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Using NBPT to Increase Efficiency and Safety of Urea Fertilizer
Introduction
Urea is the major granular fertilizer used, because it has a high N analysis, it is easy and safe to handle and store and is relatively low cost. Urea is readily soluble in water and can be used in the formulation of urea ammonium nitrate (UAN) and compound fertilizers. But, urea has two major drawbacks: 1) It is subject to volatilization losses if surface applied; 2) It can produce severe seedling damage if seed-placed levels are too high.
Banding or broadcast-incorporation application of urea will reduce seedling toxicity and volatilization loss, but this requires extra trips across the field, with extra costs and possible loss of moisture and seed-bed quality. With reduced tillage systems, the soil disturbance associated with incorporation or fertilizer banding may not be desirable, while in perennial forages or in forestry, these application methods may not be possible. Urease inhibitors may be an economical option to increase the efficiency of surface applications and reduce the toxicity of seed-placed urea.
For any urease inhibitor to prove effective in a crop production system, several conditions must be met. Firstly, the urease inhibitor must inhibit urease activity over a range of soil and environmental conditions. Secondly, potential seedling damage or volatile losses of urea-containing fertilizers must be sufficiently large to reduce crop yield or protein content. Thirdly, the inhibitor must not damage the crop, consumers of the crop or environmental health. Fourthly, the benefits derived from the inhibitor must be sufficient, over the long-term, to justify the cost of the product. One of the more promising urease inhibitors is n-(n-butyl) thiophosphoric triamide (NBPT).
Urea Reactions in the Soil
Urea itself is not directly damaging to seedlings or subject to volatilization loss. When urea is applied to the soil, it rapidly hydrolyses to ammonia (NH3) in a reaction catalysed by the enzyme urease. The more rapidly the urea hydrolyses, the higher the concentration of NH3 present The NH3 can be lost to the atmosphere when it remains near the soil surface. The NH3 will also partially convert to NH4+ and both NH3 and NH4+ can damage germinating seedlings. Seedling damage and volatilization loss both increase with increasing concentration of NH3 in the solution and vary with fertilization level, soil properties and environmental conditions.
Factors Affecting Volatile Losses of Urea
Ammonia volatilization will increase with increasing concentration of NH3 at the soil surface. Increasing rate of urea hydrolysis, increasing soil pH and carbonate content and increasing fertilizer rate increase NH3 concentration in the soil solution. Ammonia losses tend to be higher from courser than fine-textured soils, since fine-textures soils have a higher cation exchange capacity and can hold more NH4 out of solution. Losses may be higher when the urea is applied
to soil covered with organic residue as compared to bare soil, as hydrolysis of urea on the residue particles may increase NH3 release. Windy conditions will also increase moisture loss and volatilization.
Moisture effects will vary depending on the situation. Granules must dissolve before losses occur, so if granules are broadcast on the soil and the soil dries before granule dissolution, volatilization will be arrested. Once the granules have dissolved, volatilization generally increases with evaporation of water from the soil, since the moisture loss will increase concentration of NH3 at the soil surface. Therefore, losses will tend to be high if urea is applied to a moist soil and the soil dries. Significant rainfall or irrigation after fertilizer application will reduce volatilization loss, since the water will both dilute the NH3 and carry the uncharged urea into the soil, reducing NH3 concentration at the soil surface. Factors which increase evaporation will tend to increase volatilization losses, by moving NH3 to the soil surface and increasing the concentration of NH3 in the soil solution.
Use of NBPT With Surface-Applied Urea-Containing Fertilizers
Volatile losses of NH3 from urea-containing fertilizers will increase as the concentration of NH3 at the soil surface increases. Concentration of NH3 at the soil surface increases as urea hydrolysis increases, through the action of the urease enzyme. Inhibiting urease activity slows the conversion of urea to NH4+ and retains the N in the urea form for a longer period. This will reduce the concentration of NH4+ present in the soil solution, reducing the potential for volatilization losses and seedling damage. Slowing the hydrolysis of urea allows more time for the urea to diffuse away from the application site or for precipitation or irrigation to dilute urea and NH4+ concentration at the soil surface and increase its dispersion in the soil. Diffusion and leaching can move the fertilizer through surface residue and into the soil, where it will be less subject to volatilization and immobilization losses.
Reduction of Seedling Toxicity
NBPT may also increase the amount of urea-containing fertilizers that can safely be placed with the seed. Seed-placement allows seeding and fertilizer application in a single operation, reducing labour and operational costs. It reduces soil disturbance, as compared to incorporated or banded applications and reduces equipment costs and draft requirements as compared to side-banded applications. As a specialized form of banding, it has the benefits of reduced volatilization and improved fertilizer use efficiency of an in-soil band. However, seed-placed urea-containing fertilizers can lead to severe seedling damage when applied at rates required to optimize crop yield. Seedling damage from seed-placed fertilizers is related to the concentration of NH3/NH4+ in contact with the germinating seedling. Damage will increase with decreasing moisture levels during germination, so drying conditions after seeding will increase damage, while significant rainfall after seeding will decrease damage. Damage tends to be greater on coarse-textured soils or soils with low organic matter content because of their lower CEC and lower water-holding capacity. Increasing carbonate content and pH may increase damage, by increasing the proportion of NH3 present and the subsequent NH3 toxicity. Damage may be decreased by any action that decreases the concentration of urea in contact with the seed, such as increasing the width of the row over which the seed and fertilizer are spread, decreasing row spacing, or decreasing fertilizer rate.
As seedling damage is caused by NH3 and NH4+ rather than directly by the urea, slowing the hydrolysis of urea to reduce the concentration of NH4+ and NH3 in contact with the seedling could reduce toxicity. Slower hydrolysis also allows more time for the urea to diffuse away from the seed-row, decreasing NH3 and salt concentration in the seed-row.
NBPT Effects on Urease Activity and NH3 Volatilization
The effectiveness of urease inhibitors in reducing urease activity can be evaluated by measuring volatilization of NH3 from plots treated with urea-containing fertilizers, with and without the inhibitor. In studies conducted at Brandon Research Centre, NBPT reduced NH3 volatilization from mulched soils when applied with either urea or urea ammonium nitrate. Total volatile loss of N from urea over 7 days in two separate studies was reduced from 40 mg to 2 mg and from 88 mg to 12 mg by the addition of NBPT. NBPT delays urea hydrolysis, but does not eliminate it, so the effectiveness of NBPT decreases with time after application. Loss of ammonia from the NBPT treated urea began to occur 4 to 7 days after application. Use of NBPT delays the time of maximum rate of volatilization, allowing diffusion of the urea downward and increasing the chance of rain moving the urea into the soil, where it would be protected from volatilization.
NBPT Effects on Seedling Damage, Crop Yield and N Use Efficiency
While NBPT can inhibit urease activity, it will not increase crop yields and N recovery under all circumstances. Reducing volatilization losses from surface-applied urea-containing fertilizers will only increase yield if: a) Nitrogen fertility is limiting to crop yield when the NBPT is not applied and b) Volatilization losses from the applied fertilizer are sufficient to impact on crop yield. Similarly, reducing seedling damage will only increase yield if the damage is sufficient to impact on crop growth and competitiveness, or to delay maturity. Maximum benefits of NBPT use can therefore be expected where crop yield potential is high, soil N levels are low and soil and environmental conditions promote extensive volatilization losses or seedling damage.

