Nitrogen Sources and Timing of Application to Improve N-Use Efficiency of Upland Rice in Brazil.

Nitrogen (N) fertilization is one of the most important production factors of upland rice in Brazil. Urea is the most used source of synthetic N in Brazil, and NH₃ volatilization is the main mechanism that contributes to the low fertilizer N recovery by crops, mainly when the urea is applied to the mulch in a no-till system. For the efficient management of N in the cropping systems, adequate rate, appropriate source and timing of application during crop growth cycle play an important role. Using urease inhibitor and polymers covering urea granules are technological options to reduce N losses and to improve N use efficiency. A field study was carried out in a clayey Rhodic Ferralsol in 2010/11 growing season, in Brazilian savanna, aiming to evaluate the influence of N fertilizer sources and timing of application on upland rice grain yield, apparent fertilizer N recovery (Nrec), and agronomic efficiency. The experiment was carried out in a complete randomized block design with four replications and ten treatments arranged in a 3x3+1 factorial scheme: three N sources (common urea, urea with NBPT urease inhibitor, and polymer coated urea), three timing of application (100% at sowing, 50% at sowing + 50% at tillering, and 100% at tillering), and one control. We used the rate of 80 kg N ha^-1. The results showed that N fertilization significantly increased rice yields in 39%. The averages of fertilized treatments for rice yield, Nrec, and agronomic efficiency were 3,419 kg ha^-1, 52%, and 12, respectively. There was no significant difference among N sources for these measured variables. Results were similar when N fertilizers were applied 100% at sowing or divided 50% at sowing and 50% at tillering; however, the total rate applied at tillering decreased rice yield, Nrec, and agronomic efficiency for the three fertilizer sources evaluated.