Season-long elevation of ozone concentration to projected 2050 levels under fully open-air conditions substantially decreases the growth and production of soybean

• Mean surface ozone concentration is predicted to increase 23% by 2050. Previous chamber studies of crops report large yield losses caused by elevation of tropospheric ozone, and have been the basis for projecting economic loss. • This is the first study with a food crop (soybean, Glycine max) using free-air gas concentration enrichment (FACE) technology for ozone fumigation. A 23% increase in ozone concentration from an average daytime ambient 56 p.p.b. to a treatment 69 p.p.b. over two growing seasons decreased seed yield by 20%. • Total above-ground net primary production decreased by 17% without altering dry mass allocation among shoot organs, except seed. Fewer live leaves and decreased photosynthesis in late grain filling appear to drive the ozone-induced losses in production and yield. • These results validate previous chamber studies suggesting that soybean yields will decrease under increasing ozone exposure. In fact, these results suggest that when treated under open-air conditions yield losses may be even greater than the large losses already reported in earlier chamber studies. Yield losses with elevated ozone were greater in the second year following a severe hailstorm, suggesting that losses caused by ozone might be exacerbated by extreme climatic events.