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Does Soil Nitrogen Availability Limit Ecosystem Productivity Response to Atmospheric CO2 Enrichment?
Reference
Cannell, M.G.R. and Thornley, J.H.M.  2003.  Ecosystem productivity is independent of some soil properties at equilibrium.  Plant and Soil 257: 193-204.

Background
It is periodically suggested that lack of soil nitrogen will limit the productivity responses of various ecosystems to the ongoing rise in the air's CO2 content [see, for example, our Journal Reviews of Finzi and Schlesinger (2003) and Schafer et al. (2003) with respect to forests].  Is this idea correct?

What was done
In a study not specifically designed to answer this question (but clearly qualified to broach it), Cannell and Thornley changed the nitrogen mineralization constants in the Edinburgh Forest Model to study the effects this action would have on net primary productivity (NPP) and a variety of other ecosystem properties.

What was learned
The authors found that given enough time, "modeled ecosystems tend to generate amounts of soil organic matter that are able to supply nitrogen at rates which do not greatly limit plant growth."  At the two extremes, for example, they report that "when specific nitrogen mineralization rates are low, large amounts of soil organic matter accumulate, whereas when specific nitrogen mineralization rates are high, small amounts of soil organic matter accumulate."  In both situations, however, the ecosystems go on to ultimately achieve NPP rates that are constrained by other environmental variables, particularly those that define the concept of climate.

What it means
"In the short term," in the words of the authors, "NPP depends on soil organic matter and specific [nitrogen] mineralization rate; but in the long term, NPP is determined by climate rather than soil properties such as soil organic matter and specific mineralization rate because nitrogen is not greatly limiting, and in the long term soil organic matter itself depends on a climate-determined NPP and specific mineralization rate."  Hence, there is every reason to believe that the extra NPP potential provided by the ongoing rise in the air's CO2 content will ultimately be met in the very same way that the extra NPP potential provided by growth-promoting climatic conditions is met, which suggests that low soil nitrogen availability does not represent a permanent roadblock to ecosystems ultimately responding to the promise of greatly enhanced growth potential provided by atmospheric CO2 enrichment.  Low soil nitrogen levels may slow some ecosystems down for a while, but they cannot prevent the inevitable, which is destined to be a remarkable "greening of the earth" quite beyond anything man has historically encountered.

References
Finzi, A.C. and Schlesinger, W.H.  2003.  Soil-nitrogen cycling in a pine forest exposed to 5 years of elevated carbon dioxide.  Ecosystems 6: 444-456.

Schafer, K.V.R., Oren, R., Ellsworth, D.S., Lai, C.-T., Herrick, J.D., Finzi, A.C., Richter, D.D. and Katul, G.G.  2003.  Exposure to an enriched CO2 atmosphere alters carbon assimilation and allocation in a pine forest ecosystem.  Global Change Biology 9: 1378-1400.


Reviewed 3 December 2003