Date of Graduation

5-2015

Document Type

Thesis

Degree Name

Master of Science in Crop, Soil & Environmental Sciences (MS)

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor

Nathan A. Slaton

Committee Member

Trenton L. Roberts

Second Committee Member

Richard J. Norman

Third Committee Member

Edward E. Gbur

Fourth Committee Member

Richard E. Mason

Keywords

Biological sciences; Nitrogen; Simulated rainfall

Abstract

The effect of rainfall between urea application and flood establishment on N loss and grain yield of rice (Oryza sativa L.) has not been studied. The first research objective was to compare the effects of simulated rainfall amounts and N-(n-butyl) thiophosphoric triamide (NBPT) urease inhibitor rate on NH3 volatilization and rice growth. Three field experiments were conducted and NH3 volatilization was measured in two experiments for 11 days after urea application (DAU) in semi-open chambers. Urea or NBPT-treated urea (NBPT-Urea) was subjected to six simulated rainfall amounts (0-25 mm) applied 5 to 15 h after urea application and flooded 7 to 12 DAU. Cumulative NH3 loss from Urea accounted for 8.6% of the applied N with no simulated rainfall and decreased quadratically to 0.6% with 24 mm of simulated rainfall. Cumulative NH3 loss from NBPT-Urea also decreased quadratically as simulated rainfall amount increased but loss was 0.2-2.0% of the applied-N. Depending on the site, yields of rice fertilized with Urea decreased linearly or nonlinearly as simulated rainfall increased with the greatest yield produced by rice receiving no simulated rainfall. The yields of rice fertilized with NBPT-Urea were not affected by simulated rainfall amount in two trials. In the third trial, the yields of rice fertilized with NBPT-Urea decreased nonlinearly as simulated rainfall amount increased but were 8.9 to 18.1% greater than the yields of Urea-fertilized rice. Rainfall following preflood urea application appears to reduce NH3 loss but increase N loss via denitrification. Total-N loss was reduced when urea was treated with NBPT. Our second research objective was to compare the effects of simulated rainfall time and selected urea-N amendments on rice N uptake and grain yield. Two field experiments were conducted to evaluate rice growth as affected by two NBPT rates (0 and 0.89 g NBPT kg-1 urea), two nitrapyrin (NP) rates (0 and 572 g NP ha-1), and three simulated rainfall timings [no simulated rainfall (NOSR), simulated rainfall before N (SRBN), and simulated rainfall after N (SRAN)]. Yield was unaffected by simulated rainfall timing when rice was fertilized with NBPT-treated urea (7904-8264 kg ha-1). When rice was fertilized with untreated urea (no NBPT), grain yields were greater with NOSR than with SRAN or SRBN. Within each simulated rainfall timing, rice yields were 6.9 to 21.3% greater when NBPT-treated urea was applied. Nitrapyrin rate had no effect on grain yield in 2013, but, compared to untreated urea (no NP), NP-treated urea decreased yield by 5.6% in 2014. Application of untreated urea to moist soil or dry soil followed by rainfall are field environments that result in more substantial N loss than urea applied to a dry soil that remains dry until the rice field is flooded. Use of NBPT-treated urea minimized N loss and maximized grain yield in each simulated rainfall scenario examined.

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