Fertilizer N uptake and efficiency of preplant surface N and injected aqua-N
California rice growers typically apply two forms of preplant N fertilizer: aqua-N which is injected at about 5 to 10 cm below the soil surface and a surface applied ammonium fertilizer. The rational behind this is that the surface applied fertilizer is available for the plant early on, until the root system develops sufficiently to access the aqua-N. These recommendations were developed when rice straw was routinely burned following harvest. Currently, as a response to increased environmental legislation, most rice straw is incorporated. Research has shown that straw incorporation can reduce the fertilizer requirement by up to 25 kg N ha-1. Given this increase in available N, the question arises as to the need for surface applied N. On-farm experiments have been set up around the Sacramento Valley with the objective of: (1) identifying the relative contribution of soil, surface-N and aqua-N to crop N uptake, (2) identifying when rice begins to take up the deep aqua-N, (3) determining the efficiency of surface and aqua-N (efficiency is measured using the mass balance method as well as 15N), and (4) developing improved N recommendations to rice growers.