FREE-AIR CO2 ENRICHMENT OF COTTON - VERTICAL AND LATERAL ROOT DISTRIBUTION PATTERNS

The objective of this investigation was to determine how free-air carb on dioxide enrichment (FACE) of cotton (Gossypium hirsutum L.) affects root distribution in a natural soil environment. For two years cotton was grown on a Trix clay loam under two atmospheric CO2 concentration s (370 and 550 mu mol mol(-1)) and two water treatments [wet, 100% of evapotranspiration (ET) replaced and dry, 75% (1990) and 67% (1991) of ET replaced] at Maricopa, AZ. At early vegetative and mid-reproductiv e growth, 90 cm soil cores were taken at 0, 0.25, and 0.5 m perpendicu lar to row center; root variables were ascertained at three 30 cm dept h increments. The effect of water stress alone or its interaction with CO2 on measured variables during both samplings were rare and showed no consistent pattern. There was a significant CO2 x position interact ion for root length density at the vegetative stage (both years) and r eproductive stage (1990 only); the positive effects of extra CO2 were more evident at interrow positions (0.25 and 0.5 m). A CO2 x depth x p osition interaction at the vegetative phase (1990) indicated that FACE increased root dry weight densities for the top soil depth increment at all positions and at the middle increment at the 0.5 m position. Si milar trends were seen at the reproductive sampling for this measure a s well as for root length density at both sample dates in 1990. In 199 1, a CO2 x depth interaction was noted at both periods; CO2 enhancemen t of root densities (i.e., both length and dry weight) were observed w ithin the upper and middle depths. Although variable in response, incr eases for root lineal density under high CO2 were also seen. In genera l, results also revealed that the ambient CO2 treatment had a higher p roportion of its root system growing closer to the row center, both on a root length and dry wight basis. On the other hand, the FACE treatm ent had proportionately more of its roots allocated away from row cent er (root length basis only). Results from this field experiment clearl y suggest that increased atmospheric CO2 concentration will alter root distribution patterns in cotton.