The rise in atmospheric CO₂ stimulates photosynthesis in most plants, leading to an increase in plant production by approximately 20%. The stimulation of plant production should enhance soil C input, which in turn may increase soil C sequestration, thereby counterbalancing the rise in atmospheric CO₂. However, whether soils will serve as CO₂ sinks in the long-term is still debated, since it is uncertain how elevated CO₂ affects the interactions between plants and soil nutrient cycling. Using meta-analysis we showed that in long-term field experiments simultaneous increases in N demands by plants and soil microbes under elevated CO₂ leads to enhanced nutrient limitation. As a result the positive plant growth response to elevated CO₂ may not be sustained in the long-term in unfertilized ecosystems. Nonetheless, under low N availability prolonged 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 recalcitrant soil organic matter (SOM) pools through enhanced rhizodeposition. We aimed at elucidating how CO₂-induced increases in rhizodeposition affect N release from recalcitrant SOM, and how wild versus cultivated genotypes of wheat mediated differential responses in soil N cycling under elevated CO₂. To quantify root-derived soil C input, plants were exposed to continuous labeling with ¹³C under ambient and elevated CO₂. To quantify release of N from stable SOM pools, the plants were grown in soil containing ¹⁵N predominantly present in recalcitrant SOM pools.

Results/Conclusions

Root-derived soil C input to the active soil C pool increased by 53%, and microbial-¹³C was enhanced by 35% under elevated compared to ambient CO₂. Concurrently, plant ¹⁵N-uptake increased (+24%) under elevated CO_2, while ¹⁵N-contents in the microbial biomass and mineral N pool decreased. Wild genotypes allocated more C to their roots, while cultivated genotypes allocated more C to their shoots under ambient and elevated CO₂. This led to increased N acquisition for the wild genotypes, and enhanced biomass production for the cultivated genotypes. Data suggest that increased rhizodeposition under elevated CO₂ stimulates mineralization of N from recalcitrant SOM pools. In addition, contrasting C allocation patterns can explain plant mediated differential responses in soil N cycling to elevated CO₂.