The impact of elevated CO on yield loss from a C-3 and C-4 weed in field-grown soybean

Soybean (Glycine max) was grown at ambient and enhanced carbon dioxide (CO2 , + 250 muL L-1 above ambient) with and without the presence of a C3 weed ( lambsquarters, Chenopodium album L.) and a C4 weed (redroot pigweed, Amaran thus retroflexus L.), in order to evaluate the impact of rising atmospheric carbon dioxide concentration [CO2] 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 [CO2]. However, for lam bsquarters the reduction in soybean seed yield relative to the weed-free co ndition increased from 28 to 39% as CO2 increased, with a 65% increase in t he average dry weight of lambsquarters at enhanced [CO2]. Conversely, for p igweed, soybean seed yield losses diminished with increasing [CO2] from 45 to 30%, with no change in the average dry weight of pigweed. In a weed-free environment, elevated [CO2] resulted in a significant increase in vegetati ve dry weight and seed yield at maturity for soybean (33 and 24%, respectiv ely) compared to the ambient CO2 condition. Interestingly, the presence of either weed negated the ability of soybean to respond either vegetatively o r reproductively to enhanced [CO2]. Results from this experiment suggest: ( i) that rising [CO2] could alter current yield losses associated with compe tition 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 [CO2] increases.