Nitrification of ammoniated peat and ammonium sulfate in mineral soils

Soil Bio/. Biochem. Vol. 26,No.8,pp.1041-1051, 1994 Copyright 0 1994ElsevierScienceLtd Printed in Great Britain. All rights merved 0038-0717(94)E0017-T 0038-0717/94$7.00+ 0.00

Pergamon

NITRIFICATION AMMONIUM

OF AMMONIATED PEAT AND SULFATE IN MINERAL SOILS

C. ABBES, L. E. PARENT* and A. KARAM Equipe de Recherche en Sols Agricoles et Miniers (ERSAM), Department of Soil Science, Paul Comtois Building, Lava1 University, Sainte-Foy, Quebec, Canada GlK 7P4 (Accepted 25 January 1994) Sunmuuy~ur objective was to model nitrification kinetics of ammoniated peat (AP), ammonium sulfate (AS) and peat supplemented with ammonium sulfate and hydrated lime (PAS), in four different surface soils. The peat source was an acid sapric peat of high ammonia retention capacity. Nitrification rate was influenced significantly by soil type, N carrier and N application rate. Nitrification of AP showed a lag lasting longer in coarse-textured soils than in fine-textured soils, whatever N rate was applied. No lag was observed in AS- and PAS-treated soils. Initial nitrification rate was lowest in AP-treated soils at low N application rates but increased with AP application rate. High application rates of AS and PAS reduced NO; production considerably. N loss was higher for AS and PAS than for AP treatments. AP and PAS treatments significantly increased initial soil pH values. AP tended to decrease soil pH slightly at the end of the incubation. AP appeared to be an initially slow-release, thereafter a sustained-release N fertilizer.

INTRODUCTION Organo-mineral

fertilizers

containing

soluble

humic

substances derived from peat ammoniation with urea or ammonia could provide higher N, P and K recovery by crops than conventional fertilizers (Tishkovitch and Bulganina, 1980; Tishkovitch et al., 1983). The use of humus-containing fertilizers was recommended to improve soil physical properties (Lishtvan et al., 1976; Polyankov et al., 1984), to stimulate the activity of heterotrophic bacteria (Visser, 1985) and to promote plant growth (Schnitzer and Poapst, 1967; Lee and Bartlett, 1976; Chen and Aviad, 1990). Ammoniation technology of peat materials was developed to provide cost-effective active humus (Davis et al., 1935; Davis and Scholl, 1939; Lishtvan et al., 1976; Efimov et al., 1987). Peat-ammoniamineral fertilizers were prepared by ammoniating peat in bulk with NH, liquor or gas at air temperature and incorporating P and K fertilizers, or by granulating peat materials with mineral fertilizers and urea as the NH, carrier (Tishkovitch et al., 1983). Abbes et al. (1993) found that exchangeable NH: and acidhydrolyzable N were the dominant N fractions in peat ammoniated at 25°C and that ammoniation increased water-soluble and alkali-extractable carbon in peat. Much of the NHr fixed by peat at 120°C could be nitrified in the soil and absorbed by plants (Nommik, 1967). Some researchers have classified peat-based fertilizers as slow-release N fertilizers (Tishkovitch et al., 1983; Daigle et al., 1991). Thus, *Author for correspondence. saa 26,8--H

NO; loss by leaching or denitrification may be reduced by the use of peat-based fertilizers. Our objective was to investigate nitrification kinetics of ammoniated peat, ammonium sulfate and peat treated with ammonium sulfate and hydrated lime in four acid mineral soils. MATERIALSAND METHODS

