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NITROGEN MANAGEMENT

Assessing the Necessity of Surface-Applied Preplant Nitrogen Fertilizer in Rice Systems

^a Dep. of Plant Sciences, Univ. of California, Davis, CA 95616
^b Univ. of California Coop. Ext., 2279B Del Oro Ave., Oroville, CA 95965
^c Univ. of California Coop. Ext. 142A Garden Hwy., Yuba City, CA 95991
^d Natural Resources Conservation Services, Modesto, CA 95358
^e Dep. of Soil Science, Univ. of Wisconsin, Madison, WI 53706

^* Corresponding author (balinquist{at}ucdavis.edu).

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.

Abbreviations: DAS, days after sowing • NRE, nitrogen recovery efficiency

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Received for publication December 17, 2008.