Denitrification in the top soil of managed grasslands in The Netherlands in relation to soil type and fertilizer level

The objective of this investigation was to determine how free-air carbon dioxide enrichment (FACE) of cotton (Gossypium hirsulam L.) affects root distribution in a natural soil environment. For two years cotton was grown on a Trix clay loam under two atmospheric CO₂ concentrations (370 and 550 μmol 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-reproductive growth, 90 cm soil cores were taken at 0,0.25, and 0.5 m perpendicular to row center; root variables were ascertained at three 30 cm depth 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 × position interaction for root length density at the vegetative stage (both years) and reproductive stage (1990 only); the positive effects of extra CO₂ were more evident at interrow positions (0.25 and 0.5 m). A CO₂ × depth × position 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. Similar trends were seen at the reproductive sampling for this measure as well as for root length density at both sample dates in 1990. In 1991, a CO₂ × depth interaction was noted at both periods; CO₂ enhancement of root densities (i.e., both length and dry weight) were observed within the upper and middle depths. Although variable in response, increases for root lineal density under high CO₂ were also seen. In general, results also revealed that the ambient CO₂ treatment had a higher proportion 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 treatment had proportionately more of its roots allocated away from row center (root length basis only). Results from this field experiment clearly suggest that increased atmospheric CO₂ concentration will alter root distribution patterns in cotton.