Selected physical and chemical properties of the four mineral soils are shown in Table 1. Soils were air-dried, screened (c 2 mm), moistened to 80% of field capacity (water retained at -33 kPa), and placed into 530 ml polyethylene sterile bags. The bags were closed with foam stoppers and rubber bands. Nitrogenous materials later described were incubated at 25°C. Soil moisture was adjusted weekly by adding deionized water followed by thorough mixing. At the beginning of the incubation and periodically thereafter, NH:, NO; and NO; were extracted with 2 M KC1 and determined by steam distillation (Keeney and Nelson, 1982). NO; was not detected. At the beginning and at the end of the incubation, subsamples were analyzed for total N by the modified permanganate-reduced iron Kjeldahl method to include NO; and NO; (Bremner and Mulvaney, 1982) and for pH determination using a 1: 2 (v/v) soil : water ratio. Organic matter concentration was determined by the Walkley-Black procedure (Nelson and Sommers, 1982). Cation exchange capacity was measured by the cobaltihexamine method (Orsini and Remy, 1976). Water-holding capacity and bulk density of the soils were determined according

1041

C. ABE&

1042

et al.

Table 1. Selected physical and chemical propcrtics of the mineral soils Soil series Property

St Jude

Beaurivage

Chaloupe

Tilly

PH (H,G) Organic matter (g kg-‘) CEC [cmol ( + ) g kg-‘] Total N (g kg-‘) NH:-N (mg kg-‘) NO, (mg kg-‘) Clay (g kg-‘) Silt (g kg-‘) Sand (g kg-‘) WHC* (g kg-‘) Bulk density (Mg m-‘) Textural class

5.25 6.0 0.6 0.1 0.62 5.80 36.5 71 892.5 67.9 1.42 Sand

4.9s 30.6 0.9 1.24 4.88 9.81 57 229 714 169.8 1.39 Gravelly sandy loam Orthic bumoferric podzol Potato

6.55 41.5 13.4 2.04 4.81 16.31 119 674.5 206.5 314.2 1.17 Silty loam

6.50 37.4 13.8 1.7s 4.40 IS.75 139 295 566 223.0 1.21 Sand loam

Orthic-humic gleysol Fallow

Gleyic ferrohumic podzol Alfalfa

Soil group (CSSC, 1987)t Crop sampling time

Gleyic bumoferric podzol Fallow

*Water-holding capacity at -33 kPa. tCanada Soil Survey Committee (1987).

to Klute (1986) and Blacke and Hartge (1986), respectively. Sapric peat containing 600 g H,O kg-’ wet peat and 13.6 mg total N kg-’ was ammoniated with 50 g NH, kg-’ dry peat. Ammoniated peat (AP), ammonium sulfate (AS) and peat supplemented with

(NH:),,

= (NH:),=,

and

(NO; L =

1-

where N, is the added N, trt is the treatment and c is the appropriate control. Initial nitrification rates were computed from slopes of regression curves [(dNO; /dt )] using the kinetic model of Nishio and Fujimoto (1990):

a[l-exp(-(e+ 0+~W-YLexp -c+cNH~j,_ 1 (( (2)

“NH’)f=o[l -exp(-(‘+

(NHhc,,)t)]+(No_)

a +~LNHlL3exp

(( -

ammonium sulfate (PAS) were incorporated into soils at nine N rates: 0, 133, 266, 399, 532,798, 1064, 1330 or 1596 mg total N kg-’ dry soil. Such N rates corresponded to soil application rates of 0, 5, 10, 15, 20, 30, 40, 50 and 60 t AP ha-‘, respectively. Native N in peat accounted for 27% of total N in peat-based fertilizers. Raw peat (RP) was also applied to soils as control at the same rates ranging from 0 to 60 t RP ha-’ to account for peat native N. Hydrated lime was added to soils receiving PAS to raise pH close to values obtained in soils receiving similar rates of AP. The experimental design was a nested-nested split-plot (Sokal and Rohlf, 1981) with soils as main groups, N carriers as sub-groups and N rates as sub-sub-groups. There were two replications. The results were analyzed statistically using the SAS computer package (SAS Institute, 1990). Percentage conversion of added N to NO; was calculated as follows: %conversion = 100 (NO; N,, - NO,N,)/N,

(1)

P +

3

,=o

+.t

(3)

