Elevated CO2 Concentration and Drought Stress Exert Opposite Effects on Plant Biomass,Nitrogen, and Phosphorus Allocation in Bothriochloa ischaemum

Increased concentrations of atmospheric carbon dioxide (CO₂) and drought stress have greatly influenced plant growth, the status of nitrogen (N) and phosphorus (P), and N:P ratios. We identified the plant biomass, N and P distributional patterns, and N:P stoichiometry of a grass species on the Loess Plateau in China under elevated CO₂ concentration and drought stress conditions. Bothriochloa ischaemum, a C4 perennial herbaceous grass, was grown in pots at CO₂ concentrations of 400 (ambient) and 800 (elevated) μmol mol^?1 and at 60 ± 5 and 40 ± 5 % of field capacity. The elevated CO₂ concentration significantly increased plant total biomass, N concentration, N and P content, allocation of biomass to roots, and allocation of N to shoots, and increased the N:P ratios of whole plants and the shoots, especially under well-watered conditions. Drought stress significantly decreased plant biomass and plant N and P content, especially under elevated CO₂. Drought stress decreased the N:P ratios, but was only significant in the roots under ambient CO₂. Drought stress may attenuate the stimulation of plant growth and N and P acquisition by CO₂ enrichment, and projected elevated CO₂ concentrations may partially offset the negative effects of increased drought by increasing the assimilation of N and P.