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Kinetics of Transfoimation of urea in

P. K. ParRA AND J. M. JArN

a Typic Ustochrept

Abstract : The nature of urea hydrolysis in a Typic (Isto_chrept soil was studied at two revers of ureaapprica'tion. Theresults sltowed that urea hydrotysis follotved a zero order reactionduring first four hours andfirst orrler reacrion between 4-12 hours.of the urease inhibitors tested hydroxyramine proved to be quiteeffective in retardation of urea hydrolysis whereas citrullus colo-synthis cake (non-ediblel was not. The study indicates the possi-bility of regulating N rerease by means of urease tnhibttorsv'hich may restilt in better utilisation of fertiliser N by crops.

(Key worils : Urea hydrolysis, urease inhibitor)

iurea, after its incorporation into soil, undergo'es en zymatichydrolysis,leading to formation of ammonium carbonate. wherever all the ammoniumis neither held by the soil colloid nor nitrified fast enough a major loss thatmay occur is ammonia volatilisation especially in light texturld soils con-taining high cabo, with pH greater than g.0 (Fenn

-& Kissel 1973; Jain

et al. 1981). Therefore, a knowledge of;the kinetics of urea hydrolysis andmeasures to regulate it may provide useful means of conserving fertilizer Nin soil for augmenting its efficiency in crop prod.uction.

' Materials and Methods

Laboratory experiments were conducted to study (a) kinetics of ureahydrolysis and (b) effbct of a non-edible oil cake'ittrultw colosynthis(locally known as thumba) and hydroxylamine on kinetics of urea hvdrolvsis-in a Typic Ustochrept (Mehrauli series). Some of the physicocfr"*icai,characteristibs of the soil are presented in table l.

Kinetics af urea hydrolysis ; The Michaelis constant (Km) andmaximum velocity (V."*) of urea hydrolysis in the soilwere determined byusing substrate concentrations ranging from 1.665- g.3i5 pmoles/g soil bythe method of Mcllvaine (r92r). The reaction was terminaied after-one hour

Division of Soil Science cind Agricultaral Chenistry, Indian Agricultural ResearcltInstitute, New Delhi, 1ruTt2

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146 Nitrogen in Soils, Crops and Fertilizers

Table 1. Some important characteristics of the soil

Particulars Value

pH (l :2.5 soil water suspension)

Electrical conductivity (mmhos/cm) (l :2.5)CEC (m.e.1100 g soil)

Organic carbon (%)Available nitrogen (kg lha)Clay (%)

sitt (%)Sand (%)Maximum water holding capacity (gravimetric "/6)

8.4

0.1

9.8

0.6

210.0

1,5.2

12.0

7?.8

33.0

by the addition of KCI-PMA (phenyl mercuric acetate) solution (Douglas&Bremner 1970). The reaction velocity (expressed as p moles/g/h) wasobtained from the plot of S/V and S according to the Michaelis-Mentenequation :

s/v Km,s-T."- -. v*-,.I

T-- (Km+S)Y m8x

The cogcentration of attainment of maximum hydrolytic velocity was

termed as critical concentration.

The determination of kinetics of urea hydrolysis was made by measur-ing the amounts ofurea hydrolysed after l, 2,4,8,12,24,48 and 72 hoursof incubation at 37"C. The substrate concenrration was taken at criticaland at five times the critical. Urea determination was done in PMA-KCIextract by the colorimetric method of Douglas and Bremner (1970).

The inhibitory materials, hydroxylamine and Citrullus colosynthis,

were examined at two levels (10 and 2A%) wlth rate of urea application at8 p molelg soil. The amount of urea hydrolysed during the first four hours

was recorded at an interval of one hour.

Results and Discussion

The effect of substr'ate concentration on hydrolytic velocity of urea is

presented in table 2 which shows the rate of hydrolysis in a buffered system

increases from 0.216 to 0.499 p mole/g/h when the substrate concentrationis raised from 1.665 to 8.325 p moles/g beyond which there is no increase.

