Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 10, 2000
Publication Date: N/A
Interpretive Summary: Since photosynthesis and growth of plants are dependent on carbon dioxide, increases in agricultural productivity are expected as atmospheric carbon dioxide continues to increase. However, the rise in carbon dioxide may increase the response of weeds as well. To date, projected increases in agricultural productivity have been based on experiments where the crop was sgrown in monoculture and not in a realistic weed/crop mixture in the field In the experiment presented here, soybean was grown with and without the presence of either lambsquarters or pigweed at current carbon dioxide concentrations and at a future enhanced concentration (300 ppm above the current level). These weeds were chosen because of their contrasting photosynthetic pathways. Lambsquarters should respond to increasing carbon dioxide, pigweed should not. Both weeds are commonly found in commercial soybean fields. For lambsquarters, the reduction in soybean seed yield relative to the weed-free condition increased from 24% to 38% at enhanced carbon dioxide with a concomitant 65% increase in the average dry weight of lambsquarters. Conversely, for pigweed the reduction in soybean seed yield diminished with enhanced carbon dioxide from 41% to 24% with no change in the average dry weight of pigweed. However, the presence of either weed prevented soybean from responding to enhanced carbon dioxide. These data emphasize the importance of weed control and suggest that current projections of agricultural productivity in a future high carbon dioxide environment may be overestimating seed yield by not accounting for weed/crop competition.
The effect of ambient and enhanced carbon dioxide (300 ppm above ambient) was determined for field soybean with and without the presence of a C3 weed (lambsquarters, Chenopodium album) and a C4 weed (redroot pigweed, Amaranthus retroflexus) at a density of two weeds per meter of a row. A significant reduction in soybean seed yield was observed for both weed species relative to the weed-free control for either carbon dioxide level. For lambsquarters, the reduction in soybean seed yield relative to the weed-free condition increased from 24% to 38% at enhanced carbon dioxide with a concomitant 65% increase in the average dry weight of lambsquarters. Conversely, for pigweed the reduction in soybean seed yield diminished with enhanced carbon dioxide from 41% to 24% with no change in the average dry weight of pigweed. In a weed-free environment, elevated carbon dioxide resulted in a significant increase in vegetative dry weight and seed yield at maturity for soybean (33% and 24%, respectively) compared to the ambien carbon dioxide control. However, the presence of either weed negated the ability of soybean to respond either vegetatively or reproductively to enhanced carbon dioxide. Results from this experiment suggest: (1) rising carbon dioxide could alter current yield losses associated with weedy competition, and (2) weed control will be crucial in realizing any potential increase in economic yield of agronomic crops as carbon dioxide increases.