Elevated atmospheric CO2 and species mixture alter N acquisition of trees in stand microcosms

The potential for elevated atmospheric CO 2 to increase forest growth depends on how it affects plant acquisition of soil nitrogen (N) in realistic competitive settings. We grew seedling microcosms in large (0.6-m 2 ) boxes of forest soil placed outdoors in CO 2 -controlled open-top chambers. Loblolly pine (Pinus taeda L.) and sweetgum (Liquidambar styraciflua L.) were grown as single-species stands (monocultures) and as 50:50 pine:sweetgum mixtures, with a factorial combination of CO 2 (ambient, twice ambient) and soil water (dry, moist) for two growing seasons. We added N, enriched with 15 N, 2 months after planting and used N and 15 N content of microcosm components to evaluate treatment effects. Under ambient CO 2 , species mixture decreased biomass and N accumulation of pine compared with pine in monoculture. Elevated CO 2 partially to fully ameliorated this negative effect of species mixture for pine by increasing its biomass and N accumulation irrespective of competitive setting. Sweetgum biomass and N accumulation were improved in mixed culture (compared with monoculture) under moist conditions. However, only sweetgum biomass (not N) responded positively to increasing CO 2 . Our study suggests that increasing atmospheric CO 2 concentration may provide a competitive advantage to pine growing in mixture with sweetgum in low fertility forest soils.