SEQUENTIAL NLEAP SIMULATIONS TO EXAMINE EFFECT OF EARLY AND LATE PLANTED WINTER COVER CROPS ON NITROGEN DYNAMICS

In different studies conducted during the last 10 years, underground w ell water NO3--N concentrations have been found to exceed drinking wat er standards of 10 mg NO3--N L-1 (10 ppm)Sor some areas of the San Lui s Valley of south central Colorado. Same of these studies reported tha t NO3--N leaching is a factor that can contribute to these high well w ater NO3--N concentrations. Computer models are technology transfer to ols that can assess impacts of best irrigation and nutrient management practices Sequential computer simulations of the effects of winter co ver crop planting date on residual soil NO3--N concentrations have not been previously conducted. The new version 1.2 of the Nitrate Leachin g and Economic Analysis Package (NLEAP) model was used to conduct an a ssessment of the effect of winter cover crop planting date on conserva tion of water quality The NLEAP model simulated a lettuce (Lactuca sat iva L.) - winter cover wheat (Triticum aestivum L.) and lye (Secale ce reale L.) (early- or late-planted) potato (Solanum tuberosum L.) rotat ion, grown on a center-pivot irrigated Kerber loamy sand. Winter cover crops increased the system N use efficiency (NUE) (P<0.05). The effec t of early-planted winter cover craps on conservation of water quality extended into the potato growing season, increasing NUE and reducing NO3--N leaching (P<0.05).