The overall purpose of the entire study was to determine and model the response of field-grown cotton to elevated atmospheric CO₂. The specific goal of the effort summarized here was to assess root response and associated edaphic factors. Eight experimental plots were established, four at nominal CO₂ concentrations of 550 ppm and four at ambient, 350 ppm, at the University of Arizona Maricopa Agricultural Center. Free-air CO₂ enrichment (FACE) was supplied over the season (17 April to 22 September, 1989) using 22-m diameter circular standpipe arrays. On 27-29 September, 8 replicate sets of 15 soil samples each were taken from each of the study plots, four for soil water content, bulk density (gravimetric, oven-dry weight basis, 105 C), and nutrient content determinations; and eight for root length density measurements within the crop row and three from the interrow zone. Each sample was taken from a continuous 90-cm core cut into six sections of 15 cm each. A Giddings slotted sample tube (3.8 cm I.D. with a #123 cutting head) was pushed below the 90-cm depth with a Giddings hydraulic sampler mounted on a custom-made frame designed to allow lateral precision positioning of the probe over a 2 m distance with hydraulically driven treaded rods. Once obtained soil cores were transferred to a stationary tray, inspected for uniformity, and divided into 15-cm increments. Roots were washed from core increments using a hydropneumatic elutriation system (Gillison's, Inc.) and, after separating roots from trash, measured with a root length scanner (Hawker de Havilland). Eight plants, with taproots and attached lateral roots, from each study plot were pulled up after loosening with a drain spade. Geometric size and dry weights (55 C) of their various parts were ascertained. Leaf and boll numbers were both around 40% higher, stem diameter and leaf area were both 20% higher, height about 6% for the 550 ppm plots compared to controls, but node number was essentially unchanged. Leaf, stem, and boll dry weights rose 22, 48, and 32%, respectively, with elevated CO₂. Taproot length, volume, and dry weight increases were 11, 58, and 76%, respectively. The total number of lateral roots was up 36% in the CO₂ enriched treatments. Nutritionally, the soil (Trix clay loam: fine, loamy, mixed (calcareous), hyperthermic Typic Torrifluvents) appeared to be well-managed with respect to pH, P, K, Mg, and Ca; no treatment differences were observed. Percent soil water was generally the same under both regimes, as were bulk densities with a plow layer mean of 1.3 g/cm³. Appreciable differences in root length density were seen throughout the profile sampled, with an average increase of 18%. Root dry weight density for the same samples showed a 109% increase. The dropoff in root length density from center of row to interrow appeared to be much steeper for high CO₂ plots. It is readily apparent that cotton roots respond significantly to atmospheric CO₂ enrichment, and it may be inferred that other belowground processes impacted by root proliferation could also shift. Logical extensions of this work would be rhizosphere microbiology and soil carbon. A need for further and more in-depth research is certainly indicated.