Assessing fertilizer nitrogen sources and application timing for water‐seeded rice systems

Abstract

Abstract 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 alternative N management practices by testing different N sources and application times. In 2020, we tested 13 treatments and in 2021, six more treatments were added. The following N sources were evaluated: 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 four‐way split. 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 used for comparison. All fertilizer N treatments increased grain yields relative to the zero‐N control, which averaged 5093 kg ha−1. Applying aqua‐N or urea preplant to a dry soil resulted in the highest yields and ANUE. The EENF fertilizers performed similarly to or worse than urea applied alone. If the field was already flooded, splitting urea applications was the best option. Only one split N treatment was evaluated in this study; this warrants further research on fine‐tuning the best N splits for these systems.