Background
The authors begin by stating how difficult it is to predict how environmental changes will influence various ecosystem functions, partly because of the fact that "the duration of most relevant experiments, typically less than five years, is shorter than the timescale of some treatment plant-soil interactions," citing the studies of Fornara et al. (2009), Eisenhauer et al. (2011) and Reich et al. (2012). And, therefore, it can be appreciated that it can sometimes be next to impossible to correctly project what will occur over the course of many years, making such phenomena as the progressive nitrogen limitation hypothesis of Luo et al. (2004) highly controversial.

What was done
According to Mueller et al., they measured soil nitrogen (N) transformations and dissolved inorganic N concentrations for thirteen consecutive years in the well-known BioCON grassland experiment conducted in Minnesota (USA), where atmospheric CO2 concentration (ambient and ambient plus 180 ppm), herbaceous plant diversity (1, 4, 9 and 16 species), and two nitrogen fertilization treatments (unfertilized and fertilized with 4 g N m^-2 year^-1) were applied in a factorial design (Reich et al., 2001).

What was learned
Two major findings of the study were that (1) "plant species richness had increasingly positive effects on soil N transformations over time, likely because in diverse subplots the concentrations of N in roots increased over time," and because "species richness also had increasingly positive effects on concentrations of ammonium in soil, perhaps because more carbon accumulated in soils of diverse subplots, providing exchange sites for ammonium," and that (2) "in contrast with existing hypotheses, such as progressive N limitation, and with observations from other, often shorter, studies, elevated CO2 had relatively static and small, or insignificant, effects on soil inorganic N pools and fluxes."

What it means
Again, in the words of the researchers who conducted the study, "during the first 13 years of the BioCON experiment, progressive N limitation was not induced through a negative effect of elevated CO2 on net N mineralization," which is similar to what Reich and Hobbie (2013) also found to be the case. Hence, there is good real-world experimental evidence to suggest that the growth-enhancing effect of atmospheric CO2 enrichment will not gradually wind down with the passage of time due to declining availability of soil nitrogen, in strong contradiction of what many climate alarmists would have everyone believe.

References
Eisenhauer, N., Milcu, A., Sabais, A.C.W., Bessler, H., Brenner, J., Engels, C., Klarner, B., Maraun, M., Partsch, S., Roscher, Schonert, F., Temperton, V.M., Thomisch, K., Weigelt, A., Weisser, W.W. and Scheu, S. 2011. Plant diversity surpasses plant functional groups and plant productivity as driver of soil biota in the long term. PLOS ONE 6: 10.1371/journal.pone.0016055.

Fornara, D.A., Tilman, D. and Hobbie, S.E. 2009. Linkages between plant functional composition, fine root processes and potential soil N mineralization rates. Journal of Ecology 97: 48-56.

Luo, Y., Su, B., Currie, W.S., Dukes, J.S., Finzi, A., Hartwig, U., Hungate, B., McMurtrie, R.E., Oren, R., Parton, W.J., Pataki, D.E., Shaw, M.R., Zak, D.R. and Field, C.B. 2004. Progressive nitrogen limitation of ecosystem responses to rising atmospheric carbon dioxide. BioScience 54: 731-739.

Reich, P.B. and Hobbie, S.E. 2013. Decade-long soil nitrogen constraint on the CO2 fertilization of plant biomass. Nature Climate Change 3: 278-282.

Reich, P.B., Knops, J., Tilman, D., Craine, J., Ellsworth, D., Tjoelker, M., Lee, T., Wedin, D., Naeem, S., Bahauddin, D., Hendrey, G., Jose, S., Wrage, K., Goth, J. and Bengston, W. 2001. Plant diversity enhances ecosystem responses to elevated CO2 and nitrogen deposition. Nature 410: 809-812.

Reich, P.B., Tilman, D., Isbell, F., Mueller, K.E., Hobbie, S.E., Flynn, D. and Eisenhauer, N. 2012. Impacts of biodiversity loss escalate through time as redundancy fades. Science 336: 589-592.