Submitted to: Plant and Soil
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/28/2003
Publication Date: 8/1/2003
Citation: Lee, T.D., Tjoelker, M.G., Reich, P.B., 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. Interpretive Summary: The concentration of carbon dioxide has been rising in Earth's atmosphere since the late 1800s in response to burning fossil fuels, widespread timber harvesting and slash burning, and greater losses of soil organic matter as new land was cultivated. Carbon dioxide tends to trap heat that radiates from the Earth's surface and is implicated, along with other gases, in apparent global warming. Carbon dioxide is the main building block for plants to produce sugars, starch, fats, oils, proteins, and other organic compounds. This conversion is driven by photosynthesis. Plants have different capacities to fix carbon dioxide, and these differences relate to leaf structure, metabolism rate, and the supply of other nutrients and water. When legumes, such as clovers, beans, and peas, are infected by beneficial bacteria called rhizobia that have the capacity to fix nitrogen gas from the atmosphere, the plants have a generous supply of nitrogen. On the other hand, grasses and other nonlegumes depend on nitrogen supply from the soil, fertilizer, or animal manure. We found that a legume called lupin grew much better under higher carbon dioxide levels than a nonlegume, yarrow. This difference is most likely due to the high nitrogen supply in lupin, because more nitrogen gas was fixed by lupin under high carbon dioxide concentration than under normal levels. The yarrow grew better under high carbon dioxide, too, but only when more fertilizer nitrogen was added. Our results show that in the absence of nitrogen fertilizer legumes will respond better to higher carbon dioxide levels in the short term because they can fix nitrogen gas. However, these higher rates of nitrogen fixation suggest that in the longer term the nonlegumes will grow better, too, because nitrogen from the legumes will become available in the ecosystem. The results will help scientists produce better models of plant community response to global change.
Technical Abstract: With the ability to symbiotically fix atmospheric N2, legumes may lack the N-limitations thought to constrain plant response to elevated concentrations of atmospheric CO2. The responses of two perennial grassland species were compared to test the hypotheses that 1) the CO2 response of wild species is limited at low N availability, 2) legumes respond to a greater extent than non- fixing forbs to elevated CO2, and 3) elevated CO2 stimulates symbiotic N2 fixation, resulting in an increased amount of N derived from the atmosphere. This study investigated the effects of atmospheric CO2 concentration (365 and 700 umol/mol) and N addition on whole plant growth and C and N acquisition in an N2-fixing legume (Lupinus perennis) and a non-fixing forb (Achillea millefolium) in controlled-chamber environments. To evaluate the effects of a wide range of N availability on the CO2 response, we incorporated six levels of soil N addition starting with native field soil inherently low in N (field soil + 0, 4, 8, 12, 16, or 20 g N/m2/yr). Whole plant growth, leaf net photosynthetic rates, and the proportion of N derived from N2 fixation were determined in plants grown from seed during one growing season. Both species increased growth with CO2 enrichment; however, the response of the non-fixer, Achillea, depended on mineral N supply as growth enhancements under elevated CO2 increased from 0% in low N soil to +25% at the higher levels of N addition. In contrast, Lupinus plants had 80% greater biomass under elevated CO2 regardless of N treatment. Elevated CO2 increased plant total N yield by 57% in Lupinus but had no effect on Achillea. The increased N in Lupinus came from symbiotic N2 fixation, which resulted in a 32% greater proportion of N derived from fixation relative to other sources of N. These results suggest that compared to non-fixing forbs, N2-fixers exhibit a larger response to increased atmospheric CO2 that is independent of soil N supply. Increased N2 fixation under elevated CO2 may help alleviate N-limitations for N2-fixing species as well as that of neighboring non-fixing species, at least over the short-term, in N-poor grassland communities under projected rising atmospheric CO2.