Evaluation of added nitrogen interaction effects on recovery efficiency in irrigated cotton

Studies were conducted in 1997 and 1999 at the University of Arizona Maricopa Agricultural Center to evaluate the added nitrogen (N) interaction or 'priming effect' on the determination of N recovery efficiencies (NRE) in Upland cotton (Gossypium hirsutum L.). Overall growth patterns of the crop were significantly different between the two years. In 1997, the crop experienced a favorable balance between reproductive and vegetative growth. The NRE estimates were 34.7% and 34.7% for Treatment 1 (recommended fertilizer N rate, 168 kg N ha (RecN)) and 25.2% and 26.0% for Treatment 2 (twice recommended fertilizer N rate, 336 kg N ha (RecN 2×)) for the difference and isotopic dilution technique, respectively. In 1999, however, the crop experienced very poor fruit load and vigorous vegetative growth. This resulted in a crop that produced much more vegetative dry matter at the expense of reproductive dry matter (yield). Higher NRE estimates were observed using the difference technique (50.8% and 40.5% for RecN and RecN 2×, respectively) when compared with the isotopic dilution technique (32.3% and 35.2% for RecN and RecN 2×, respectively). Higher amounts of soil N taken up by the plant were also observed in 1999 when compared with those in 1997. The results presented from these studies indicate that plant uptake of indigenous soil N was much higher in 1999 than in 1997, which is evidence of an added N interaction. However, this increase does not seem to have been stimulated by the addition of fertilizer N but rather the increased vegetative growth and root exploration that occurred in 1999.