Background
The authors write that "because multifactor manipulation experiments test the anticipated changes in climate and CO2 simultaneously, as opposed to single factor manipulative experiments," the former "can be expected to yield more relevant benchmarking data," citing Shaw et al. (2002), Dukes et al. (2005) and Larsen et al. (2011) in this regard. And, therefore, they synthesize "published effects of warming and elevated atmospheric CO2 concentration, applied separately and in combination," on terrestrial ecosystem biomass production and nitrogen mineralization, the latter of which factors is said by them to be the "main limiting factor of plant CO2 uptake in most natural northern ecosystems."

What was done
Dieleman et al., as they describe it, "collected data from 150 manipulation experiment sites across a range of different ecosystems and climates, reporting data on total biomass, aboveground biomass, root biomass, fine root biomass, soil carbon heterotrophic respiration, soil respiration, and soil nitrogen availability and foliar nitrogen content, resulting in 821 entries for the meta-analysis," which procedure employed techniques using MetaWin 2.1 software, as described by Rosenberg et al. (2000).

What was learned
The 19 researchers, hailing from 8 different countries, report that "responses to single factor treatments were rarely additive," and that "in a combined treatment, effects of elevated CO2 often dominated the response, suggesting a larger sensitivity of terrestrial ecosystems to rising CO2 compared to rising temperatures." In addition, they found that "mineral nitrogen availability declined less in the combined treatment than in the CO2-only treatment, possibly due to the warming-induced acceleration of decomposition."

What it means
In light of the first of their two major findings, Dieleman et al. conclude that "ecosystem models should ideally be tested against results from multifactor experiments to optimize their model structures." And in light of the second of their major findings, they conclude that "progressive nitrogen limitation (PNL) may not occur as commonly as anticipated from single factor CO2 treatment studies." In fact, they state that "due to the growing intensity and global distribution of atmospheric deposition of reactive nitrogen compounds," as described by Galloway et al. (2004), "more and more extra-tropical ecosystems are shifting from a state of nitrogen (co-) limitation to a state of nitrogen-saturation," citing Aber et al. (1998). And they say that these ecosystems will be "less prone to exhibit PNL with rising atmospheric CO2."

References
Aber, J., Mcdowell, W., Nadelhoffer, D., Magill, A., Berntson, G., Kamakea, M., McNulty, S., Currie, W., Rustad, L. and Fernandez, I. 1998. Nitrogen saturation in temperate forest ecosystems - Hypotheses revisited. BioScience 48: 921-934.

Dukes, J.S., Chiariello, N.R., Cleland, E.E., Moore, L.A., Shaw, M.R., Thayer, S., Tobeck, T., Mooney, H.A. and Field, C.B. 2005. Responses of grassland production to single and multiple global environmental changes. PLoS Biology 3: 1829-1837.

Galloway, J.N., Dentener, F.J., Capone, D.G., Boyer, E.W., Howarth, R.W., Seitzinger, S.P., Asner, G.P., Cleveland, C.C., Green, P.A., Holland, E.A., Karl, D.M., Michaels, A.F., Porter, J.H., Townsend, A.R. and Vöosmarty, C.J. 2004. Nitrogen cycles: past, present, and future. Biogeochemistry 70: 153-226.

Larsen, K.S., Andresen, L.C., Beier, C., Jonasson, S., Albert, K.R., Ambus, P., Arndal, M.F., Carter, M.S., Christensen, S., Holmstrup, M., Ibrom, A., Kongstad, J., Van Der Linden, L., Maraldo, K., Michelsen, A., Mikkelsen, T.N., Pilegaard, K., Prieme, A., Ro-Poulsen, H., Schmidt, I.K., Selsted, M.B. and Stvenbak, K. 2011. Reduced N cycling in response to elevated CO2, warming, and drought in a Danish heathland: synthesizing results of the CLIMATE project after two years of treatments. Global Change Biology 17: 1884-1899.

Rosenberg, M.S., Adams, D.C. and Gurevitch, J. 2000. Metawin: Statistical Software for Meta-Analysis. Sinauer Associates, Inc., Sunderland, Massachusetts, USA.

Shaw, M.R., Zavaleta, E.S., Chiariello, N.R., Cleland, E.E., Mooney, H.A. and Field, C.B. 2002. Grassland responses to global environmental changes suppressed by elevated CO2. Science 298: 1987-1990.