Cotton Root Response to Free-Air Carbon Dioxide Enrichment

Hugo H. Rogers and Stephen A. Prior


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.

Reprinted from 1990 Proceedings: Beltwide Cotton Production Research Conferences pg. 721
©National Cotton Council, Memphis TN

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Document last modified Sunday, Dec 6 1998