extra N that has been needed to facilitate the 22% increase in the net primary production of this particular sweetgum plantation in response to the mere 40% increase in the atmospheric CO2 concentration to which it has been exposed since April of 1998.

References
Finzi, A.C., DeLucia, E.H., Hamilton, J.G., Richter, D.D. and Schlesinger, W.H.  2002.  The nitrogen budget of a pine forest under free air CO2 enrichment.  Oecologia 132: 567-578.

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

Johnson, D.W. and Lindberg, S.E.  1992.  Atmospheric Deposition and Forest Nutrient Cycling: A Synthesis of the Integrated Forest Study.  Springer-Verlag, New York, NY, USA.

Johnson, D.W., Ball, J.T. and Walker, R.F.  1997.  Effects of CO2 and nitrogen fertilization on vegetation and soil nutrient content in juvenile ponderosa pine.  Plant and Soil 190: 29-40.

Johnson, D.W., Cheng, W. and Ball, J.T.  2002.  Effects of [CO2] and nitrogen fertilization on soils planted with ponderosa pine.  Plant and Soil 224: 99-113.

Johnson, D.W., Hungate, B.A., Dijkstra, P., Hymus, G., Hinkle, C.R. and Stiling, P.  2003.  The effects of elevated CO2 on nutrient distribution in a fire-adapted Scrub Oak Forest.  Ecological Applications 13: 1388-1399.

Norby, R.J., Ledford, J., Reilly, C.D., Miller, N.E. and O'Neill, E.G.  2004.  Fine-root production dominates response of a deciduous forest to atmospheric CO2 enrichment.  Proceedings of the National Academy of Sciences USA 101: 9689-9693.

Paul, E.A. and Clark, F.E.  1989.  Soil Microbiology and Biochemistry.  Academic Press, New York, NY, USA.

Strain, B.R.  1985.  Physiological and ecological controls on carbon sequestering in terrestrial ecosystems.  Biogeochemistry 1: 219-232.


Reviewed 13 October 2004