Impact of nitrogen fertilizer on maize evapotranspiration crop coefficients under fully irrigated, limited irrigation, and rainfed settings

One of the common methods for estimating actual evapotranspiration (ET_a) is the two-step approach, which relates crop-specific crop coefficients (K_c) to a reference surface ET, typically alfalfa or grass (ET_r and ET_o, respectively). Minimal, if any, study has reported K_c values for water, nutrient, and both water and nutrient deficiencies. In this study, alfalfa (K_cr) and grass (K_co) reference maize (Zea mays L.) K_c values were developed as a function of growing degree days (GDDs) for 0, 84, 140, 196, and 252 kg ha^-1 nitrogen (N) treatments under fully irrigated (FIT), limited irrigation (75% FIT), and rainfed conditions at the University of Nebraska-Lincoln South Central Agricultural Laboratory (SCAL) near Clay Center, Nebraska, for the 2011 and 2012 growing seasons. The research also investigated a stress factor (K_stress) to assess the reduction in crop water use as compared with a nonlimiting water and N treatment (reference). In 2011, maximum K_cr values ranged from 0.95 to 1.27 and occurred between GDD values of 995 and 1,163°C (late July to early August), which corresponded to the R1 to R3 growth stages, whereas in 2012 (much drier), maximum Kcr values ranged from 0.84 to 1.19 for 75% FIT and FIT and existed between GDD values of 1,111 and 1,267°C (R2 to R4 growth stages). On average, greater K_cr values existed at higher N rates (e.g., 196 and 252 kgNha^-1) compared with lower N rates. Lower N treatments typically reached their maximum K_cr value earlier in the growing season and began to decrease towards harvest. Rainfed and 75% FIT experienced a greater reduction in K_stress as compared with FIT as well as lower N rate treatments as compared with higher N treatments. A water stress factor (K_w) was calculated to determine the portion of K_stress attributed with water stress alone. The monthly average values often experienced lower K_stress compared with K_wvalues, indicating that K_w alone was unable to account for the total reduction in K_cr from a nonlimiting water and N reference. Thus, an N stress factor (K_n) was also quantified by assuming Kstress was the product of water and N stress (e.g., K_stress = K_w × K_n). The seasonal average K_n was 1.15 in 2011 and 0.64 in 2012. Values of K_n were always lower in the drier year in 2012 than in 2011, ranging from 0.45 towards the end of the season in 2012 to a maximum of 1.27 in August 2011. In general, K_n decreased as N rate decreased and K_n had a decreasing trend (e.g., greater N stress) throughout the growing season, especially in the drier year in 2012. The reduction in K_n over time was due to the temporal reduction in readily available N as well as compounding effects of reduced N on plant growth and consequently crop water uptake over the growing season.