Assessing the necessity of surface-applied preplant nitrogen fertilizer in rice systems
California rice (Oryza sativa L.) growers typically use two forms of preplant N fertilizer: aqua NH3 applied 7 to 10 cm below the soil surface (subsurface N) and surface-applied N. The rational for applying about 25% of the total N rate to the surface is to provide a readily available N source for young rice seedlings; however no research has been done to verify this. On-farm field studies were conducted over a 3-yr period (12 site-years) with the specific objectives of determining when rice begins to use subsurface N and to compare the efficiency of surface and subsurface applied N. Rice seedlings began accumulating subsurface N within 2 wk after sowing at some sites. When a portion of the N rate was applied to the surface, early season plant biomass and N uptake was higher than when all of the fertilizer-N was applied subsurface. In contrast, grain yields were higher when all of the N fertilizer was applied subsurface. Averaged across all sites, the fertilizer-N recovery efficiency of surface-applied N was 38% compared to 53% when only subsurface N was applied. As aqua NH3 is less expensive than NH4+ based fertilizers and the application of surface N requires an additional field operation, there is no justification to recommend the practice of applying surface N fertilizer in these rice systems. Instead, all of the preplant N should be applied subsurface as aqua NH3.