Temporal Dynamics of Soil Nitrogen (NH4+ and NO3-) in Corn Fields Under Tilled and No-Tillage Field with Varying Levels of Pre-Plant Nitrogen Fertilization.

Understanding dynamics of inorganic nitrogen (NH4⁺ and NO3^-) from pre-plant N fertilization in corn (Zea maize) fields is imperative for effective and efficient in-season N fertilization program. An attempt was made in 2016 by sampling soils from the upper 12 inches of tilled and non-tilled corn fields in Oklahoma from replicated plots having varying levels of pre-plant N (0, 40, 80, 120, 160 and 200 lbs/acre). Soil samples were sequentially taken a week prior to pre-plant fertilization, and at four (4) vegetative growth stages V2, V4, V8 and VT. Soils collected were then analyzed for inorganic NH4⁺, NO3^- and total N (NH4⁺ + NO3^-). Pre-plant application of 200 lbs/acre N consistently showed significantly higher concentrations of total N (NH4⁺ and NO3^-) at V2 (72.94 ppm, P <0.001) and V4 (56 ppm, P <0.001)   in no-till and in all vegetative stage in tilled field with P values of 0.034, 0.012, 0.006 and <0.001 for V2, V4, V8, and VT respectively. Interestingly, total N left at VT for 200 lbs/acre pre-plant N in tilled field was twice the amount (57.75ppm) as compared to no-till field (25.22 ppm). Steep decline of total N at V8 can be attributed to the increased rate of N uptake by the plant in both fields. NO3^- concentrations were found to follow the same trend as with total N. Concentration of NH4⁺ in both field peaked at V2 and decreased at V4 for all rates of pre-plant N. The study clearly showed that inorganic N from pre-plant fertilization can be dynamic throughout the growing stage of the corn plant. It can also be described by a polynomial model across rates of pre-plant N with average R² of 0.911 and 0.835 for no- till and tilled field respectively.