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Ball. Indian Soc. Soil Sci. 13, 1984 117

Table 2. Deterntination of critical concentration for Y^o* of urease in the soil

p moles of. urea-Naddedlg soil

(s)

p moles of urea-N S/Vhydrolysed/g soil/h

(v) (h)

1 .665

3.330

4.995

6.66A

8. 325

a.216

0.336

0.412

a.473

a.499

7.698

9.90s

D.A96

14.055

I 6.683

Thus hydrolytic reaction of urea attains maximum velocity at a concentra-tion of 8.325 p moles/g soil, which is the critical concentration for this typeof soil. The increase in velocity of hydrolysis of urea with concentrationis in accordance with the law of mass action because the other parameters

such as urease activity, pH, temperatvte, etc., which are believed to governthe hydrolysis were maintained constant during the experiment. It can furtherbe observed from the table that if the rate of 0.499 p molelglh is maintain-ed, it would take 16.683 h for the complete hydrolysis of urea but in actualpractice the time taken is much larger than the theoretically calculatedvalue (S/V).

Table 3 shows periodic hydrolysis of urea applied at critical concen-

tration and at five times the critical concentration (8.325 and 41.625 pt

moles/g soil). The rate of urea hydrolysis remains more or loss constant

for first four hours and. can be determined by the equation f which holds

good for a zero order'reaction. The table further showsthattherateremains constant up to a concentration of 6.993 p moles/g soil below whichthere is a steep fall in thdrate from 0.458 to 0.222 p mole/g/h and the dropin the rate continues till a concentration of 3.63 p moles/g soil is reached

after which the rate again becomes constant and remains steady at 0.086 pmole/g/h. It can further be seen from table 3 that only 75'l of appliedurea is hydrolysed aftet 72 hours. The remaining 25'lof urea wouldbehydrolysed during another Ah tf the rate of 0.086 pomolelglhismain-tained, thus the total time taken for complete hydrolysis of urea would be

96 h when applied at erirical concentration.

However at higher concentratiott also, the hydrolysis again follows

zero order reaction as is shown by the constancy of{, thou8h the rate is

much higher than that at lower concentration. At this concentration only

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r48Nitrogen in Soils, Crops and Fertilizers

Table 3. Effect of urea-N concentration on the kinetics of its hydrolysis

p rnoles of P, moles ofurea-N urea-N

added/ foundigg soil soil after

hydrolysis(c)

g. moles of Pt moles ofurea-N urea- F{

hydrolysedi hYdrolYsed,t

g soil g soil/tr

(x) (x/t)

Log CTime ofhydrolYsis(hour) (t)

1

2

4

8

t?24

48

72

I1

4

8

17

1L

48

72

s.325

,,

,t

t,

,t

tt

tt

tt

4r.625

,,

,,,,

,,t,

,,

tt

7.963

7.643

6.993

6.544

5.984

5.795

. 4.363

2.A87

40.659

39.654

37.763

36.564

35.765

33.167

31.369

28.172

o,462

0.882

1,832

1.781

2,331

2.530

3.962.

6.243

0.966

1.971

3.862.

5.061

5.860

8.458

ro.256

13.453

0.462

0.44r

0,458

a.772

a.$40.105

0.082

0.086

a.966

0.985

0.965

a.632

0.488

0.352o.213

0.1 86

0.9012

0.8832

0.8446

0.8 1 58

0.7777

0.9630

0.6397

0.31 82

1.6090

1.5982.

r.5770

1.5630

1.5534

1.52A7

1.4965

2.4498

33"flofapptiedurea gets hydrolysed "f:'-l? h and the remainine 67'/'

would take anothe, f Si ft if 'n" "t"

of 0'186 p mole/g/h is maintained'

The rate of urea hydrolysis follows first order reactiou during 4-12

hoors ut both the subsirate concentrations as is indicated by the constancy

oflogC.Betweenl|andaghitd.oesnotexhibitanyregularpatternbutit is interesting to note ,t

" ,n"r 4g h, the hydrolysis of urea again shifts to

zero order reaction *iri"t iUen coritinues titt it. completion of hydrolysis'

The retardation in tf'" -'utt

of urea hydrolysis may be the consequence of

reduced concentration and increase{ pH as observed by sinha and Prasad

(1967)andSahraw*trg-80).ltsee.msthalureahydrolysismaynotfollowany regular trend andit should not be expected in view of involvement of a

large nrunb", of f*tott ;ht"h control tle activity and nat,re of urease in

the soil. Thus the ,r"*ptr-ri iJation and. cha'acterisation of soil urease

may prove usefut in re'g'lutioo of urea hydrolysis in soil so that |T-t'*Ii i"rJ"* of NHa-N from urea commensurates the optimal plant requre-

ment.

