Effects of elevated atmospheric CO₂ and soil water availability on root biomass, root length, and N, P and K uptake by wheat

Abstract

We investigated interactions between the effects of elevated atmospheric carbon dioxide concentrations ([CO₂]) and soil water availability on root biomass, root length and nutrient uptake by spring wheat (Triticum aestivum cv. Tonic). We grew plants at 350 and 700 µmol mol⁻¹ CO₂ and with frequent and infrequent watering (‘wet’ and ‘dry’ treatments, respectively). Water use per plant was 1·25 times greater at 350 than at 700 µmol CO₂ mol⁻¹, and 1·4 times greater in the ‘wet’ than in the ‘dry’ treatment. Root biomass increased with [CO₂] and with watering frequency. Elevated [CO₂] changed the vertical distribution of the roots, with a greater stimulation of root growth in the top layers of the soil. These data were confirmed by the video data of root lengths in the ‘dry’ treatment, which showed a delayed root development at depth under elevated [CO₂]. The apparent amount of N mineralized appeared to be equal for all treatments. Nutrient uptake was affected by [CO₂] and by watering frequency, and there were interactions between these treatments. These interactions were different for N, K and P, which appeared to be related to differences in nutrient availability and mobility in the soil. Moreover, these interactions changed with time as the root system became larger with [CO₂] and with watering frequency, and as fluctuations in soil moisture contents increased. Elevated [CO₂] affected nutrient uptake in contrasting ways. Potassium uptake appeared to be reduced by the smaller mass flow of water reaching the root surface. However, this might be countered with time by the greater root biomass at elevated [CO₂], by the greater soil moisture contents at elevated [CO₂], enabling faster diffusion, or both. Phosphorus uptake appeared to be increased by the greater root biomass at elevated [CO₂]. We conclude that plant nutrient uptake at elevated [CO₂] is affected by interactions with water availability, though differences between nutrients preclude generalizations of the response.