Abstract
The biological nitrogen fixation (BNF) process in grasses is caused by diazotrophic bacteria, particularly Azospirillum brasilense. However, studies are lacking on BNF efficiency to define how much mineral nitrogen (N) can be applied to achieve more sustainable high yields. Furthermore, there should be an analysis of whether urea with the urease enzyme inhibitor NBPT is less harmful, benefiting BNF in grasses. The objective of this study was to evaluate the effect of N sources and N rates associated with inoculation with Azospirillum brasilense regarding foliar diagnosis and leaf chlorophyll index (LCI), agronomic efficiency (AE), and corn grain yield in the Cerrado (Brazilian tropical savanna) region. The experiment was conducted in a no-tillage system in a Latossolo Vermelho Distroferrico (Oxisol). A randomized block experimental design was used with four replications in a 2 × 5 × 2 factorial arrangement as follows: two N sources - urea and Super N, urea with urease enzyme inhibitor NBPT [N - (n-butyl thiophosphoric triamide)]; five N rates (0, 50, 100, 150, and 200 kg ha-1) applied in topdressing; and two seed inoculation treatments, one with and one without A. brasilense. N rate positively influenced the LCI and concentrations of N, S, and Mn in leaves, and may increase the concentrations of P, Cu, and Fe; however, higher N rates can reduce AE. The N sources had similar effects, and therefore urea is recommended for N fertilization. Inoculation with A. brasilense decreased leaf concentration of Fe and increased LCI, leaf concentration of P, AE, and corn grain yield; the use of this diazotrophic bacterium is therefore viable even when high rates of N are applied.
Highlights
Brazil is the world’s third largest corn producer, despite the fact that, in general, Brazilian soil does not contain enough N for this crop to thrive
The objective was to evaluate the effect of N sources and N rates associated with inoculation with Azospirillum brasilense regarding foliar diagnosis and leaf chlorophyll index (LCI), agronomic efficiency, and corn grain yield in the Brazilian Cerrado region
The maximum concentration was obtained from 155 kg ha-1 of N in 2013/14 and 185 kg ha-1 of N in 2014/15 (Figures 2a and 2b)
Summary
Brazil is the world’s third largest corn producer, despite the fact that, in general, Brazilian soil does not contain enough N for this crop to thrive. Nitrogen fertilization is one of the highest costs of the production process of non-leguminous crops (Nunes et al, 2015). Corn, and rice crops utilize approximately 60 % of the N fertilizer produced in the world (Espíndula et al, 2014). The use of N fertilizer must be carefully controlled to ensure good yield and manage N in the soil; N fertilizer increases production costs for farmers (Teixeira Filho et al, 2014). Corn yield may be reduced because of NH3-N volatilization. 10 kg ha-1 of grain is lost for each 1 % of N that is volatilized (Lara Cabezas et al, 2000). Urea is unlikely to be replaced by other sources of N because it has the lowest cost per kilogram of N
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