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Bull. Indian Soc. Soil Sci.13, 1984 149

The effect of Citrullus colosynthis on urea hydrolysis is presented intable 4 which shows that it does not affect the rate of hydrolysis at lowerconcentration but at higher concentrations, it shows the tendency of sup-

pression of urea hydrolysis. Table 4 further brings out that f, again is fairly

constant and ureahydrolysis.follows zero order reaction during first fourhours even in the presence of Citrullus colosynthis, a nitrification retarder(lain et a/. 1980).

Table 4. Effect of Citrullus colosynthis cake on uree hydrolysis

Time ofhydrolysis(h)

Urea N Citrullusadded colosynthis

ftr moles/ added (%)g soil)

Urea N {Jrea Nhydrolysed hydrolysed(p moles/g) (p moles/g/h)

I23

41

7

3

4

8.08.08.08.08.08.08.08.0

1010

IO1020202A

2A

0.M90.8781.3491.7800.4380.8681.3281.760

a.449o.4394.449a.4450.430o,434o.443o.Mo

The effect of hydroxylamine'is much more prbnounced in the retarda-tion of urea hydrolysis (Table 5). It can be seen that fhe rate of hydrolysisslumpsdowndrasticallyfrom0.46Zto 0.250 p molelglh, which again isconstant over a period offour hours. It may be inferred that hydroxyla-mine (NHroH) is a more efficient retarder of urea hydrolysis than citrulluscolosynthis. The superiority of NH,OH may be attributed to its high pHwhich adversely affects the urease activity. The effect of pH on ul€aseactivity may be quite prominent in light-textured soils having poor bufferingcapacity than in heavy textured soil with high buffering capacity.

The possibility of chemical reaction between urea and hydroxylamine,which might be another cause of reduction of urea hydrolysis, cannot beruled out. The reaction between urea and NHzoH may be represented bythe following e'quation.

- NOH

NH, - Co - NH, -f NH,oH s NH, - I - NHz * HroThough such a reaction may occur, it is to be confirmed by furtherstudies.

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Nitrogen in Soils, Crops and Fertilizers

Table 5" Effect of hydroxylamine (HA) on' urea hydrolysis

Time of Urea N HA HYdrolysed Urea Nhydrolysis added added (%) (rr molesig hydrolysed

(h) (6r, mole I soil) (P, mole lelh)g soil)

8.0 10 0.2s 0.250

8.0 l0 0.49 0.245

8.0 10 a.74 0.248

8.0 10 1.00 0.250

8.0 2A 0.30 0,300

8.0 20 0.48 0.240

8.0 za 0.84 0.280

8.0 2A 1.A2 0.255

The study shows the possibility of reducing N losses from tJ:e soil bya careful regUlation ofurea hydrolysis in the soil. The regnlation ofureahydrolysis may require a screening of a large number of natural and synthe-

tic compounds with no harmful after-effects in the soil and on plant.

Acknowledgements

The authors are grateful to Dr. N. N. Goswami, Head of theDivisionof Soil Sciencp and Agricultural Chemistry for his keen interest during the

course of the investigation. The authors are also thankful to Mr. M. N-Datta for rendering laborafory assistance. The first author is also thankful

to Director, IARI, for granting him a junior fellowship.

References

Douglas, L. A. & Bremner, J. M. (1970) Proc. Soil Sci. Soc. Am.34, 859.

Fenn, L. B. & Kissel, D. E. (1973) Proc. Soil ^Scf, Soc. Am.37, 855.

Jain, J. M., Narayanasamy, G., Sarkar, M. C. & Datta, M, N. (1980) l. Indian Soc.

Soil ,Sci. 28, 480.

Jain, J. M., Sarkar, M. C. &Deori, M. L. (1981) J. Indian Soc, Sotl Sci.29r97.

Mcllvaine, T. C" (1921) J. biol" Chem.49, 183.

Sahrawat, K. L. (1980) Fertil. Atrews, 25(i2),5A.

Sinha, H. & Prasad, K. (1967) J. Indian Soc, Soil Sci. 15r 28I.

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