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.