What was done
The authors grew well-watered one-year-old seedlings of nitrogen-fixing alder (Alnus hirsuta Turcz.) trees in five-liter pots filled with a 1:1 mixture of pumice and clay loam supplied with three concentrations of soil-nitrogen (N) - 52.5 mg N/pot/week (High-N), 5.25 mg N/pot/week (Low-N), and (No-N) - for 100 days within phytotron chambers exposed to natural daylight and maintained at atmospheric CO2 concentrations of either 360 ppm (ambient) or 720 ppm (elevated). On day 59 of the study, they measured the trees' leaf net photosynthetic rates; while at the end of the experiment the trees were harvested and the dry mass and N content of each plant organ determined. In addition, leaf litter from each seedling was collected daily as leaves senesced when temperatures declined from September to November.

What was learned
In response to the experimental doubling of the air's CO2 concentration, mid-season light-saturated net photosynthetic rates were increased by 21%, 12% and 25% in the High-N, Low-N and No-N treatments, respectively, while whole-plant dry mass (including nodule mass) at the end of the study was boosted by 26%, 18% and 32% in the same respective treatments. In addition, whole-plant N content at the end of the season was found to be 12%, 18% and 12% greater in the CO2-enriched High-N, Low-N and No-N treatments, respectively; and the Japanese scientists report that the enhanced whole-plant N uptake in the No-N elevated CO2 treatment "means that N2 fixation increased under elevated CO2." Last of all, they report that because elevated CO2 increased total leaf area but had no effect on area-based leaf litter N in all N-treatments, total per-plant leaf litter N was also increased.

What it means
On the basis of what they observed in their experiment, Tobita et al. concluded that in a high-CO2 world of the future "A. hirsuta would accumulate a greater biomass N through increased N2 fixation by increased nodule mass, and may increase soil N availability by increased leaf litter N [input to soil] under elevated CO2." These linked phenomena, in turn, would likely produce still further biological benefits, for as the four researchers noted in the introduction to their study, "forest ecosystems are usually nitrogen-limited (Vitousek and Howarth, 1991)," and "N2-fixing species, including Alnus hirsuta Turcz., can contribute significant amounts of fixed N to temperate forest ecosystems (Dawson, 1983; Koike et al., 1997)." This latter phenomenon, when intensified by elevated CO2, helps earth's forests to more readily overcome the growth-deterring effect of insufficient soil nitrogen that plays a key role in the progressive nitrogen limitation hypothesis, which discredited concept (see our Subject Index) is often wrongly invoked by climate alarmists as providing an insurmountable counterforce to the growth-promoting aerial fertilization effect and water-conserving anti-transpiration effect of atmospheric CO2 enrichment.

References
Dawson, J.O. 1983. Dinitrogen fixation in forest ecosystems. Canadian Journal of Microbiology 29: 979-992.

Koike, T., Izuta, T., Lei, T.T., Kitao, M. and Asanuma, S. 1997. Effects of high CO2 on nodule formation in roots of Japanese mountain alder seedlings grown under two nutrient levels. In: Ando, T., Fujita, K., Mae, T., Matsumoto, H., Mori, S. and Sekiya, J., Eds. Plant Nutrition - For Sustainable Food Production and Environment, Kluwer Academic Publishers, Japan, pp. 887-888.

Vitousek, P.M. and Howarth R.W. 1991. Nitrogen limitation on land and in the sea: How can it occur? Biogeochemistry 13: 87-115.