Microsite reactions of urea-nbtpt fertilizer on the soil surface

Soil Biol. Biochem.Vol. 25, No. 8, pp. I 107-II 17,1993 Printedin Great Britain. All rights reserved

MICROSITE

0038-0717/93$6.00+ 0.00 Copyright 0 1993Pcrgamon Press Ltd

REACTIONS OF UREA-nBTPT ON THE SOIL SURFACE

FERTILIZER

C. B. CHRISTIANSON, W. E. BAETHGEN, G. CARMONA and R. G. HOWARD International Fertilizer Development Center, P.O. Box 2040, Muscle Shoals, AL 35662, U.S.A. (Accepted 25 Jamuwy 1993) Summary-Nitrogen transformations that occur at the microsite of urea granule placement in soil as affected by the use of the urease inhibitor n-(N-butyl) thiophosphoric triamide (nBTPT) were studied. Urea granules containing 0, 0.5 or 0.05% w/w nBTPT were placed on the soil surface in plastic cups for up to 6 days. Soils were then frozen in liquid N, and a 0.9 cm thick vertical slice was cut through the fertilizer placement site. A 3.6 cm wide x 2.Ocm deep section of this slice was cut into 45 squares (0.4 x 0.4 cm) and analyzed for soil pH and extractable ammonium, nitrate and urea concentrations at the microsite where the fertilizer had been placed. In a sandy soil @H 5.2), it was found that urease inhibitors lowered soil pH and soil NH: concentrations at the placement site compared to urea alone and allowed more diffusion of urea away from the fertilizer microsite. In a clay soil @H 8.2), the effect of nBTPT was not as pronounced, and high concentrations of NH,+(450 Pg N g-r) developed in a zone of high pH. The experiments show that a significant factor in the effectiveness of urea inhibitors is their capacity to improve diffusion of NH:-N away from the zone of high soil pH associated with urea hydrolysis. The effectiveness of these inhibitors depends on the capacity of the soils to permit diffusion.

ITVTRODUCX’ION Although urea applied to the soil surface is prone to loss by NH, volatilization, such loss has been shown to be reduced significantly by using urease inhibitors (Voss, 1984). Laboratory and field research has identified n-(N-butyl) thiophosphoric triamide (nBTPT) and other related phosphoric triamides as some of the most effective urease inhibitors presently available for use with urea (Bremner and Chai, 1986; Clay er al., 1990; Carmona ef al., 1990; Bronson et al., 1989, 1990). The effectiveness of this class of inhibitors has been evaluated in laboratory studies by measuring the relative rates of hydrolysis of high urea concentrations in soil samples with or without urease inhibitor (McCarty et al., 1989; Bremner and Chai, 1986; Lu et al., 1989). However, Christianson et al. (1990) found that, although urease inhibitors had decreased urea hydrolysis in some soils by only 25% over 6 days, NH, losses from the inhibitor-treated samples were reduced by 90% during the same period. This indicated that the mode of action of these inhibitors was not simply one of delaying urea hydrolysis per se. Soil column studies have shown that urea treated with urease inhibitors diffused to greater depths than did untreated urea. This improved diffusion would be expected to reduce subsequent NH3 volatilization. Mixing urease inhibitors with urea can also reduce the rise in soil pH that normally occurs concurrently with urea hydrolysis (Clay et al., 1990). Ammonia volatilization is significantly affected by the reactions that take place at the microsite that develops in the zone of fertilizer granule placement (Hauck, 1984). Studies were initiated to look at the effect of urease inhibitors on the microsite chemistry

of urea and to determine if use of these products would cause significantly different patterns of N distribution and soil pH profiles in the zone of fertilizer placement. MATERIALS AND METHODS

