Nitrogen Fertility Management and Soil Nitrogen Dynamics in Alternative Rice Establishment Systems.

Alternative rice establishment systems which utilize minimum tillage and stale seedbed practices have been developed in California to help manage herbicide-resistant weed populations.  Although alternative and conventional establishment systems have similar yield potentials, changes in preseason water management, tillage, and nitrogen (N) application practices may impact N cycling in alternative systems and improved N management guidelines are needed.  The objectives of this study were to (i) determine the optimum rate, timing, and source of N fertilizer applications to improve grain yields and N recovery efficiencies, and (ii) investigate soil N dynamics and preseason N losses with respect to N management in alternative establishment systems. 

Nitrogen fertility trials were conducted at the California Rice Experiment Station (RES) in Biggs, CA and at four on-farm locations in the Sacramento Valley over a two year period (2008-2009).  N fertilizer sources included urea and ammonium sulfate, either applied prior to the permanent flood or split between pre-flood and mid-tillering.  Nine N rates ranging from 0-224 kg N/ha were evaluated at each site.  Soil N dynamics were assessed during field preparation activities and early season growth at the 0-15 cm depth.  Yield, N uptake, and N recovery efficiencies were determined at harvest.     

Results from the RES suggest that minimum tillage, stale seedbed systems have greater N fertility requirements than conventional systems by up to 50 kg N/ha.  Pre-flood N applications produced yields and N recovery efficiencies similar to split N applications, suggesting that a single N application prior to the permanent flood is sufficient.  Urea is a more reliable N source than ammonium sulfate to maintain yields, although results varied between sites and systems.  Soil N availability and N uptake was significantly greater in conventional systems, which may help explain the increased N fertility requirements of alternative rice establishment systems.