1. Oklahoma State University Department of Plant and Soil Sciences Oklahoma State University Department of Plant and Soil Sciences Nitrogen Loss from Urea Fertilizer Applications on Hot, Dry Soils R.W. Mullen, G.V. Johnson, K.W.Freeman,K.J. Wynn, W.E. Thomason, and W.R. Raun R.W. Mullen, G.V. Johnson, K.W.Freeman,K.J. Wynn, W.E. Thomason, and W.R. Raun

2.  Urea is the most widely used solid fertilizer N source in the world; 23,610,991 Mt used for cereal production in 1998 (www.FAO.org).  Volatilization losses from surface applications of urea-based fertilizers are thought to occur in sizeable quantities (Ernst and Massey, 1960; Fenn and Hossner, 1985).  Urea is the most widely used solid fertilizer N source in the world; 23,610,991 Mt used for cereal production in 1998 (www.FAO.org).  Volatilization losses from surface applications of urea-based fertilizers are thought to occur in sizeable quantities (Ernst and Massey, 1960; Fenn and Hossner, 1985). Introduction

3.  Urea fertilizers must be hydrolyzed by the enzyme urease before urea-N becomes plant available.  Increases in urease activity can result in more N loss due to volatilization.  Urease activity is moisture and temperature dependent.  Urea fertilizers must be hydrolyzed by the enzyme urease before urea-N becomes plant available.  Increases in urease activity can result in more N loss due to volatilization.  Urease activity is moisture and temperature dependent. Introduction

4.  Urea hydrolysis is maximized when moisture levels are optimum for plant growth (Havlin et al., 1999).  Volk (1966) reported that 14 d after application of prilled urea to air-dry soil 80% had not been hydrolyzed.  McInnes et al. (1986) found hydrolysis was minimal following urea application to dry soil, but following wetting, urea hydrolysis increased.  Urea hydrolysis is maximized when moisture levels are optimum for plant growth (Havlin et al., 1999).  Volk (1966) reported that 14 d after application of prilled urea to air-dry soil 80% had not been hydrolyzed.  McInnes et al. (1986) found hydrolysis was minimal following urea application to dry soil, but following wetting, urea hydrolysis increased.

5. Introduction  Urease activity has been observed from 2ºC to 37ºC (Havlin et al., 1999).  Urease activity is positively correlated with temperature (Clay et al., 1990)  Fisher and Parks (1958) found that hydrolysis was 50 and 85% complete at temperatures 10 and 20ºC, respectively, two weeks after urea application.  Urease activity has been observed from 2ºC to 37ºC (Havlin et al., 1999).  Urease activity is positively correlated with temperature (Clay et al., 1990)  Fisher and Parks (1958) found that hydrolysis was 50 and 85% complete at temperatures 10 and 20ºC, respectively, two weeks after urea application.

6. Introduction  Mechanical incorporation of urea fertilizers can effectively decrease N-loss due to volatilization.  Increased soil surface area to absorb released NH 3 (Havlin et al., 1999).  Palma et al. (1998) reported N-loss decreased from 8.6% to 5.4% when urea was incorporated.  Mechanical incorporation of urea fertilizers can effectively decrease N-loss due to volatilization.  Increased soil surface area to absorb released NH 3 (Havlin et al., 1999).  Palma et al. (1998) reported N-loss decreased from 8.6% to 5.4% when urea was incorporated.

7.  Rainfall or irrigation can result in subsurface incorporation thus minimizing volatilization losses.  Significant precipitation (>.25 cm) 3 to 6 d after application can efficiently reduce volatilization losses (Havlin et al., 1999).  Rainfall or irrigation can result in subsurface incorporation thus minimizing volatilization losses.  Significant precipitation (>.25 cm) 3 to 6 d after application can efficiently reduce volatilization losses (Havlin et al., 1999). Introduction

8. Objectives  To determine decreases in winter wheat yield which could be attributed directly to N-loss via volatilization.  To determine the effect of surface application or incorporation on winter wheat N uptake.  To determine decreases in winter wheat yield which could be attributed directly to N-loss via volatilization.  To determine the effect of surface application or incorporation on winter wheat N uptake.

9. Materials and Methods  Two experimental sites:  Stillwater and Tipton, OK  Experimental design: RCBD  Eight treatments with three replications  Three N sources both surface applied and incorporated and one injected source  Urea (46-0-0), Urea ammonium nitrate (AN; 28- 0-0), and ammonium nitrate (AN; 34-0-0)  AA (82-0-0)  Two experimental sites:  Stillwater and Tipton, OK  Experimental design: RCBD  Eight treatments with three replications  Three N sources both surface applied and incorporated and one injected source  Urea (46-0-0), Urea ammonium nitrate (AN; 28- 0-0), and ammonium nitrate (AN; 34-0-0)  AA (82-0-0)

10. Materials and Methods  All N was applied at a 112 kg N ha -1 rate.  Urea and AN were applied using conventional dry- fertilizer spreader.  UAN was broadcast applied using spray applicator.  AA was injected 15 cm below soil surface using custom built applicator.  Incorporated treatments received a 15-cm tillage pass.  All N was applied at a 112 kg N ha -1 rate.  Urea and AN were applied using conventional dry- fertilizer spreader.  UAN was broadcast applied using spray applicator.  AA was injected 15 cm below soil surface using custom built applicator.  Incorporated treatments received a 15-cm tillage pass.

11.  Winter wheat variety ‘Jagger’ planted in 21- cm rows at a rate of 88 kg ha -1.  Harvested area: 2.0 x 6.1 m.  Grain sample taken from each plot for N analysis using Carlo-Erba dry combustion analyzer (Schepers et al., 1989)  Winter wheat variety ‘Jagger’ planted in 21- cm rows at a rate of 88 kg ha -1.  Harvested area: 2.0 x 6.1 m.  Grain sample taken from each plot for N analysis using Carlo-Erba dry combustion analyzer (Schepers et al., 1989) Materials and Methods

12. Results and Discussion  1998-1999 Yield response at Stillwater SED=266SED=266 N Source Yield kg ha -1

13. Results and Discussion  1998-1999 N uptake at Stillwater SED=8.1SED=8.1 N Source N Uptake kg ha -1

14. Results and Discussion  1998-1999 Yield response at Tipton SED=320SED=320 N Source Yield kg ha -1

15.  1998-1999 N uptake at Tipton SED=8.2SED=8.2 N Source N Uptake kg ha -1 Results and Discussion

16.  1999-2000 Yield response at Stillwater Results and Discussion SED=238SED=238 N Source Yield kg ha -1

17.  1999-2000 N uptake at Stillwater Results and Discussion SED=9.1SED=9.1 N Source N Uptake kg ha -1

18.  1999-2000 Yield response at Tipton Results and Discussion SED=296SED=296 N Source Yield kg ha -1

19.  1999-2000 N uptake at Tipton SED=6.6SED=6.6 N Source N Uptake kg ha -1 Results and Discussion

20. Conclusions  Incorporation of urea fertilizers tended to increase yields when compared to surface applications.  Losses due to volatilization may have been depressed due to rainfall within a week of fertilizer application in both years.  Incorporation of urea fertilizers tended to increase yields when compared to surface applications.  Losses due to volatilization may have been depressed due to rainfall within a week of fertilizer application in both years.