Effects of nitrapyrin, nitrogen rates and nitrogen timing on nitrous oxide emissions and nitrogen use efficiency in maize fertilized with urea ammonium nitrate

Nitrogen losses as N2O-N to the atmosphere from soils in maize cropping systems can be minimized by adopting optimum N fertilizer management practices regarding rates, timing, and nitrification inhibitors. Ultimately, the goals of such strategies are to increase N recovery by the crop, and to derive maximum grain production per unit of N lost to the environment, via integrated genetic and management technology approaches for specific environments. In this research, field experiments were established to study integrated N technologies following the recent commercial release of a reformulated nitrification inhibitor (i.e. nitrapyrin) that could be co-applied with liquid N fertilizers such as urea ammonium nitrate (UAN). Our specific objectives were to determine: i) the impact of N management practices (timing, rate and nitrification inhibitor) and environmental variables on N2O fluxes; ii) superior treatment combination(s) to both reduce N2O emissions and increase maize grain yields; and iii) the impacts of integrated N management practices on maize whole-plant N uptake, and nitrogen use efficiency (NUE) and its components. The experiments near West Lafayette (2010 & 2011) and Wanatah (2011), IN evaluated three N rates (0, 90, 180 kg N ha-1 applied as UAN), nitrapyrin (presence or absence), and timing of N application (pre-emergence or side-dress at V6 stage). Greenhouse gas flux measurements on 7 to 10 day intervals at West Lafayette, IN, confirmed that N2O-N emissions increased with N rate, but were reduced by nitrapyrin presence. Cumulative N2O-N emissions (Q10 corrected), averaged across years and other-treatment factors, were increased by N rate ( 0.8, 1.57, and 2.96 kg N2O-N ha-1 for the 0, 90, and 180 kg N ha-1 rates, respectively), and increased by ∼37 % with side-dress N application (from 1.49 after pre-emergence UAN to 2.06 kg N2O-N ha-1 after side-dress UAN). The use of nitrapyrin reduced cumulative N2O-N emissions by ∼ 27 % (from 2.05 without to 1.5 kg N2O-N ha-1 with nitrapyrin). Grain yields from the various treatment means ranged from 6.3 to 12.1 Mg ha-1, primarily in a positive response to N rates, but were not significantly affected by nitrapyrin presence at any N rate or UAN application timing. Yield-scaled N2O-N emissions (g of N 2O-N released/Mg of harvested grain) behaved similarly to cumulative N2O-N emissions: they were increased by increasing N rate or side-dress application of the treatments, and were reduced by the presence of nitrapyrin. Total plant N uptake at physiological maturity (NU) was increased by N rates (112, 158, and 196 kg N ha-1 for rates of 0, 90, 180 kg N ha -1), and by delaying the timing of application from pre-emergence to V6 (from 147 to 163 kg N ha-1). The use of nitrapyrin significantly enhanced NUE (from 33.8 to 39.5 kg grain/kg N applied) while increasing N rates from 90 to 180 kg N ha-1 had the opposite effect (i.e. a reduction in NUE, from 40.6 to 32.7 kg grain/kg N applied). Nitrogen recovery efficiency (NRE) was improved from 0.44 to 0.55 kg N uptake/kg N applied by the inclusion of nitrapyrin (when averaged for years, N rates, and UAN timings). The NRE was also enhanced by delaying the UAN application to V6 stage (from 0.41 to 0.59 kg N uptake/kg N applied). The joint outcomes of N2O-N emissions and maize NUE in these production environments were, therefore, influenced by the main treatment factors of N rates and nitrification inhibitor presence. Additionally, cumulative N2O-N emissions were affected by the timing of UAN application. Neither response outcome was ever significantly affected by interactions between/among the N treatment factors of rate, timing and nitrapyrin presence