Wednesday, August 8, 2007 - 11:10 AMCOS 75-10: Elevated CO2 increases N rhizodeposition and microbial immobilization of root-derived N
Marie-Anne De Graaff, Johan Six, and Chris van Kessel. University of California, Davis
It is unclear whether enhanced plant production under elevated CO2 can be sustained in the long-term. Using meta-analysis we showed that in long-term field experiments microbial N-immobilization increases under elevated CO2. These data suggest that elevated CO2 increases nutrient limitation. Nonetheless, under low N availability elevated CO2 still stimulated plant production by ~10%. A possible explanation for this effect may be that CO2 increases nutrient availability in the rhizosphere by stimulating N release from soil organic matter through enhanced rhizodeposition. However, it is yet uncertain to which degree changes in rhizodeposition affect microbial regulation of soil N availability. With this study, we aimed at determining how elevated CO2 affects rhizodeposition and the cycling of rhizodeposited N under C3 and C4 plants. In addition, we tested how cultivated genotypes of Triticum turgidum (wheat) and Zea mays (maize) versus their wild relatives responded to elevated CO2. By constructing an N transfer experiment using 15N we could directly assess cycling of the rhizodeposited N, and trace the fate of root derived N in the soil and into receiver plants. No effect of elevated CO2 on shoot-, root growth and rhizodeposition was found for maize, a C4 species. In contrast, elevated CO2 stimulated root- and shoot production on average by 38% and increased rhizodeposition and microbial immobilization of rhizodeposited N by 30% in wheat, a C3 species. Concurrently, elevated CO2 reduced mineral 15N and re-uptake of the root derived N by 50% for wheat. Since there were no significant genotypic differences with regard to rhizodeposition and N cycling in response to elevated CO2, plant breeding appears to have limited effect on genotypic responses to elevated CO2. Results showed that elevated CO2 may enhance N limitation by increasing N rhizodeposition in C3 species and subsequent immobilization of the root derived N by microbes.