Background
Biospheric pessimists have long contended that the ongoing rise in the air's CO2 content will likely result in little, if any, increase in soil carbon contents; but as Jastrow et al. (2005) have demonstrated, they significantly err in this contention, as over three dozen carefully conducted experiments, when properly analyzed, show just the opposite to be the case.  One of the things put forth by the pessimists to explain their position on the matter is their secondary contention that soil supplies of mineral nitrogen, especially where present in limited quantities, will be swiftly depleted by the initial large growth stimulation provided by the rising atmospheric CO2 concentration, as well as by tying up ever more needed nitrogen in soil microbial biomass where it cannot be accessed by plants, producing a significant negative feedback that reduces vegetative growth rates and their carbon inputs to the soils on which they grow.  Now, however, in their insightful and important paper, Jastrow et al. additionally demonstrate that this contention is also wrong.

What was done
Jastrow et al. describe and further analyze the pertinent findings of the first five years of the deciduous forest FACE study that is being conducted at Oak Ridge, Tennessee, USA (Norby et al., 2001) plus the entire eight years of the prairie grassland open-top chamber study that was conducted at Manhattan, Kansas, USA (Owensby et al., 1993).

What was learned
Jastrow and colleagues found that CO2-induced increases in the soil carbon contents of the two ecosystems were accompanied by CO2-induced increases in their soil nitrogen contents.  Specifically, they state that the CO2-induced "soil C accrual was accompanied by significant increases in soil N at average rates of 2.2 ± 0.6 g N m^-2 yr^-1 in the forest and 3.4 ± 1.3 g N m^-2 yr^-1 in the prairie."  Thus, as they continue, "in addition to enhancing C inputs to soil through increased primary production, elevated CO2 apparently also caused some combination of reduced N losses, stimulated N fixation, and redistribution of subsurface N via greater root exploration at depth."

What it means
Jastrow et al. conclude that "these results challenge the hypothesis that N cycling feedbacks will constrain ecosystem C accumulation (Zak et al., 1993; Luo et al., 2004)."  Rather, as they put it, "in these ecosystems the N cycle appears sufficiently flexible - at least for the duration of these experiments - to support soil C accrual in response to CO2 enrichment."

References
Luo, Y., Su, B., Currie, W.S. et al.  2004.  Progressive nitrogen limitation of ecosystem responses to rising atmospheric carbon dioxide.  BioScience 54: 731-739.

Norby, R.J., Todd, D.E., Fults, J. et al.  2001.  Allometric determination of tree growth in a CO2-enriched sweetgum stand.  New Phytologist 150: 477-487.

Owensby, C.E., Coyne, P.I., Ham, J.M. et al.  1993.  Biomass production in a tallgrass prairie ecosystem exposed to ambient and elevated CO2.  Ecological Applications 3: 644-653.

Zak, D.R., Pregitzer, K.S., Curtis, P.S. et al.  1993.  Elevated atmospheric CO2 and feedback between carbon and nitrogen cycles.  Plant and Soil 151: 105-117.