Global Change Biology (1997) 3, 513–521 Effects of elevated atmospheric CO 2 in agro-ecosystems on soil carbon storage H. ALLEN TORBERT,* HUGO H. ROGERS,† STEPHEN A. PRIOR,† WILLIAM H. SCHLESINGER‡ and G. BRETT RUNION§ *USDA-ARS, Grassland, Soil and Water Research Laboratory, 808 East Blackland Rd., Temple, TX 76502, USA, †USDA-ARS National Soil Dynamics Laboratory, Box 3439, Auburn, AL 36831–3439, USA, ‡Duke University, Box 90340, Durham, NC 27708–0340, USA, §School of Forestry, Auburn University, Auburn, AL 36849, USA Abstract Increasing global atmospheric CO 2 concentration has led to concerns regarding its potential effects on the terrestrial environment. Attempts to balance the atmospheric carbon (C) budget have met with a large shortfall in C accounting (μ1.4 X 10 15 gCy –1 ) and this has led to the hypothesis that C is being stored in the soil of terrestrial ecosystems. This study examined the effects of CO 2 enrichment on soil C storage in C3 soybean (Glycine max L.) Merr. and C4 grain sorghum (Sorghum bicolor L.) Moench. agro ecosystems established on a Blanton loamy sand (loamy siliceous, thermic, Grossarenic Paleudults). The study was a split-plot design replicated three times with two crop species (soybean and grain sorghum) as the main plots and two CO 2 concentration (ambient and twice ambient) as subplots using open top field chambers. Carbon isotopic techniques using δ 13 C were used to track the input of new C into the soil system. At the end of two years, shifts in δ 13 C content of soil organic matter carbon were observed to a depth of 30 cm. Calculated new C in soil organic matter with grain sorghum was greater for elevated CO 2 vs. ambient CO 2 (162 and 29 g m –2 , respectively), but with soybean the new C in soil organic matter was less for elevated CO 2 vs. ambient CO 2 (120 and 291 g m –2 , respectively). A significant increase in mineral associated organic C was observed in 1993 which may result in increased soil C storage over the long-term, however, little change in total soil organic C was observed under either plant species. These data indicate that elevated atmospheric CO 2 resulted in changes in soil C dynamics in agro-ecosystems that are crop species dependent. Keywords: carbon cycle, carbon dioxide, δ 13 C, Glycine max, soil carbon dynamics, Sorghum bicolor Received 24 August 1996; revision accepted 3 January 1997 Introduction The rise of CO 2 in the atmosphere is well documented Dahlman 1991; Rogers et al. 1994; Goudriaan & Zadoks (Keeling et al. 1989); what has not been documented are 1995). Carbon fixed within biomass ultimately enters the the sinks for this C. With 5.5 X 10 15 gC y –1 emitted from soil where it may reside for hundreds of years (Parton fossil fuel and 1.1 X 10 15 g C y –1 from land-use change et al. 1986; Wallace et al. 1990). (tropical deforestation), 5.2 X 10 15 gCy –1 are accounted The ability of the soil to store C, however, is a highly for by atmospheric increase and by oceanic uptake, debated scientific question. Changes in plant morphology leaving an unknown sink of 1.4 X 10 15 gCy –1 (Schimel (Thomas & Harvey 1983; Prior et al. 1995), physiology et al. 1995). One hypothesis that has been forwarded is (Amthor 1991; Rogers & Dahlman 1991; Amthor et al. that C is being stored in terrestrial ecosystems (Tans et al. 1994; Rogers et al. 1994), and phytochemistry (Lekkerkerk 1990; Fisher et al. 1994) as a result of higher plant et al. 1990; Liljeroth et al. 1994) as a result of increasing productivity induced by elevated CO 2 . Increases in plant levels of atmospheric CO 2 will likely have dramatic growth are well documented experimentally (Rogers & impacts on plant–microbe interactions and, thus, on C cycling and the potential for C storage in soil. Schlesinger Correspondence: H.A. Torbert, fax +1 817-7706561, e-mail (1986, 1990) found little evidence for soil C storage. Torbert@brcsun0.tamu.edu Lamborg et al. (1983) have argued that increased soil © 1997 Blackwell Science Ltd. 513