Nitrogen (N), as a critical element for microbial metabolisms, recycles rapidly in the euphotic ocean. Oxidation by nitrifiers is a competing pathway for phytoplankton assimilation of regenerated N (NH4+ and urea). Sharing the overlapping substrates may result in the competitive exclusion, thus, niche separation for the two assemblages. Both pathways are sensitive to light, but whether light intensity will intensify or alleviate such resource competition in the euphotic zone remains poorly explored in the field at community level. By using 15N labelling techniques, paired kinetic responses of uptake and oxidation were conducted in one single bottle under manipulated light intensities for both NH4+ and urea. In contrast, light effects were opposite for oxidation kinetics (Rm and αO). As irradiance increased, the rapid increase in αU and concomitant decrease in αO imply a distinctive competition advantage of photosynthetic organisms over oxidizers under substrate limited environments. The ratio of αU/αO for NH4+ ranged from 0.8 to 3089 (5.8−46788 for urea) showing a distinct increasing pattern as ambient light increases, demonstrating that phytoplankton overwhelm nitrifiers throughout the oligotrophic euphotic zone driving down concentrations and maintaining short turnover times of the two regenerated N. Moreover, phytoplankton relied equally on NH4+ and urea, yet, nitrifiers preferred NH4+ to urea. In the nitrate-depleted euphotic ocean, light acts as a crucial driver for utilization pathways of regenerated N and vertical niche separation. This field study in the open ocean showing that light can shape the specific affinity of assimilators and oxidizers toward NH4+ and urea. Moreover, an enhanced photosensitivity under very low substrate levels was observed, providing an insight into the joint control of light and substrate on the distribution of N recycling pathways. This information may contribute to N-based ocean biogeochemical models.