Intensive cropping and conservation tillage can increase soil organic C (SOC) and improve soil quality, however, economic reality often dictates cotton ( Gossypium hirsutum) monoculture. We conducted a study on a Compass loamy sand (coarse-loamy, siliceous, subactive, thermic Plinthic Paleudults) from 1998-2001 to compare an intensive conservation cropping system to standard cotton production systems used in the southeastern USA (Alabama). The system uses sunn hemp ( Crotalaria juncea) and ultra-narrow row (UNR; 8-inch drill) cotton in a rotation with wheat ( Triticum aestivum) and maize ( Zea mays). The standard systems used continuous cotton (both standard 40-inch rows and ultra-narrow row) and a maize-cotton rotation with standard row widths. A cover crop mixture of black oat ( Avena strigosa [ A. nuda])/rye ( Secale cereale) was used in all systems preceding cotton and a white lupin ( Lupinus albus)/crimson clover ( Trifolium incarnatum) mix was used before maize in the maize-cotton and intensive system. All systems were tested under conservation and conventional tillage in a split plot design of four replications; main plots were cropping systems and subplots were tillage. We used extension budgets to calculate net returns over variable costs and determined C balance of all residues returned to the soil. At the end of the experiment, soil C was determined by dry combustion (0-0.4, 0.4-2, 2-4, 4-8, and 8-12 in depths). Cropping system had a more consistent effect on cotton yield than tillage system. Four-year average lint yields were 872, 814, 711 and 663 lbs acre -1 for continuous UNR, intensive, maize-cotton, and continuous 40-in cotton systems, respectively. The UNR systems with conservation tillage had the highest net returns ($105 acre -1 year -1 (continuous) and $97 acre -1 year -1 (intensive)) while the conventional tillage continuous 40-in system had the lowest returns ($36 acre -1 year -1). Conservation tillage increased SOC concentration in the top 2 inches of soil 46% compared to conventional tillage. Cropping system affected SOC levels to the 4-in depth and the maize-cotton rotation resulted in the lowest SOC levels of all systems. Results suggest that small grain cover crops and wheat for grain in the intensive system were the dominate factor in SOC changes. For these drought-sensitive soils, UNR cotton production systems with conservation tillage and small grain cover or cash crops have the potential to rapidly increase soil organic matter; improving soil productivity and enhancing economic sustainability of cotton production in the southeastern USA.