Table 1: Effect of seed-placed urea, with and without a urease inhibitor (UI), on stand density (plants/metre) and grain yield (tonnes/ha) of barley on a fine sandy loam, 1994-96.

N Rate

Stand

Grain Yield

Straw Yield

(kg ha-1)

NI

UI

NI

UI

NI

UI

0

24.9

24.9

2870

2870

2710

2710

20

26.1

27.0

3091

3095

2940

2773

40

24.0

25.3

3165

3462

2854

3343

60

19.9

23.5

3244

3283

3170

3090

80

18.7

23.4

3008

3298

3409

3264

100

15.3

23.4

2741

3910

3053

3513

Studies over three years on two soils near Brandon evaluated the effect of seed-placed urea on barley stand density and grain yield. In the absence of NBPT, barley stand density declined on both soils when urea was seed-placed at levels above 40 kg N ha-1 on the fine sandy loam soil and 60 kg N ha-1 on the clay loam soil (Tables 1 and 2). With NBPT, stand decline from seed-placed urea was slight.
On the fine sandy loam soil, grain yield was similar with and without the inhibitor until the N level exceeded 60 kg N ha-1. Above this level, grain yield declined with increasing N applications in the absence of NBPT, but not when NBPT was added. On the clay loam, grain yield was higher when the inhibitor was used when N levels exceeded 20 Kg N ha-1 (Table 2).

Table 2: Effect of seed-placed urea, with (UI) and without (NI) a urease inhibitor, on stand density (plants/metre) and grain yield (tonnes/ha) of barley on a clay loam soil, 1994-96.

