Urea persistence in floodwater and soil used for flooded rice production

Abstract

AbstractUrea is a common fertilizer in delayed‐flood rice production in the United States, and its use worldwide has increased dramatically in recent decades. This study aimed to directly quantify urea‐N persistence in floodwater and soil used for rice production. We conducted a set of three laboratory experiments to investigate urea‐N presence in the floodwater and soil. Untreated urea was applied to dry or wet soil and flooded immediately or urea treated with the urease inhibitor N‐(n‐butyl)‐thiophosphoric triamide (NBPT), or untreated urea was applied to dry soil and flooded after a 5‐day delay. Urea‐N was analysed colorimetrically (using the microplate‐adapted, diacetyl monoxime method) in the floodwater, and at 2‐cm intervals in soil after 10‐cm long, silt‐loam soil columns were flooded for 12, 24, 48 and 96 h. The only management practice that led to insignificant urea‐N concentrations in floodwaters was the application of urea followed by a 5‐day delay before flooding. Urea‐N can persist in floodwaters for an estimated 98 and 120 h after immediately flooding dry‐soil‐applied or wet‐soil‐applied untreated urea, respectively. Urea‐N concentrations in floodwaters were up to 33 times less when dry‐soil‐applied than wet‐soil‐applied. Average NBPT‐treated urea‐N concentrations in soil ranged up to 63 mg/kg after 24 h of flooding and were <1 mg/kg after 96 h of flooding. The 5‐day delay resulted in ≤1 mg urea‐N/kg soil when untreated urea was applied. Generally, the threat of N entering adjacent waterways in the form of urea is likely to be limited because of its short‐term persistence (≤120 h) in rice floodwater.