Elevated CO2 effects on nutrient competition between a C3 crop (Oryza sativa L.) and a C4 weed (Echinochloa crusgalli L.)

Abstract

Rice (a C3 crop) and barnyard grass (Echinochloa crusgalli L.) (a C4 weed) were grown in a 1:1 mixture in a paddy field in ambient condition and with supplemented free air carbon dioxide enrichment (FACE, CO2 concentration + 200 μmol mol−1), in order to evaluate the impact of rising atmospheric carbon dioxide on nutrient competition between rice crop and weed. Results showed that elevated CO2 significantly enhanced the biomass, tillers, leaf area index (LAI) and net assimilation rate (NAR) of rice, but reduced those of barnyard grass after elongation. Tissue nitrogen (N) concentrations were decreased in both competitors, but their phosphorus (P) and potassium (K) concentration were increased. The increase in tissue P concentration of rice was greater than that in barnyard grass. Furthermore, the absolute uptake of C, N, P, K by rice were increased while those of barnyard grass decreased. As a result, significant increase of the ratios of rice/barnyard grass of biomass and absolute nutrient uptake were observed under elevated CO2. The results suggest that rising atmospheric CO2 concentration could alter the competition between rice and barnyard grass in paddy fields in favor of rice. The ability of rice to compete more successfully for nitrogen and phosphorous under elevated CO2 is likely an important factor underlying this response. More generally, the results suggest that elevated CO2 may have varying implications on nutrient dynamics between different elements of overall plant biomass and the soil nutrients pool.