Agroecosystems Lab

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Determining starter fertilizer N uptake and its influence on total N uptake under differing N and straw management practices

Kaden Koffler, Chris Hartley, Bruce Linquist, Fernando Martins, Johan Six, Cass Mutters, Chris Greer, William Horwath, and Chris van Kessel
Department of Plant Sciences, University of California, Davis

Photo of isotopic Nitrogen application Starter fertilizers are used by 95% of California rice growers (2003 Fertility Management Survey) and are an important tool in rice fertility management, providing the seedlings with a readily available source of nutrients to help insure rapid early growth, improving competitiveness with weeds and increasing pest and disease resistance. Strong early uptake of nitrogen (N) through the use of starter fertilizer may increase the overall N uptake capacity of rice plants throughout season. That is, starter N uptake may give rice plants the necessary jumpstart to increase total N uptake from aqua-ammonia-N and native soil N sources. Nonetheless, there are currently no standard recommendations for the selection, timing or rate of application of starter fertilizer materials. Consequently, rice growers have expressed the need for better information on starter N management, particularly under varying straw and water management practices.

Achieving optimal starter N availability and uptake efficiency in rice fields requires an understanding of early season N uptake and N loss under differing straw and water management practices. Starter N is either surface applied or lightly incorporated, making it readily available to newly emerged rice plants. However, because starter N is applied near the soil surface, it is susceptible to loss resulting from changes in soil-water status.

Photo of soil sampling
To begin addressing the uncertainties around starter N management, we set up on-farm starter N trials across the Sacramento Valley in 2005. Five sites (10 fields) were chosen in the Richvale and Princeton rice growing areas which had side by side straw burned and incorporated fields or checks, allowing us to compare the interaction between starter N and straw management. An additional trial was conducted near Arbuckle where rice is rotated with other crops (in this case tomatoes were planted the previous year). This site in Arbuckle allowed us to evaluate the interaction of starter N with crop rotations, however only one site was established for this comparison, so results need to be evaluated with caution.

In microplots, starter fertilizer 15N was used to directly measure starter fertilizer N uptake and its effect on total N uptake, biomass production, and yield. The effects of different starter N rates were examined under a range of aqua-ammonia-N rates with a straw burning, straw incorporating, or crop rotation management system. Whole plants were sampled from the starter N treatments at mid-tillering, panicle initiation, and harvest to measure yield, and starter and total N accumulation in biomass and grain. At each plant sampling date, soil cores were also taken to evaluate soil N dynamics in relation to starter N application rate and uptake, and calculate total starter N recovery.

By conducting this research on starter N use, we hope to help fine tune starter fertilizer N recommendations under differing straw and fertility management systems, and across different regions of the Sacramento Valley.