CO2 Enrichment Effects on Soil Nitrogen: Positive or Negative?
Mueller, K.E., Hobbie, S.E., Tilman, D. and Reich, P.B. 2013. Effects of plant diversity, N fertilization, and elevated carbon dioxide on grassland soil N cycling in a long-term experiment. Global Change Biology 19: 1249-1261.
Against this backdrop, Mueller et al. 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).
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."
Summarizing their work, 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 have suggested.
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