Two contrasting soils (Table 1) were air dried and sieved (< 2 mm). Sufficient soil was placed in tapered plastic specimen cups (top i.d. 6.2 cm, base i.d. 5.0 cm, 6.3 cm depth) to provide a smooth soil layer 0.5 cm below the upper edge of the cup. A filter paper was placed on the soil surface and water was added drop-wise to the paper. The filter paper was then removed and the soils were kept for 3 days at 25°C in a desiccator containing water to ensure uniform soil moisture and bulk density conditions in the soil samples and to prevent cracking at the soil surface. Prior to the start of the experiments, representative soils were sampled in 0.5 cm layers in order to measure moisture profiles in the cups. In both soils, moisture was shown to be uniform within the surface 3.0 cm at the time of application of the fertilizers. Urea containing urease inhibitors was prepared by dissolving appropriate amounts of nBTPT in methanol and thoroughly mixing it with finely ground urea. After drying in a vacuum oven, the fertilizers (including urea without inhibitor) were compressed into small (12 mg) uniform cylindrical granules using a Coulter press equipped with a 0.24 cm die. Concentration of nBTPT in the fertilizer granule was determined by high-performance liquid chromatography (HPLC) using 15% methanol : water mobile phase with D4 modifier and pH adjusted to

1107

1108

C.

B. CHRISTIANSONet al.

Table I. Properties of soils used Organic

matter Sand Series

Texture

Subgroup

PH

(%)

(%)

Clay (%)

Vernon

Clay Silty loam

Typic Ustochrepts Typic Fragiudults

8.2 5.2

0.6 0.5

19 48

46 20

Savannah

3.0. Flow rate was 1.5 ml min-’ using a C-18 Resolve column. Samples were found to contain 0.05 and 0.5% nBTPT (w/w). After water had been added and the soil kept at 25°C for 7 days, a single preweighed fertilizer granule was placed on the soil surface in the exact center of each cup. Fertilizer-soil samples were then kept at 25°C for 2,4 or 6 days in desiccators containing water to maintain the relative humidity at 100%. Cups were removed from the desiccator and dipped in liquid N, for 1 min. The frozen block of soil was removed from the plastic cup, and a vertical slice 0.9 cm wide was made through the center of the frozen soil using an electric band saw. The soil block was laid on its side and allowed to thaw. A 3.6 x 2.0 cm grid made of interwoven guitar strings forming 45 precise squares (each 0.4 x 0.4 cm) was laid on the side of the soil section such that the center of the top of the grid lay over the location of the original granule placement. The grid was pushed into the soil, thereby cutting 45 soil subsamples. Because it was assumed that diffusion had occurred symmetrically about a vertical axis, samples equidistant from the center line but on opposite sides were removed from the grid and combined in a test tube. The sample was mixed with 5 ml of 2N KCI containing phenyl phosphorodiamidate (PPDA). This procedure was repeated with at least four sample cups for each soil x fertilizer combination on each sampling date. The pH of the soil-KC1 solution (1: 15, soil: solution-w/v) was analyzed using a thin electrode placed in the test tube. The soil was allowed to settle, and the extract was decanted; the urea, NH: and NO; contents were determined by autoanalysis (Technicon, 1973, 1974). Periodic sampling did not reveal the presence of NO; -N. Data were collated; an analysis of variance (ANOVA) was performed on each treatment x date combination, and least significant difference was determined. RESULTS

Savannah soil

This soil was sandy (Table 1) with a relatively low pH (5.2). As such, its buffering capacity was lower than that of the Vernon soil resulting in more pronounced shifts in soil pH as urea hydrolysis and nitrification took place. Two days after fertilizer application, the urea concentrations were significantly higher in the treatments containing nBTPT than in the urea-only samples, indicating that the inhibitor was retarding urea hydrolysis at the site of

