CO₂ enrichment on biomass accumulation and nitrogen nutrition of spring wheat under different soil nitrogen and water status

Atmospheric CO₂ increase may bring unexpected consequences on field crops with less nitrogen (N) and water supply. This study investigated biomass accumulation and N nutrition of spring wheat (Triticum aestivum L.) grown under two atmospheric CO₂ concentrations (350 and 700 μmol mol^-1), two soil moisture treatments (well-watered and droughted) and five N fertilizer treatments (0, 50, 100, 150, 200 mg kg^-1). Results showed that the effects of elevated [CO₂] on shoot and total biomass accumulation depended largely on the soil N levels. Shoot and total biomass increased significantly in responding to CO₂ enrichment only in moderate to high N treatments. With either the droughted or the well-watered treatments, CO₂ enrichment didn't show any significant enhancement in biomass accumulation. Thus, elevated CO₂ concentrations did not relieve any adverse effects of droughted and N stress on the dry matter accumulation. Enriched [CO₂] levels reduced the shoot N contents, shoot and total N uptake significantly but only slightly decreased the root N content and uptake. Shoot critical N concentrations were lower for spring wheat grown at 700 μmol mol^-1 than at 350 μmol mol^-1 CO₂ concentration in both well-watered and droughted treatments. The critical N concentrations were 1.6% and 2.1% for the well-watered and droughted treatment at enriched [CO₂], and 1.9% and 2.6% at ambient [CO₂], respectively. Our results showed that CO₂ enrichment increased both the water use efficiency (WUE) and nitrogen use efficiency (NUE), possibly due to the reduced mass flow to the roots and thus reduced shoot and root N concentrations. The decrease in N uptake by spring wheat due to enriched [CO₂] could not be explained in this experiment.