Do rhizosphere priming effects enhance plant nitrogen uptake under elevated CO2?

• We tested rhizosphere priming effects on plant N availability under elevated CO2.
• We used a dual-isotope labelling technique with 13C-depleted CO2 and 15N-enriched soil.
• eCO2 enhances plant N availability via accelerated SOM decomposition.
• Enhanced available N from SOM may support sustained plant growth under elevated CO2.

Numerous studies suggest that rhizosphere priming can mediate decomposition of soil organic matter (SOM), but direct evidence of priming-induced soil SOM decomposition on plant N uptake under elevated CO2 (eCO2) is very rare. By using a continuous dual labelling technique with 13C-depleted CO2 and 15N-enriched soil, we investigated priming of SOM decomposition and its relationship with plant N uptake of C4 and C3 grasses from a grassland ecosystem under eCO2. We observed that eCO2 induced increases in plant biomass, plant N uptake, rhizosphere priming, and total SOM decomposition in both grasses at an early plant life stage. Increased total SOM decomposition was positively related with plant N uptake by both C4 and C3 grasses under eCO2. However, the C3 grass was more dependent on N acquired from rhizosphere priming of SOM than the C4 grass. Our findings highlight that plant N uptake could be enhanced under eCO2 via accelerated SOM decomposition, and rhizosphere priming effects on SOM decomposition could play a more important role in N availability of the C3 grass in comparison with the C4 grass.