fertilizer placement (Fig. 1). Persistence of N in the form of urea during this period allowed significant N diffusion away from the placement site, and urea concentrations in excess of 100 pg N gg’ soil were found 1 cm away from the placement zone after 2 days exposure. By day 4, only small amounts of urea remained in the 0.5% nBTPT treatment and other treatments showed only trace amounts of urea. By day 6, all urea had been hydrolyzed in all treatments. Concomitant with the rapid urea hydrolysis in the urea-only sample was a significant rise in NH: concentration at the granule placement site. By the second day, a strong concentration gradient had developed from the placement site (180 pg N g-r soil) to a point 2.0 cm away and 2.0 cm deep where NH: concentrations had declined to 3Opg N g-i soil (Fig. 2). This gradient declined on day 4 as nitrification started, and by day 6 NH: concentrations were uniformly low. In contrast, very little NH: was found in either of the treatments containing inhibitor on day 2, although a slight gradient in NH,+ concentration did develop in the 0.05% treatment in day 4 and became more pronounced on day 6. Similar trends developed with the 0.5% inhibitor treatment although maximum NH: concentrations and NH: concentration gradients were less pronounced. After 2 days, nitrification commenced in the zones farthest from the granule placement. At this stage, nitrification was most rapid in the 0.5% treatment. In all cases, a shallow NO;-N concentration gradient had developed and increased from the zone of fertilizer placement to the periphery of the sampling zone, indicating that in all treatments nitrification was proceeding most rapidly in the zone of the lowest NHiconcentrations (Fig. 3). Similar trends in nitrification of banded fertilizers were found by Pang et al. (1975) in column studies. Soil pH profiles were similar in shape to those of soil NH:and a steep gradient developed in the ureaonly treatment declining from a maximum of 7.2 in the immediate zone of fertilizer placement to <: 6.0 at the edge of the sampling zone (Fig. 4). Though these gradients declined during the 6 days of the experiment, pH at the center of the granule placement zone remained at 6.6. In contrast, the pH the treatments with inhibitor did not exceed 6.0 at the placement zone. Soil pH rapidly declined during the following 4days as nitrification took place. This decline was more pronounced in the 0.5% inhibitor treatment than in the 0.05% samples, showing the effect of nBTPT concentration on urea hydrolysis rates and the associated rise in soil pH.

0 s

i

I

0.8

1.6

Distance (cm)

1.2

2.0

4 Daye, nBTPT = 0.0

M8tance (cm)

Fig. 1. Soil urea-N concentrations

O 0.4

100

160

200

2 Days. nBTPT= 0.0

Savannah Soil - Urea-N

0.8

1.2 DtstaNe

bm)

1.6

2.0

’ / /

___)

4

0.4

;

jj

:

0.8

nBTPT = 0.6

I

1

I.6

+’

Distnnce km)

1.2

L__._

I

T--+_..-_

:

.__._

_

2.0

soil.

I

1 __

_+___-.I ~-

/

!

Day& nBTPT = 0.6

Dwance (cm)

Day,

i!;Li :

A

2

on days 2 and 4 in treatments containing urea or urea plus 0.05 or 0.5% nBTPT-Savannah

0.4

Day& nBTPT = 0.06

4 Day% nBTPT = 0.06

2

fi?wame bIIQ

Diatanoe bmf

Fig. 2, Soil NH,+-N cmcsntrations on days 2, 4 and 6 in treatments containing urea OF urea pIus 0.05 or 0.5% nBTPT----Savannah soil.

SavannahSoil- N&N

1111

.i

II

.:_ I

\

-T

ri f

d

i

5.5 5.0

:::

*

5.O

8.0 7.5 7.0

5 f

8.0 7.5 7.0 8.5 6.0 5.5

08

1.2 t38tance

bllt

l.%

2.0

Distanes k#nk

M8tanc9

km)

__._-__.c_..__

f- -- “-.---$6i.._.._ :__-_.-_.. ___-._:___..-, .

4 Days, nBTPT = 0.0%

0.8

% Day& riBTPT = 0.05

O-4

2 Days, nBTPT = 0.05

6 Days, nBTPT = 0.0

Distanoehn~

4 Days. nBTPT = 0.0

I.2 1.8 2.0 Distanca bln~

pH

0.4

3.2 Distanw

@ml

1.6

Mstanca (cm)

% Days, nBTPT = 0.5

Dmame km)

soil.

2*0

4 Days, nBTPT = 0.S

0.8

2 Days, nBTPT = 0.5

Fig, 4. Soil pH concentratians on days 2, 4 and 6 in treatments containing urea or urea plus 0.05 or 0.5% nBTPT-Savannah

0.4

2 Days, nBTP7 * 0.D

Savannah Soil - Soil

.,.“4

,_I

,,_.”

. ^.+.“_-__~ .

.

.

A_

---..

E

Fig, 6, soil NH:-N

/I -j

/1---

wn0entrations

on days 2, 4 and 6 in trcatrwnts

_

.’

,,,l.

‘-

.

.~

Q.4

0‘8

,

..,

_,, ,”

1.2 1.S 2.0 Dwance km)

._l..,

_.,_

.

soil.

