EFFECTS OF ELEVATED ATMOSPHERIC CO2 IN AGROECOSYSTEMS ON SOIL CARBON STORAGE

Increasing global atmospheric CO2 concentration has led to concerns re garding its potential effects on the terrestrial environment. Attempts to balance the atmospheric carbon (C) budget have met with a large sh ortfall in C accounting (approximate to 1.4 x 10(15) g C y(-1)) and th is has led to the hypothesis that C is being stored in the soil of ter restrial ecosystems. This study examined the effects of CO2 enrichment on soil C storage in C3 soybean (Glycine max L.) Merr. and C4 grain s orghum (Sorghum bicolor L.) Moench. agroecosystems established on a Bl anton loamy sand (loamy siliceous, thermic, Grossarenic Paleudults). T he study was a split-plot design replicated three times with two crop species (soybean and grain sorghum) as the main plots and two CO2 conc entration (ambient and twice ambient) as subplots using open top field chambers. Carbon isotopic techniques using delta(13)C were used to tr ack the input of new C into the soil system. At the end of two years, shifts in delta(13)C content of soil organic matter carbon were observ ed to a depth of 30 cm. Calculated new C in soil organic matter with g rain sorghum was greater for elevated CO2 vs. ambient CO2 (162 and 29 g m(-2), respectively), but with soybean the new C in soil organic mat ter was less for elevated CO2 vs. ambient CO2 (120 and 291 g m(-2), re spectively). A significant increase in mineral associated organic C wa s observed in 1993 which may result in increased soil C storage over t he long-term, however, little change in total soil organic C was obser ved under either plant species. These data indicate that elevated atmo spheric CO2 resulted in changes in soil C dynamics in agro-ecosystems that are crop species dependent.