An Approach to Estimate 15N Recovery from a Blend of Fertilizer Nitrogen Sources.

Blending coated urea with regular urea is one recommended alternative to improve fertilizer nitrogen (N) use efficiency (FNUE). Two widespread techniques to asses FNUE are the difference method (which requires the use of a 0-N plot to be compared with the N-fertilized plots), and the isotopic technique using ¹⁵N. However, when two fertilizer N sources are blended, the reliability of each individual technique can be questionable. The objective of this study was to propose an approach to estimate the fertilizer N recovery by corn (Zea mays) using a blend of two N fertilizer sources. A field study was performed during the 2015-2016 cropping season in the state of Sao Paulo, Brazil. Nitrogen rates (0, 60, 120, 280, 240 and 300 kg ha^-1) were applied and incorporated before corn planting, as a blend of polymer-sulphur coated urea (PSCU) and non-coated urea (NCU) in a 70:30 ratio, respectively. Only the NCU was ¹⁵N-labeled (1.6% atoms ¹⁵N). Using the ¹⁵N recovery values and the non-fertilized plots N uptake values, the difference method was used to calculate the N recovery from the blend of PSCU and NCU. The N derived from NCU fitted a linear regression, while N derived from the blend PSCU+NCU fitted in a quadratic regression curve. In average, 10% of the N uptake by corn was derived from NCU and 33% was derived from PSCU, while 57% was derived from the soil. An average of 39 and 46% of the N applied as NCU and PSCU, respectively, were recovered by corn plants. The approach evaluated in this study could be used as a tool to estimate the fate of N fertilizer under various cropping systems utilizing slow release N fertilizers as an alternative to improve FNUE.