.-

tsa O‘O$ - g.a ~_._...“,,

~1~ .,‘.

urea or urea plus 0.05 or 0.5% nBTPT--Vernon

1__. _.L:-q

containing

_,__~.___-_._.

91DAYS, nBTPT m 03

6 DAY$, nWPT = 0.05

S DAYS, IlSTPT = 0.0

x

Dkmncekml

Olatance(ad

Distance kmz

Diatanw (Amy

kanl

4 DAYS, nBTPT = 6.9

txawlce

2 DAYS, nBTPT = O,t5

4 DAY!& n@TPT= 0.06

1.2 1.S 2.0 Diatanca (cm!

2 DAYS, nBTPf = 0.05

4 DAYS, nBTPT r* 0.0

08% 0,8

2 DAYS, nBTPT m 0.0

Vernon Soil - NH&N

Fig.

6 Days, nBTPT = 0.05

km)

6 Days, nBTPT = 0.0

DManM 4 Daya, nBTPT = 0.05

/I

4 Days, nBTPT = 0.0

1

2 Days, nISTPf = 0.06 n

fan)

6 Days, nBTPT = 0.50

4 Days. nBTPT - 0.50

Dhhnce

2 Uaus. nBTPT =0.50

7. Soil NOT-N concentrations on days 2, 4 and 6 in treatments containing urea or urea plus 0.05 or 0.5% nBTPT-Vernon

A

2 Daya, nI3TPT* 0.0

VernonSoil - NUi-N

soil.

1

8.0

8.5

7.0

v) 7.5

I s

-YI\ t-W

7.5

8.0

0.5

9.0

0.4

t.2 t.t! 2.0 Distance kan)

Day& nBTPT = 0.0

fClId

--

‘-“‘

0.4

iCd

t.2 1.8 DkJt8me hII)

2.0

Distance km)

6 Days, nBTPT = 0.05

0.8

4 Days, nBTPT = 0.05

~8t&MEe

2 bays, nBTPT = 0.05

-‘”

‘-*”

0.4

0.4

12 1.8 2.0 Distance bml

20

Distance km) soil.

8 Day% nBTPT= 0.5

t.2 ‘I.8 Distance km)

4 Days. nBTPT = 0.5

0.8

0.8

2 Daya. nBTPT = 0.5

Fig. 8. Soil pH values on days 2, 4 and 6 in treatments containing urea or urea plus 0.05 or 0.5% nBTPT-Vernon

!3ihMlC8

6 Days, nBTPT= 0.0

0.8

4

Distance km)

2 Days. nBTPf = 0.0

Vernon Soil - Soil pti

-.-

-.-

‘----I

Microsite reactions Vernon

soil

Urea hydrolysis was rapid in this soil, and no measurable amounts of urea were detected at 2 days in the urea-only samples. Soils treated with ureanBTPT retained small amounts of fertilizer in the form of urea at the site of granule placement (Fig. 5). However, by day 4, no urea was found in any samples. Very high concentrations of NH$were found at the site of granule placement in all treatments, and the concentrations decreased as distance and depth increased. As in the Savannah soil, NH: concentrations were highest for the treatment without inhibitor and reached 690 peg N gg’ soil (Fig. 6). The inhibitors were less effective in reducing NH,+ concentrations in the Vernon soil than they were in the Savannah soil. As a result, NH: concentrations in the 0.05 and 0.5% treatments were 450 and 310 pg N g -I soil, respectively, at 2 days. Concentrations of NH: at the periphery of the placement site were similar for all treatments though they declined over the following 4 days as nitrification proceeded. As seen with the Savannah soil, nitrification was most rapid in the 0.5% nBTFT treatment and, as a result, concentrations of NO; were highest near the periphery of the granule placement site (Fig. 7). This indicates that nitrification was starting in zones of lowest NH: concentrations and proceeding towards the zone of highest NH: concentration. This trend persisted on day 4 though differences between treatments were not great. By day 6, concentrations of NO; were relatively uniform throughout the soil samples. In all samples, pH at the soil surface rose from background values of 8.2 to values in excess of 8.5 (Fig. 8). No significant difference was found between treatments on day 2. However, by day 6, a reduction in NH: concentrations due to nitrification had caused some soil acidification. As a result, soil pH had declined slightly in the nBTF’T treatments; the decline was most pronounced at the highest concentration of inhibitor. DLSCUSSION

