Background/Question/Methods Carbon sequestration via plant biomass, which can potentially mitigate increasing atmospheric CO₂, depends on nutrient availability. In turn, nutrient uptake depends partly on carbon allocation to root biomass for soil exploration and to root exudates for stimulation of microbial activity. Because a plant may encounter heterogeneous nutrient environments during its growth, rapid localized response to pulses of nutrients may be important in stressful conditions. We determined how the plant carbohydrate export to root responded to short-term changes in nutrient availability under enhanced atmospheric CO₂. We used pulses of the short-lived radiotracer ¹¹CO₂ generated at the Triangle Universities Nuclear Laboratory to measure the dynamic transport of recently fixed carbon to root sinks, exudates, and respiration in two grass species. An annual crop, Hordeum distichum (Hd), and a native perennial, Scirpus olneyi (So), were grown hydroponically at two atmospheric [CO₂] (380 and 600 µmol mol^-1) in environmental growth chambers for 15 days in the Duke Phytotron. The short-lived radiotracer method allowed two sequential measurements per day on the same plant: (1) in a control nutrient solution and (2) immediately followed by a switch to a 10-fold increase or decrease in nutrient concentration.

Results/Conclusions For both species, elevated CO₂ reduced the proportion of belowground ¹¹C lost to root respiration regardless of nutrient treatments (74% and 62% reduction for Hd and So, respectively). In Hd, elevated CO₂ enhanced the proportion of belowground C lost to exudates (2.7% vs. 5.0% for ambient and elevated CO₂, respectively). However for So, the proportion of belowground C lost to exudates was reduced under elevated CO₂ (2.8% vs. 0.9% for ambient and elevated CO₂, respectively). In both species, root exudates were significantly greater in low than high nutrient solutions (71% and 28% for Hd and So, respectively) regardless of switching direction. Switching from low to high nutrient treatments resulted in a greater enhancement of root exudation with elevated CO₂. These data showed that root exudation is enhanced under short-term nutrient deficiency suggesting a more rapid root response to nutrients than the reported general breakdown in plant metabolism due to chronic nutrient deficiency. Furthermore, the data showed a stronger root exudate response in plants acclimated to stressful nutrient conditions in elevated CO₂ suggesting that these can adjust more efficiently their metabolism to stimulate microbial biomass, hence potential nutrient turnover, than plants in optimal soil conditions.