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Growth Responses of an N2-Fixing Legume and a Non-N2-Fixing Forb to Elevated CO2
Lee, T.D., Tjoelker, M.G., Reich, P.B. and Russelle, M.P.  2003.  Contrasting growth response of an N2-fixing and non-fixing forb to elevated CO2: dependence on soil N supply.  Plant and Soil 255: 475-486.

What was done
The authors investigated the effects of atmospheric CO2 concentration (365 and 700 ppm) and nitrogen fertilization (low-N field soil + 0, 4, 8, 12, 16 and 20 g N m-2 year-1) on leaf net photosynthesis, whole plant growth, and carbon and nitrogen acquisition in an N2-fixing legume - wild lupine (Lupinus perennis L.) - and a non-N2-fixing forb - yarrow (Achillea millefolium L.) - in controlled-environment chambers, where plants were grown from seed in pots (10 seeds of a single species in each 2600 cm3 pot) for one full growing season.

What was learned
Mean rates of leaf net photosynthesis of Lupinus and Achillea plants growing in the CO2-enriched chambers were, respectively, 39% and 37% greater than those of plants growing in the ambient-air chambers, irrespective of N treatment.

Total plant biomass of Lupinus at final harvest was 80% greater in the CO2-enriched chambers than in the ambient-air chambers, again irrespective of N treatment.  In Achillea, however, there was no CO2-induced biomass increase in plants grown in the low-N soil, but there was a positive CO2-induced biomass response that increased in linear fashion with increasing N application to +25% at the highest rate of N application.

Elevated CO2 increased plant total N by 57% in Lupinus, but it had no such effect in Achillea.  In Lupinus, the extra N came from extra symbiotic N2 fixation, apparently, in the words of the authors, as "a result of an increased number and overall mass of nodules, rather than changes in specific nitrogenase activity."  They also report that "although partial photosynthetic acclimation to CO2 enrichment occurred, both species maintained significantly higher rates of photosynthesis and more efficient carbon capture per unit leaf N (average + 60%) in elevated compared to ambient CO2," indicative of a substantial CO2-induced increase in nitrogen use efficiency.

What it means
Noting that compared to non-N2-fixing forbs, "N2-fixers exhibit positive photosynthetic and growth responses to increased atmospheric CO2 that are independent of soil N supply," Tali et al. conclude that "the enhanced amount of N derived from N2 fixation under elevated CO2 presumably helps meet the increased N demand in N2-fixing species," which could not be done by the non-N2-fixers of this study.  Hence, they suggest that this advantage enjoyed by N2-fixers may lead to significant changes in species interactions in grassland communities, "especially those that are inherently N-poor, under projected rising atmospheric CO2."  Over time, however, much of the extra nitrogen supplied to such communities by its N2-fixers may ultimately become available to other species as well, thereby significantly boosting the productivity of the entire ecosystem even more.

Reviewed 3 December 2003