Effect of Soil Moisture on the Efficiency of Pre-Flood N Applications in Rice.

Soil moisture at the time of fertilizer nitrogen (N) application can effect in N use efficiency (NUE) of preflood applications in rice production. Losses of N can occur from ammonia volatilization and nitrification/denitrification. Field studies were conducted to: 1) evaluate NUE and grain yield response when preflood fertilizer N is applied on a dry, moist and flooded soil; and 2) quantify N volatilization losses of preflood fertilizer N when applications are made on a dry, moist and flooded soil.

Two field yield and volatilization trials were conducted in 2014 and 2015. Trials consisted of three soil moisture conditions at fertilization: 1) dry, 2) moist, and 3) flooded. N treatments included urea, Agrotain Ultra-treated urea (AU-U; 26.7% NBPT), manufactured Agrotain urea (MAU; NBPT concentration unknown), and SuperU (NBPT + DCD incorporated urea). Volatilization was measured in the field over a 15-d period of time after fertilization.

Cumulative volatilization losses over the 15-day period were 23.6, 25.2, and 5.9% when applied on a dry, moist, or flooded soil, respectively, in 2015 and were 23.0, 19.8, and 4.5% when applied on a dry, moist, or flooded soil, respectively, in 2014. Higher volatile N losses were observed from urea when applied on a dry or moist soil than applications into a standing flood, regardless of fertilizer source. When the fertilizer source was AU-U, MAU or SuperU, applications on a dry or moist soil were significantly reduced as compared to untreated urea; however, applications into a standing flood resulted in volatilization losses similar to untreated urea.

Increased nitrification/denitrification and volatilization losses will occur when fertilizer applications are made onto moist soils or into flooded rice fields, as compared to applications on dry ground, when rice is at the 4- to 5-leaf stage of development. Preflood fertilizer applications should always be applied on a dry soil.