The impact of elevated CO₂ on yield loss from a C₃ and C₄ weed in field‐grown soybean

Summary Soybean ( Glycine max ) was grown at ambient and enhanced carbon dioxide (CO 2 , + 250 μL L −1 above ambient) with and without the presence of a C3 weed (lambsquarters, Chenopodium album L.) and a C4 weed (redroot pigweed, Amaranthus retroflexus L.), in order to evaluate the impact of rising atmospheric carbon dioxide concentration [CO 2 ] on crop production losses due to weeds. Weeds of a given species were sown at a density of two per metre of row. A significant reduction in soybean seed yield was observed with either weed species relative to the weed‐free control at either [CO 2 ]. However, for lambsquarters the reduction in soybean seed yield relative to the weed‐free condition increased from 28 to 39% as CO 2 increased, with a 65% increase in the average dry weight of lambsquarters at enhanced [CO 2 ]. Conversely, for pigweed, soybean seed yield losses diminished with increasing [CO 2 ] from 45 to 30%, with no change in the average dry weight of pigweed. In a weed‐free environment, elevated [CO 2 ] resulted in a significant increase in vegetative dry weight and seed yield at maturity for soybean (33 and 24%, respectively) compared to the ambient CO 2 condition. Interestingly, the presence of either weed negated the ability of soybean to respond either vegetatively or reproductively to enhanced [CO 2 ]. Results from this experiment suggest: (i) that rising [CO 2 ] could alter current yield losses associated with competition from weeds; and (ii) that weed control will be crucial in realizing any potential increase in economic yield of agronomic crops such as soybean as atmospheric [CO 2 ] increases.