Elevated Atmospheric CO2 in Agroecosystems: Residue Decomposition in the Field

Elevated atmospheric CO2 concentration can increase biomass production and alter tissue composition. Shifts in both quantity and quality of crop residue may alter carbon (C) and nitrogen (N) dynamics and management considerations in future CO2-enriched agroecosystems. This study was conducted to determine decomposition rates of the legume soybean [Glycine max (L.) Merr.] and nonlegume grain sorghum [Sorghum bicolor (L.) Moench.] residue produced under two levels of atmospheric CO2 (ambient and twice ambient) on a Blanton loamy sand (loamy siliceous, thermic, Grossarenic Paleudults) in Auburn, Alabama, USA, managed using no-till practices. At maturity, harvested plants were separated into component parts for dry weight determination and tissue analysis. Mass, C, and N losses from residues were determined using the mesh bag method. Biomass production was significantly greater for soybean compared to sorghum and for elevated versus ambient CO2-grown plants. The CO2 level had little affect on the C/N ratio of residue (probably because the tissue used was senesced). Elevated CO2 concentration did not affect percent residue recovery; however, greater biomass production observed under elevated CO2 resulted in more residue and C remaining after overwintering. The higher total N content of soybean residue, particularly when grown under elevated CO2, indicated more N may be available to a following crop with lower N inputs required. Results suggest that in a high CO2 environment, greater amounts of residue may increase soil C and ground cover, which may enhance soil water storage, improve soil physical properties, and reduce erosion losses. The global rise in atmospheric CO2 concentration (Keeling and Whorf 1994) has raised questions concerning the dynamics of carbon (C) in terrestrial ecosystems (Houghton and others 1990, 1992). Studies have demonstrated that CO2 enrichment can increase aboveground (Kimball 1983, Rogers and Dahlman 1993, Wittwer 1995, Kimball and others 2002) and belowground biomass production (Prior and others 1994, Rogers and others 1994) and induce changes in residue quality (Amthor 1995, Poorter and others 1997, Torbert and others 2000, Norby and Cotrufo 1998). Lower tissue nitrogen (N) concentration in conjunction with other shifts in the chemical makeup of plant tissue (e.g., lignin) due to high CO2 can be important factors influencing residue C and N dynamics in highly managed agricultural systems (Torbert and others 2000). However, the impact of these CO2-induced changes on residue decomposition can be species dependent (Wood and others 1994, Torbert and others 1995, 1998, Prior and others 1997b); differential changes in the quantity and quality of plant tissue may affect nutrient turnover and soil C storage as well as soil properties. Consideration of these factors along with farm residue management practices may become important in predicting future soil C dynamics in agroecosystems (Follett 1993, Kern and Johnson 1993, Potter and others 1998, Lal and others 1998a). This will be especially true as more farms adopt conservation tillage systems that not only promote more soil C sequestration (Follett 1993, Kern and Johnson 1993, Paustian and others 1997, Lal and others 1998b), but can also improve soil structure (Blevins and others 1984, Campbell and Zentner 1993) and water storage (Hudson 1994) while decreasing erosional losses (Unger and McCalla 1980, Phillips and others 1980, Griffith and others 1986). The fate of residue C in CO2-enriched agroecosystems is a highly relevant issue since the potential for C storage in agricultural systems is of special interest in the current climate change policy debate. Improved predictions on how changes in the global environment will impact agroecosystems will depend on obtaining realistic field data. More CO2 research with major crops is required to accurately determine if shifts in the quantity and quality of residue will influence decomposition processes in highly managed agricultural systems. Furthermore, the amount of crop res