Towards a rhizo‐centric view of plant‐microbial feedbacks under elevated atmospheric CO 2

Forum 664 Commentary Commentary Towards a rhizo-centric view of plant-microbial feedbacks under elevated atmospheric CO 2 The stimulatory effects of elevated CO 2 on plant productivity have been reported for many ecosystems (Ainsworth & Long, 2005), but whether such effects will persist in the face of increasing nutrient limitation is unclear. In nitrogen (N)-limited ecosystems, elevated CO 2 has been hypothesized to decrease nutrient availability as the N becomes sequestered in plant and soil pools with slow turnover rates. Alternatively, an elevated CO 2 may increase nutrient availability by stimulating N release from soil organic matter (SOM), resulting in positive feedbacks to primary production. Previous reports on the effects of elevated CO 2 on N cycling have been variable, with reports of increases, decreases or no change in soil N dynamics under elevated CO 2 (Zak et al., 2000). One major source of uncertainty is the degree to which potential changes in root-derived C affect the microbial regulation of soil N availability. In this issue of New Phytologist (pp. 778– 786), de Graaf et al. describe a novel approach to examining the effects of elevated CO 2 on root-derived inputs of N to soil. Their results support an emerging 'rhizo-centric' view, whereby root–microbial interactions may be the central processes in controlling the magnitude and duration of plant productivity responses under elevated CO 2. '… roots and rhizosphere microbes play a more important role than has been previously considered in mediating soil N availability under elevated CO 2 .' By labeling plants with 15 N via foliar uptake, de Graaf et al. quantified the magnitude and fate of N from rhizo-deposition in wild and cultivated genotypes of wheat and maize exposed to ambient and elevated levels of CO 2. Their study reported that rhizodeposition was a strong sink for foliar-applied N in all plants (5–10% of the total uptake from leaves), and that elevated CO 2 increased this flux in those plants that also increased in total biomass (e.g. wheat but not maize). Moreover, this study reports that CO 2-induced increases in rhizodeposition decreased soil N availability, as greater amounts of root-derived N were immobilized in the rhizosphere of the labeled plants and lesser amounts were taken up by unlabeled, 'receiver' plants growing in the same pots. Increased C fluxes from roots to soil under elevated CO 2 have been reported previously but the consequences of such changes for soil N …