Effects of Warming on a Tallgrass Prairie of the U.S. Great Plains
Niu, S., Sherry, R.A., Zhou, X., Wan, S. and Luo, Y. 2010. Nitrogen regulation of the climate-carbon feedback: evidence from a long-term global change experiment. Ecology 91: 3261-3273.
Working in a tallgrass prairie of the U.S. Great Plains located in McClain County, Oklahoma -- which was dominated by C4 grasses and C3 forbs that had not been grazed for the prior forty years -- the authors conducted a warming experiment, where infrared heaters were used to elevate soil temperature at a depth of 2.5 cm by an average of 1.96°C from 2000 to 2008, and where they say that "yearly biomass clipping mimicked hay or biofuel feedstock harvest." So what did the authors learn?
Niu et al. report that the experimental warming "significantly stimulated carbon storage in aboveground plant, root, and litter pools by 17%, 38%, and 29%, respectively, averaged over the nine years (all P < 0.05) but did not change soil carbon content or nitrogen content in any pool." In addition, they determined that the "plant carbon:nitrogen ratio and nitrogen use efficiency increased in the warmed plots compared to the control plots, resulting primarily from increased dominance of C4 plants in the community."
The five researchers concluded that increased plant nitrogen use efficiency played a more important role than soil nitrogen availability in regulating carbon cycling in this particular ecosystem, since the tallgrass prairie experienced a significant increase in productivity that was caused solely by the warming of its soil and not promoted by any addition of nitrogen to it. They explained this result by stating that "increased inputs of more recalcitrant [higher carbon:nitrogen ratio] material into soil counterbalanced any direct warming stimulation of carbon release, leading to little change in soil carbon stock and no apparent feedback to climate warming [italics added]."