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Does Enhanced Transfer of Carbon to Soil Microbes Increase Nitrogen Immobilization and Restrict the Long-Term Growth Response of Grasslands to Atmospheric CO2 Enrichment?
Richter, M., Hartwig, U.A., Frossard, E., Nosberger, J. and Cadisch, G.  2003.  Gross fluxes of nitrogen in grassland soil exposed to elevated atmospheric pCO2 for seven years.  Soil Biology & Biochemistry 35: 1325-1335.

The productivity of earth's temperate grasslands is often limited by the availability of soil nitrogen (Vitousek and Howarth, 1991); and both empirical and modeling studies have suggested that the magnitude and duration of grassland growth responses to rising levels of atmospheric CO2 may be constrained by limiting supplies of soil nitrogen (Rastetter et al., 1997; Luo and Reynolds, 1999; Thornley and Cannell, 2000).  In light of this mix of real-world observations and theoretical implications, it would seem only natural to believe, as Richter et al. (2003) hypothesize, "that increased below-ground translocation of photoassimilates at elevated pCO2 would lead to an increase in immobilization of N due to an excess supply of energy to the roots and rhizosphere," which would ultimately lead to a reduction in the size of the growth-promoting effect of elevated atmospheric CO2 that is nearly universally manifest in short-term CO2 enrichment experiments.

What was done
To test this hypothesis, Richter et al. measured gross rates of N mineralization, NH4+ consumption and N immobilization in soils upon which monocultures of Lolium perenne and Trifolium repens had been exposed to ambient (360 ppm) and elevated (600 ppm) concentrations of atmospheric CO2 for seven years in the Swiss FACE study conducted near Zurich.

What was learned
In the words of the authors, "after seven years of exposure to elevated CO2, gross mineralization, NH4+ consumption and N immobilization in both the L. perenne and the T. repens swards did not show significant differences."  In addition, they note that the size of the microbial N pool and immobilization of applied mineral 15N were not significantly affected by elevated CO2.

What it means
Richter et al. note that "the results of this study did not support the initial hypothesis and indicate that below-ground turnover of N, as well as N availability, measured in short-term experiments are not strongly affected by long-term exposure to elevated CO2."  They thus conclude "that differences in plant N demand and not changes in soil N mineralization/immobilization are the driving factors for N dynamics in these meadow grassland systems."  Hence, as in the studies of Finzi and Schlesinger (2003) and Schafer et al. (2003), Richter et al.'s work provides no evidence that the growth responses of earth's temperate forests and grasslands to atmospheric CO2 enrichment will ever be significantly reduced from what is suggested by moderate-term studies of a few to several years' duration.

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.

Luo, Y. and Reynolds, J.F.  1999.  Validity of extrapolating field CO2 experiments to predict carbon sequestration in natural ecosystems.  Ecology 80: 1568-1583.

Rastetter, E.B., Agren, G.I. and Shaver, G.R.  1997.  Responses of N-limited ecosystems to increased CO2: a balanced-nutrition, coupled-element-cycles model.  Ecological Applications 7: 444-460.

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.

Thornley, J. and Cannell, M.  2000.  Dynamics of mineral N availability in grassland ecosystems under increased [CO2]: hypotheses evaluated using the Hurley Pasture Model.  Plant and Soil 224: 153-170.

Vitousek, P.M. and Howarth, R.W.  1991.  Nitrogen limitation on land and in the sea: how can it occur?  Biogeochemistry 13: 87-115.

Reviewed 3 December 2003