Urea persistence in floodwater and soil used for flooded rice production

Urea 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 96h. 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 120h 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 63mg/kg after 24h of flooding and were <1mg/kg after 96h of flooding. The 5-day delay resulted in 1mg 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 (120h) in rice floodwater.