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Acquisition of Soil Nitrogen by Different Tree Genotypes and Species Growing in Mixed Stands
Reference
Zak, D.R., Holmes, W.E., Pregitzer, K.S., King, J.S., Ellsworth, D.S. and Kubiske, M.E. 2007. Belowground competition and the response of developing forest communities to atmospheric CO2 and O3. Global Change Biology 13: 2230-2238.

Background
The authors note that "competition for soil nitrogen (N) is especially important because it is the nutrient most often constraining the growth of temperate forests," and that future increases in atmospheric CO2 and ozone (O3) could alter the acquisition of soil N to a different extent among different temperate tree genotypes and species, "presenting the possibility that it could alter belowground competition for this limiting resource."

What was done
To explore this possibility and its potential implications, Zak et al. grew three aspen (Populus tremuloides Michx.) genotypes in mixed stands, as well as aspen and paper birch (Betula papyrifera Marsh.) in other mixed stands, at the Rhinelander, Wisconsin (USA) FACE facility for a period of eight years, where the trees were exposed to factorial combinations of CO2 (ambient and 560 ppm) and O3 (ambient ~ 30-40 and 50-60 ppb). At the start of the last year of the study, they also applied a small amount of 15NH4+ to the soils of the four treatments, while at the conclusion of the final year they "assessed N acquisition by measuring the amount of 15N tracer contained in the plant canopy (i.e., recent N acquisition), as well as the total amount of canopy N (i.e., cumulative N acquisition)."

What was learned
The six researchers discovered that "exposure to elevated CO2 and O3 differentially altered N acquisition among aspen genotypes, increasing it in some and decreasing it in others," while "in the aspen-birch community, elevated CO2 enhanced the ability of birch to forage for N in soil to a greater extent than aspen, suggesting that birch had become more competitive for soil resources."

What it means
In a conclusion that bodes well for the ability of earth's forests to continue to accrue and sequester carbon over the long-term, Zak et al. state that "if CO2-respnsive and O3-tolerant species and genotypes attain greater dominance over time," which is what one would expect as they become more competitive, "then such a response could potentially sustain the CO2 enhancement of forest net primary productivity."

Reviewed 6 February 2008