N Rate

Stand

Grain Yield

Straw Yield

(kg ha-1)

NI

UI

No UI

UI

No UI

UI

0

24.8

24.8

2517

2517

2927

2927

20

23.9

25.9

2691

2699

2870

2930

40

23.4

25.9

2555

2905

3225

3763

60

22.1

25.0

2991

3232

3840

3783

80

18.9

25.5

2652

3401

3995

4005

100

16.1

22.8

2856

3172

4653

4313

Use of NBPT can reduce the risk of seedling damage from seed-placed urea. On both soils, the inhibitor NBPT reduced seedling damage and increased grain yield of barley at N levels up to 100 kg N ha-1. This would be of particular benefit in a zero-till system, where disturbance from band application of fertilizer may be undesirable.

Table 3: Stand density (plants per metre) and grain yield (T ha-1 ) of barley as affected by placement of 50 kg N ha-1 as urea, with (UI) and without (NI) NBPT under conventional (CT) and zero tillage (ZT) on a fine sandy loam soil (1994-96)

Treatment

Stand Density

Seed Yield

CT

ZT

CT

ZT

NI

UI

NI

UI

NI

UI

NI

UI

Control

36

39

39

39

2019

2019

2125

2125

Broadcast

38

36

35

34

2053

2145

2325

2694

Pre-Plant Band

33

34

30

37

2244

2024

2819

2664

Seed-placed

28

32

30

33

2463

2064

2913

2648

Fall Band

35

34

33

34

2404

2257

3256

2839

In other studies, broadcast, banded and seed-placed applications of 50 kg N ha-1, with and without the addition of NBPT were compared under ZT and CT management in barley on two soil types near Brandon (Tables 3 and 4). A relatively low rate of N (50 kg N ha-1) was used to ensure that the crop would be under N stress, so differences in efficiency would be apparent. Therefore, crop yields were generally low, as the N level was restricting yield.
Stand density of barley on the fine sandy loam was higher in the control treatment than when N was applied (Table 3). Use of NBPT led to higher stand density in the CT, when urea was seed-placed and under ZT when the urea was either seed-placed or pre-plant banded. Stand density was also higher when the urea was broadcast than when it was seed-placed or, in the CT, pre-plant banded. Barley yield was increased by N application on both the CT and the ZT, but the effect was much greater under ZT, where yield potential was apparently increased by the extra moisture conservation. Under CT, yield tended to be slightly lower on when NBPT was used with banded fertilizer applications and higher when used with surface urea applications. There was no significant effect of NBPT under ZT, but numerically the pattern was similar to that observed under CT.
On the clay loam soil, stand density of barley was reduced with seed-placed N applications, in spite of the relatively low N rate used (Table 4). Seed-placed application also produced lower stand density than band applications under ZT. Use of the NBPT led to higher stand density under ZT. Grain yield increased by 20 to 30% with N application. Under ZT, grain yield tended to be higher with the use of NBPT than in its absence, with the greatest effects when the fertilizer was broadcast or fall banded. Under CT, there was no significant effect of use of NBPT. Under CT, highest grain yields were obtained with seed-placed or fall band applications, while under ZT, highest grain yields were obtained with fall-band or broadcast applications, if NBPT was used.

Table 4: Stand density (plants per metre) and grain yield (T ha-1 ) of barley as affected by placement of 50 kg N ha-1 as urea, with (UI) and without (NI) NBPT under conventional (CT) and zero tillage (ZT) on a clay loam soil (1994-96)

Treatment

Stand Density

Seed Yield

CT

ZT

CT

ZT

NI

UI

NI

UI

NI

UI

NI

UI

Control

32

32

30

30

2573

2573

2298

2298

Broadcast

32

32

31

32

3086

2934

2937

3265

Pre-Plant Band

32

30

31

32

3083

3068

3006

3025

Seed-placed

27

28

28

30

3238

3083

2920

2948

Fall Band

30

31

31

32

3115

3119

2999

3207

While the use of NBPT shows promise in improving the efficiency of surface applications of urea-containing fertilizers under no-till systems, it is not always effective. Where environmental conditions are wet during seeding, losses of ammonia from surface applications may be minimal and the NBPT would not be required. Where dry conditions over the summer lead to very low crop yield, the requirement for N is decreased and conserving N from volatilization would not increase in crop yield. These conditions occurred in studies conducted from 1995 to 1997 on no-till wheat, at two locations near Brandon (Table 5). Treatments of UAN and urea were applied as broadcast or dribble-banded applications with and without NBPT. Ammonium thiosulphate was also tested as a urease inhibitor in the UAN solution. On the fine sandy loam soil, where drought stress was severe, there was no response to N application at all. On the clay loam soil, banded application of urea or UAN was more effective than broadcast applications, particularly with UAN and particularly where no NBPT was used. There was no overall benefit to using NBPT in these studies, although use of NBPT did improve the yields with broadcast UAN. Volatile losses of N from broadcast UAN were likely high because the UAN would begin volatilizing immediately upon application, as it was in solution already. Also, the UAN solution would adhere to surface residue, where urease activity is high and so rate of loss would be rapid initially. There was no benefit to using ammonium thiosulphate as an inhibitor in this study.

