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Title: WIDESPREAD FOLIAGE N-15 CONCENTRATION DEPLETION UNDER ELEVATED CO2: INFERENCES FOR THE NITROGEN CYCLE

Author
item BASSIRIRAD, HORMOZ - UNIV OF IL AT CHICAGO
item CONSTABLE, JOHN - CA STATE UNIV FRESNO
item LUSSENHOP, JOHN - UNIV OF IL AT CHICAGO
item Kimball, Bruce
item NORBY, RICHARD - DGE NATL LAB TN
item OECHEL, WALTER - SAN DIEGO ST UNIV
item REICH, PETER - UNIV OF MINNESOTA
item SCHLESINGER, WILLIAM - DUKE UNIV
item ZITZER, STEPHEN - DESERT RESEARCH INST NV
item SEHTIYA, HARBANS - UNIV OF IL AT CHICAGO
item SILIM, SALIM - UNIV OF IL AT CHICAGO

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2003
Publication Date: 8/15/2003
Citation: Bassirirad, H., Constable, J.V., Lussenhop, J., Kimball, B.A., Norby, R.J., Oechel, W.C., Reich, P.B., Schlesinger, W.H., Zitzer, S., Sehtiya, H.L., Silim, S. 2003. Widespread foliage n-15 concentration depletion under elevated co2: inferences for the nitrogen cycle. Global Change Biology. 9:1582-1590

Interpretive Summary: The CO2 concentration in the atmosphere is increasing and expected to double near the end of this century. The increased levels of CO2 are generally expected to increase rates of photosynthesis and change the carbon metabolism of plants. However, plants also need nitrogen to grow, and whether or not they can attain the full benefit of stimulated photosysnthetic rates is linked to the rates at which they can obtain nitrogen from the soil. The heavy stable isotope of nitrogen, N-15, has often been used as a tracer to study soil uptake and the N nutrition of plants. Surprisingly, measurements of foliage N-15 from several free-air CO2 enrichment experiments around the U.S. showed that growth in elevated CO2 was decreasing the N-15 concentrations of the foliage, more so than the overall N concentrations. These data suggest that future elevated levels of CO2 may affect the discrimination among the isotopes of N in ways not suspected before and that the N nutrition of plants may also be affected in ways not suspected before. Understanding the linked effects of elevated CO2 and nitrogen nutrients of plants will benefit all persons and other living organisms.

Technical Abstract: Leaf N-15 signature is a powerful tool that can provide an integrated assessment of the nitrogen (N) cycle and whether it is influence by rising atmosphericCO2 concentration. We tested the hypothesis that elevated CO2 significantly changes foliage N-15 concentration in a wide range of plant species and ecosystem types. This objective was achieved by determining the N-15 concentration of foliage of 27 field-grown plant species from six free=air CO2 enrichment (FACE) experiments representing desert, temperate forest, Mediterranean-type, grass-land prairie, and agricultural ecosystems. We found that within species, the N-15 concentration of foliage produced under elevated CO2 was significantly lower (P<0.038) compared with that of foliage grown under ambient conditions. Further analysis of foliage N-15 concentration by life form and growth habit revealed that the CO2 effect was consistent across all functional groups tested. The examination of two chaparral shrubs grown for 6 years under a wide range of CO2 concentration (25-75 Pa) also showed a significant and negative correlation between growth CO2 and leaf N-5 concentration. In a select number of species, we measure bulk soil N-15 concentration at a depth of 10 cm, and found that the observed depletion of foliage N-15 concentration in response to elevated CO2 was unrelated to changes in the soil N-15 concentration. While the data suggest a strong influence of elevated CO2 on the N cycle in diverse ecosystems, the exact site(s) at which elevated CO2 alters fractionating processes of the N cycle remains unclear. We cannot rule out the fact that the pattern of foliage N-15 concentration responses to elevated CO2 reported here resulted from a general drop in N-15 concentration of the source N, caused by soil-driven processes. There is a stronger possibility, however, that the general depletion of foliage N-15 concentration under high CO2 may have resulted from changes in the franctionating processes within the plant/mycorrhizal system.