What was done
The authors studied the long-term effects of warming and nutrient fertilization on the carbon balance of wet sedge tundra ecosystems located in Alaska, USA, for a period of eight years.  Some of the experimental plots established on the tundra were subjected to enclosure by greenhouses, which warmed their ambient air temperatures by an average of nearly 6°C, while other plots received yearly fertilization with soil nitrogen and phosphorus additions.

What was learned
After eight years of treatment, it was clearly evident that fertilization had a much greater impact than warming on ecosystem dynamics.  Plant ground cover, for example, increased only slightly in response to warming, but nearly tripled in response to nutrient fertilization.  Similarly, gross ecosystem photosynthesis was unresponsive to warming, while it increased three-fold in response to fertilization.  Interestingly, and contrary to the authors' expectations, warming had no significant effect on ecosystem respiration, whereas fertilization increased it by approximately the same degree that it increased plant ground cover.  Thus, in the arctic tundra, nutrients, by increasing plant photosynthesis and growth, appear to control ecosystem respiration much more than warming.  In fact, in an attempt to analyze the relative contribution of plants to whole-ecosystem respiration, the authors performed a series of measurements that suggested that plants were responsible for about 90% of the total carbon respired.  Thus, plant, and not soil, processes appear to be responsible for controlling carbon fluxes in tundra ecosystems.

What it means
Some individuals have suggested that if air temperatures continue to rise, regardless of the cause, many ecosystems may become sources of carbon, due to predicted increases in respiration resulting from increased temperature.  However, the data of this study suggest that warmer air temperatures have little influence on carbon dynamics in arctic tundra.  Moreover, the present data indicate that plant, and not soil, processes likely control carbon fluxes in tundra ecosystems, and that plant processes are controlled by nutrient status, not temperature.  Therefore, as the CO2 content of the air continues to rise, it is likely that its aerial fertilization effect will positively impact arctic tundra plants in such a way as to increase their carbon sequestering power, regardless of any change in air temperature.