Responses of Agricultural Crops to Free-Air CO₂ Enrichment

Over the past decade, free-air CO₂ enrichment (FACE) experiments have been conducted on wheat, perennial ryegrass, and rice, which are C₃ grasses; sorghum, a C₄ grass; white clover, a C₃ legume; potato, a C₃ forb with tuber storage; and cotton and grape, which are C₃ woody perennials. Elevated CO₂ increased photosynthesis, biomass, and yield substantially in C₃ species, but little in C₄. It decreased stomatal conductance in both C₃ and C₄ species and greatly improved water-use efficiency in all crops. Growth stimulations were as large or larger under water stress compared to well-watered conditions. At low soil N, stimulations of nonlegumes were reduced, whereas elevated CO₂ strongly stimulated the growth of the clover legume at both ample and low N conditions. Roots were generally stimulated more than shoots. Woody perennials had larger growth responses to elevated CO₂, but their reductions in stomatal conductance were smaller. Tissue N concentrations went down, while carbohydrate and some other carbon-based compounds went up, with leaves being the organs affected most. Phenology was accelerated slightly in most but not all species. Elevated CO₂ affected some soil microbes greatly but not others, yet overall activity was stimulated. Detection of statistically significant changes in soil organic carbon in any one study was nearly impossible, yet combining results from several sites and years, it appeared that elevated CO₂ did increase sequestration of soil carbon. Comparisons of the FACE results with those from earlier chamber-based results were consistent, which gives confidence that conclusions drawn from both types of data are accurate.

Introduction

The increasing CO₂ concentration of Earth's atmosphere and associated predictions of global warming (IPCC, 1996) have stimulated research programs to determine the likely effects of the future elevated CO₂ levels on agricultural productivity and on the functioning of natural ecosystems (e.g., Dahlman et al., 1985). However, even predating the global change concerns, the effects of atmospheric CO₂ enrichment have been studied for more than a century in greenhouses, controlled-environment chambers, open-top chambers, and other enclosures to confine the CO₂ gas around the experimental plants (e.g., Drake et al., 1985; Enoch and Kimball, 1986; Schulze and Mooney, 1993). The results of these many chamber-based experiments have been reviewed by Kimball (1983, 1986, 1993), Morison (1985), Cure (1985), Cure and Acock (1986), Kimball and Idso (1983), Poorter (1993), Idso and Idso (1994), Ceulemans and Mousseau (1994), Wullschleger et al. (1997), Cotrufo et al. (1998), Norby et al. (1999), Nakagawa and Horie (2000), Curtis and Wang (1998), and Wand et al. (1999) (although the latter two also included a few observations from recent nonchamber open-field experiments).

However, the environment inside enclosures is not generally like that outside (e.g., Kimball et al., 1997; McLeod and Long, 1999); thus, there have been many concerns that the results from such enclosure-based CO₂-enrichment experiments might not be representative of future open fields and forests. Therefore, various attempts were made to develop techniques which could maintain the CO₂ concentrations over open-field plots at elevated levels despite the challenges imposed by open-field winds causing rapid dispersal of the CO₂ (Allen, 1992; Norby et al., 2001). Eventually, engineers from Brookhaven National Laboratory (Upton, New York) working cooperatively with scientists from the U.S. Department of Agriculture, Agricultural Research Service, and from Tuskegee University, as well as others, were able to adapt a “vertical vent pipe” technology that could adequately maintain the desired high levels of CO₂ over open-field plots all growing-season long (Hendrey, 1993; Norby et al., 2001). The first such experiment with publishable biological data was conducted on cotton in 1989 at Maricopa, Arizona. After success with cotton, the Brookhaven group moved their engineering efforts to the forest, and they were able to increase the scale of the apparatus to accommodate 14-m-tall trees (e.g., Delucia et al., 1999). Once it was demonstrated that such free-air CO₂ enrichment (FACE) experiments were feasible, several other research groups also initiated similar experiments in both managed and natural ecosystems. To date, there are about 30 active or planned FACE sites (http://cdiac.esd.ornl.gov/programs/FACE/face.html; http://www.face.bnl.gov/; http://gcte-focus1.org/co2.html).

The purposes of this paper are (i) to compile the available data from the FACE experiments on agricultural crops; (ii) to determine the relative responses of the several crops to elevated CO₂ with regard to their physiology, growth, yield, water relations, and soil processes; (iii) to search for similarities and differences among species and plant functional types; and (iv) to compare these FACE results with those from prior chamber-based studies.