The mode of action of nBTPT appears to be one of slowing urea hydrolysis long enough for urea to diffuse away from the placement zone. The lowered concentration of urea in the soil allows enhancement of the rate of nitrification, thereby lowering both the concentration of NH: and pH at the placement site. In a poorly buffered soil such as Savannah, which is prone to NH, loss, the inhibitors can be quite effective in modifying the fertilizer microsite sufficiently to reduce volatilization. In the Vernon clay soil, pH was so high that diffusion of N away from the microsite could not significantly reduce NH, loss. It is expected that urease inhibitors would be relatively ineffective in such soils.

1117

The data indicate that the amount of inhibitor required to achieve a significant reduction in N loss depends greatly on the diffusion characteristics of the urea in the soil and the urease activity, and this will have a significant effect on the economics of inhibitor use in the field. In poorly-buffered soils that allow rapid urea diffusion, NH, loss may be reduced with much less urease inhibitor than is required in heavytextured soils of high pH in which urea diffusion is retarded.

REFERENCES

Bremner J. M. and Chai H. S. (1986) Evaluation of n-butyl phosphorothioic triamide for retardation of urea hydrolysis in soil. Communications in Soil Science and Plant Analysis

17, 337-35 1.

Bronson K. F., Touchton J. T., Cummins C. G. and Hendrickson L. L. (1990) Use of the unease inhibitor N-(n-butyl) thiophosphoric triamide in corn production on a loamy sand. Journal of Fertilizer Issues 7, 31-34. Bronson K. F., Touchton J. T., Hiltbold A. E. and Hendrickson L. L. (1989) Control of ammonia volatilization with N-(n-butyl) thiophosphoric triamide in sandy soils. Communications 14391451.

in Soil Science and Plant Analysis

20,

Carmona G., Christianson C. B. and Bymes B. H. (1990) Temperature and low concentration effects on the action of the urease inhibitor N-(n-butyl) thiophosphoric triamide (nBTPT) in relation to ammonia volatilization from urea in unsaturated soil. Soil Biology & Biochemistry 22, 933-937.

Christianson C. B., Bymes B. H. and Carmona G. (1990). A comparison of the sulfur and oxygen analogs of triamide urease inhibitors in reducing urea hydrolysis and ammonia volatilization. Fertilizer Research 26, 21-27. Clay D. E., Malzer G. L. and Anderson J. L. (1990) Ammonia volatilization from urea as influenced by soil temperature, soil water content, and nitrification and hydrolysis inhibitors. Soil Science Society of America Journal 54, 263-266.

Hauck R. D. (1984) Significance of nitrogen fertilizer microsite reactions in soil. In Nitrogen in Crop Production (R. D. Hauck, Ed.), nn. 507-520. American Society of Agronomy, Madison.- _ Lu W.. Lindau C. W.. Pardue J. H.. Patrick W. H. Jr. Reddv K. R. and Khind ‘C. S. (1989) Potential of phenylphoiphorodiamidate and N-(n-butyl) thiophosphoric triamide for inhibiting urea hydrolysis in simulated oxidized and reduced soils. Communications in Soil Science and Plant Analysis 20, 775-788.

McCarty G. W., Bremner J. M. and Chai H. S. (1989) Effect of N-(n-butyl) thiophosphoric triamide on hydrolysis of urea by plant, microbial, and soil urease. Biology and Fertility

of Soils 8, 123-127.

Pang P. C., Cho C. M. and Hedlin R. A. (1975) Effects of pH and nitrifier population on nitrification of band-applied and homogeneously mixed urea nitrogen in soils. Canadian Journal of Soil Science 55, 15-2 1. Technicon (I 973) Ammonia in water and wastewater. Technicon Industrial Method 98-70W. Technicon Instrument Corp., Tarrytown. Technicon (1974) Urea nitrogen. Technicon method SE40001 FD4. Technicon Instfllment Corp., Tarrytown. Voss R. D. (1984) Potential for use of urease inhibitors. In Nitrogen in Crop Production (R. D. Hauck, Ed.), p. 571. American Society of Agronomy, Madison.