Effect of nitrogen fertilizer rate and nitrapyrin on leaf chlorophyll, leaf nitrogen concentration, and yield of three irrigated maize hybrids in Nebraska

A study was conducted from 1986 through 1988 on a Crete silt loam soil (fine, montmorillonitic, mesic Pachic Argiustolls) using three Pioneer brand maize hybrids (3377, 3475, 3551) to examine the effects of increasing fertilizer N rates (0, 75, 150, 300 kg N ha$\sp{-1}$), a nitrification inhibitor (nitrapyrin, 0 and 0.56 kg ha$\sp{-1}$), and tillage on leaf N, leaf chlorophyll, dry matter production, N uptake, N leaching, and yield. To characterize the relationships of applied N and yield components, soil and plant samples were taken at about one week before and one week after anthesis. Soil samples were taken to a depth of 0.45 m and leaf N samples collected using a punch to obtain 10 mm disks from the laminar portion of the leaf. Additional soil samples to a depth of 1.8 m were taken before planting and after harvest. Leaf disks were analyzed for chlorophyll using 80% acetone extraction and spectrophotometric colorimetry and for N concentration by modified semi-micro Kjeldahl technique. Chlorophyll and leaf N concentrations were correlated in 1987, but chlorophyll varied significantly with environmental conditions in other years. Laminar leaf punch N concentration was linearly related to midseason stover N concentration. Leaf N concentration was not consistently correlated with grain yield, although a significant association was observed in 1987. Midseason dry matter production, N uptake, and N concentration increased with applied N in every year. Grain yields were not influenced by tillage practice. Grain yields in 1986 were not increased by N applications beyond 75 kg N ha$\sp{-1}$ due to residual N effects. Plots incurred hail damage in 1987 but yields were better than 1986 with maximum yields (12 Mg ha$\sp{-1}$) occurring near the 150 kg N ha$\sp{-1}$ rate. Because of heat stress, 1988 grain yields were 30% lower than 1987, but dry matter production, N uptake, and N concentration in the stover at harvest were all increased. Grain N removal was near the amount applied at rates below 150 kg N ha$\sp{-1}$. Nitrapyrin decreased leaf N concentration in 1987, but did not affect yield. Nitrapyrin increased preanthesis inorganic soil N in all years. Postharvest soil samples exhibited significantly higher amounts of inorganic N at the highest N rate indicating that N not used by the crop is subject to leaching