Elevated CO2 increases N rhizodeposition and microbial immobilization of root-derived N

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.