Monday, November 5, 2007

Urea Persistence in a Ponded Silt-Loam Soil Used for Rice Production.

Mary Savin, Aaron Daigh, Peter Tomlinson, and Richard Norman. University of Arkansas, University of Arkansas, 115 Plant Science Building, Fayetteville, AR 72701

Urea is a common fertilizer in delayed-flood rice production in eastern Arkansas. Although the movement of urea into soil upon initiation of a flood is critical to retaining N in the system, urea persistence and mobility are rarely measured because urea is assumed to undergo rapid hydrolysis to ammonia. However, low levels of urea have been implicated as an N source for algae in aquatic ecosystems. To address the potential of urea moving out of terrestrial systems and thereby potentially contributing to harmful algal blooms, urea was analyzed directly by a colorimetric method in surface water and 10-cm length soil cores that were ponded for 12, 24, 48, and 96 hrs. Dry and muddy soil was ponded immediately after urea application, and dry soil was ponded 5 day after urea or Agrotain, a slow-release source of urea, application to the soil surface. Although concentrations decreased with increased ponding time, urea was measured in the floodwater 96 hours after ponding muddy soil, whereas much lower concentrations were measured 24 hours after ponding dry soil. Little to no urea (≤ 1 μg N/g) was measured in floodwater and soil if there was a 5-day delay between fertilizer application and ponding. Urea was measured in soil when applied as Agrotain. Concentrations were variable and averaged 3.4 to 63 μg N/g at different 2-cm depth intervals after 24 hrs, but were down to 5 μg N/g after 96 hrs of ponding. Urea remaining in floodwater may be a concern if applied to wet soil.