The Impact of Elevated CO2 on Rhizodeposition and Microbial N Mineralization Under Wild and Cultivated Wheat Genotypes.
Chris van Kessel, Marie-Anne de Graaff, and Johan Six. University of California-Davis, Dept. of Plant Sciences, One Shield Avenue, Davis, CA 95616
It is unclear whether enhanced plant production under elevated carbon dioxide can be sustained in the long-term. Using meta-analysis we showed that in long-term field experiments microbial N-immobilization increases under elevated carbon dioxide, which suggests that elevated carbon dioxide increases nutrient limitation. Nonetheless, under low N availability elevated carbon dioxide still stimulated plant production by ~10%. A possible explanation for this effect may be that carbon dioxide increases nutrient availability in the rhizosphere by stimulating N release from soil organic matter (SOM) 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 carbon dioxide affects rhizodeposition of C and how that feeds back to microbial N release from SOM under both wild and cultivated genotypes of Triticum durum (wheat). An airtight growth chamber allowed us to use continuous labelling with 13C for measuring rhizodeposition during a 5 week study. In addition, we used soil that had received 15N fertilization for 10 consecutive years to determine N release from SOM and concomitant plant N uptake. To ensure that the 15N was predominantly present in the more stable SOM pools, the soil was leached with a weak nutrient solution containing all nutrients except for N for 6 months prior to the study. Elevated carbon dioxide significantly increased aboveground plant production by 24% and 40% for the wild and cultivated genotypes, respectively. Belowground plant production was stimulated by 40% and 36% for the wild and cultivated genotypes, respectively. The impact of such changes under elevated carbon dioxide on rhizodeposition, microbial N transformations and plant N uptake will be discussed in more detail.