Table 5: Grain and straw yield of wheat as influenced by applications of urea or urea ammonium nitrate (50 kg N ha-1), with or without the use of N-(n-butyl) thiophosphoric triamide or ammonium thiosulphate as a urease inhibitor (1995-97).

Treatment

Grain Yield

Straw Yield

FSL

CL

Mean

FSL

CL

Mean

---------t*ha-1-------

1) Control

1.77

2.52

2.15

2.48

3.98

3.23

2) Urea, broadcast

1.95

2.85

2.40

2.72

4.91

3.82

3) UAN, broadcast

1.71

2.53

2.12

2.23

4.45

3.34

4) ATS, broadcast

1.68

2.67

2.18

2.44

4.04

3.24

5) Urea + NBPT, broadcast

1.90

2.88

2.39

2.59

4.85

3.72

6) UAN + NBPT, broadcast

1.88

2.97

2.43

2.69

4.65

3.67

7) UAN + ATS, broadcast

1.81

2.81

2.31

2.81

4.51

3.66

8) Urea, dribble

1.84

3.06

2.45

2.74

5.18

3.96

9) UAN, dribble

1.90

2.93

2.42

2.65

4.83

3.74

10) ATS, dribble

1.76

2.38

2.07

2.53

3.79

3.16

11) Urea + NBPT, dribble

2.00

3.01

2.51

2.69

5.19

3.94

12) UAN + NBPT, dribble

1.79

2.83

2.31

2.80

4.77

3.79

13) UAN + ATS, dribble

1.56

2.81

2.19

2.36

5.15

3.76

Mean

1.81

2.79

2.30

2.59

4.64

3.62

Environmental conditions are very important in determining volatilization losses and so in influencing the effectiveness of NBPT with surface applications. Since weather conditions normally become warmer and drier as we get later in the spring, risk of volatilization losses and seedling damage increases as fertilizer applications and seeding operations are delayed to later in May or into June. Therefore, there is likely to be less benefit of using NBPT early in the season and increasingly greater benefits as the season progresses.
Again, because of the effect of temperature and moisture, use of NBPT may be beneficial when in-crop applications of fertilizer are being applied either to top-up fertilizer rates when growing conditions improve after seeding, or to enhance protein content of wheat in anticipation of a protein premium. Since these type of applications occur when the weather in generally hotter and drier, use of NBPT may reduce losses and enhance the effectiveness of this type of application. Studies are currently underway at Brandon to evaluate the effectiveness of NBPT with urea and UAN in in-crop applications for protein enhancement.
Conclusions
Use of the urease inhibitor NBPT shows promise for use in crop production systems in the Canadian prairies. Where environmental conditions are conducive to NH3 volatilization, delaying hydrolysis can reduce concentration of NH3 at the soil surface, reducing the amount of fertilizer urea lost from surface applications. Immobilization of surface-applied urea may also be reduced, by allowing movement of urea below the surface residues, where it would be less prone to tie-up as the residues decomposed. In field studies near Brandon, use of NBPT generally increased crop yield when applied with broadcast urea. Under CT or when the urea was banded, there was generally no benefit of NBPT.
A response to NBPT cannot be expected every year, since volatile losses and yield response to N fertilizer are both greatly affected by environmental conditions. Increases in yield by the use of NBPT with surface-applied urea-containing fertilizers will only occur where: a) N deficiencies are limiting yield; b) Volatile losses of urea-containing fertilizers would be significant; and c) The crop is able to respond to N preserved by reducing volatilization losses. Use of NBPT is likely to be more beneficial under reduced tillage than conventional tillage systems.
Damage from seed-placed urea-containing fertilizers can be reduced by use of NBPT. In studies with barley, stand density and grain yield were consistently higher when NBPT was applied with high rates of seed-placed urea than in the absence of NBPT. A benefit from using NBPT will only occur where damage from seed-placed fertilizer is sufficient to produce a loss in yield, grade, maturity or competitiveness with weeds.
Although benefits from use of NBPT, either with surface-applied or seed-placed urea-containing fertilizers will not occur every year, due to variation in environmental conditions, NBPT can be a useful tool in managing risk. Since we can not effectively predict far in advance when environmental conditions will occur that will lead to either volatilization losses or seedling damage, use of NBPT can help to reduce the risk of damage, if weather conditions become detrimental. This will help improve the long-term economics of crop production.

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