Impact of elevated CO₂ on grain nutrient concentration varies with crops and soils - A long-term FACE study

The impact of elevated CO₂ (eCO₂) on grain nutrient concentration is becoming a global concern in terms of future human nutrition. Previous research has shown that eCO₂ can alter the availability and uptake of nutrients in crops. However, the interactive effects of long-term eCO₂ and soil types on the concentrations of nutrients in grain are poorly understood. By understanding such effects, we are able to develop management practices that maintain grain nutritional quality while improving crop yield in response to future climatic conditions. We conducted a seven-year experiment of free air CO₂ enrichment (FACE) with a rotation of wheat, field pea and canola grown in a Chromosol (Luvisol), Vertosol (Vertisol) and Calcarosol (Calcic Xerosol) under ambient CO₂ (aCO₂) (390 ± 10 μmol mol^-1) or eCO₂ (550 ± 30 μmol mol^-1). The concentration and amount of five macro- and four micro-nutrients in grain over the seven years were determined. Compared to aCO₂, the concentrations of N, P and Zn decreased by 6%, 5% and 10% under eCO₂, respectively, irrespective of soil, crop and year. A greater decrease in N concentration was found in canola and wheat compared to field pea. The reduction in P and Mg concentration of canola was significant in Chromosol, but not in the Vertosol nor Calcarosol soils. The concentrations of K, Fe, Mn and Cu were not affected by eCO₂ in any crop grown in the soils tested. Furthermore, eCO₂ significantly decreased soil labile N and P and exchangeable Mg and Cu due to greater nutrient uptake, which was in part ascribed to the decreased nutrient accumulation in crop grains. It appears that eCO₂ lowers the nutritional quality (nutrient concentration) in grains of non-legume crops, and that the extent of this decrease was greater in relatively fertile than infertile soils.