UC Davis

In California’s water-seeded rice systems, both high yields and nitrogen (N) use efficiency are achieved when the pre-plant N fertilizer (usually aqueous-ammonia; aqua-N) is injected into a dry seedbed before flooding and planting. However, there are situations where applying N fertilizer in this manner is not possible. The objective of this study was to evaluate N management practices by testing different N sources and application times. Over two growing seasons (three site-years) we evaluated different N sources including aqua-N, granular urea, ammonium sulfate, and three enhanced efficiency nitrogen fertilizers (EENFs). Application times for the EENFs, urea, and ammonium sulfate were one day after flooding and 2 weeks after flooding. Urea and ammonium sulfate were also applied in a 4-way split (ratio of 20:30:30:20 applied every two weeks). In 2021, additional treatments were added with aqua-N, ammonium sulfate, and urea applied before flooding; and urea applications at 3, 4, and 5 weeks after flooding. Grain yield and agronomic nitrogen use efficiency (ANUE) were measured and used to make comparisons. All fertilizer N treatments increased grain yields and ANUE relative to the zero-N control. The EENF treatments performed similarly to or worse than urea applied alone. Splitting urea applications was the best option for applying N fertilizer after the field was flooded. Only one split N treatment was evaluated in this study; this warrants further research on fine-tuning the best N splits for these systems.