Fertilizer N Uptake and Efficiency of Surface and Deep Applied N in California Rice Systems.
Bruce Linquist, Chris Van Kessel, Kaden Koffler, and Chris Hartley. Dept of Plant Sciences, Univ of California, One Shield Ave, Davis, CA 95616
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 burned. 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 N, the question arises as to the need for surface applied N. A replicated study was conducted on five grower fields with the objective of: (1) identifying when the rice crop begins to take up the aqua-N and (2) determining the efficiency of surface and aqua-N. Efficiency was measured using the mass balance method as well as 15N. Our results indicate that the uptake of the deep aqua-N begins before three weeks. Applying surface N resulted in greater N uptake and biomass early in the season, and at two of the sites, also resulted in higher grain yields. Using the mass balance method, the N uptake efficiency (NUE) of the aqua-N was similar across sites and averaged 56%. The NUE of surface applied N varied widely between sites and ranged from 0 to 99%. Using the 15N method, the efficiency of starter N ranged from 8 to 26% and was poorly correlated with the mass balance method. Fertilizer N recovery using 15N ranged from 29-71%. The differences in these approaches will be discussed as well as N uptake over time and the relative contributions of soil and fertilizer N to total N uptake.