In-season nitrogen status sensing in irrigated cotton: I. yields and nitrogen-15 recovery

Nitrogen recommendations for Upland cotton (Gossypium hirsutum L.) in the western USA are based on spring soil N[O.sup.-.sub.3]-N tests. In-season monitoring of plant N status is another approach. Our primary objective was to test spectral reflectance and chlorophyll meter measurements as in-season N decision aids for irrigated cotton, and to compare these with soil test-based N management. The secondary objective was to determine the fate of [sup.15]N as affected by N management and irrigation modes. Urea ammonium nitrate was applied with low energy precision (LEPA) center-pivot, surface drip, and subsurface drip irrigation. Microplots received 3 atom% [sup.15]N. Soil test N application was based on 0-to 60-cm soil N[O.sup.-.sub.3]-N and 1400 kg lint [ha.sup.-1] expected yield. Thirty-four kilograms of N per hectare was applied when green vegetative index (GVI) or chlorophyll meter readings relative to well-fertilized plots were