17

cNHyj,=,

where (NH: ), and (NO; ), are the respective concentrations of ammonium and nitrate in the soil (mg kg-‘) at time t (days), p is the apparent specific growth rate constant (day-‘), a is a proportionality factor (mg N kg-’ day-‘), and i is the net mineralization or denitrification rate (mg N kg-’ day-‘). Initial NH: concentrations used in the mathematical model were those recovered as exchangeable NH:. Constants a, ,u and i were determined by the optimization method of Powell (1964). Combined model (equations 2 and 3) showed highly significant R2 values varying from 0.88 to 0.99 (P c 0.01). RESULTS

Changes in NH: and NO; concentrations in AP-, AS- and PAS-treated soils at the lowest (133 mg N kg-‘) and highest (1596mg N kg-‘) N application rates are illustrated in Figs 1 and 2, respectively. The amount of exchangeable NH: (2 M KClextractable) depended on the soil type, N application rate, N sources and incubation time. Generally,

Nitrification of ammoniated peat

i?

z

(b) 1596 mg N kg-’

(a) 133 mg N kg-’

I >

Tilly

,-

silty

loam loam loam

sandy

sand

sandy

Chaloupe

Beaurivage

Q .I

St-Jude

1596 mg N kg-’

.+

(b)

AS

i

I

(b) 15% mg N kg-’

(a) 133 mg N kg-’

PAS

14

28

42

56

70

84 98

0

14

42

56

70

Time of incubation (days)

28

84

98

0

14

28

42

56

70

84

_-_

.

--

-.

-._

_ - - ._ .”

.. - -

98

,.-IIx-~~^“.I. - _.

Fig. 2. Temporal changes in NO;-N concentration in AP-, AS- and PAS-treated soils at lower [(a) 133 mg N kg-‘] and highest rates [(b) 1596 mg N kg-l] of added N.

0

0-l - , . , - , . , - , . , . , 2: . , . , - , . , - , . , . , ; - , . , - , . , . , . , - ,

200

400

600

800

1400

1600

125

150 I

AP

-

1045

Nitrification of ammoniated peat Table 2. Effect of soil type and N sources (N rates confounded) on nitrate conccotratioa during the incubation Incubation time (day) Treatment

1

14

28

42

56

IO

84

98

Mean NOT-N concentration St Jude sand AP AS PAS

6A IOA

9A IA 12A

9A IA 14A

12A 8A 15A

IIA 8A 16A

32A 11 B 1lB

35A 10B 17 B

39A II B 18B

Beaurivage sandy loam AP AS PAS

19A 12A 23 A

24A l3A 26A

36A 14 B 29A

93A l5B 33 B

122A 16B 368

142A 17B 40B

155A 118 428

157A 1lB 43 B

Chaloupe silty loam AP AS PAS

25 B 51 A 56A

53 B 82A 91 A

ll4A loac ll9B

242 A l25C 143 B

282 A l4ac 1608

296 A 159c 114B

314A l65C la3 B

324 A l7OC 190 B

Tilly sandy loam AP AS PAS

26 B 56A 53A

39 B 68A 13 A

168 A 85c lO2B

223 A 104C 127 B

250 A 119c 148 B

248 A l3OC 16OB

289 A l3ac 172B

303 A l49C 178 B

*. **

.** ***

** .I.

l ** It*

Source of variation Main effect Soils N sources (soils)

v-6

6At

-a

I

Significance of F values

l*z

.**

l l

*. ***

l

** *** l

l

l l

* *

tin each column and for each soil, means followed by a different letter are significantly different using contrasts (P < 0.05). ~Significant at l**p < 0.001 and l*P < 0.01, respectively.

recovery of exchangeable NH: decreased with increasing rates of AS or PAS. Recovery of NH: in AS-treated soils on day 0 ranged from 87 for lowest added N for 94% at highest added N in the Beurivage soil, 9698% in the St Jude soil, 78-91% in the Chaloupe soil and 69-87% in the Tilly soil. In PAS-treated soils, recovery at day 0 varied from 65 to 70% in the Beaurivage soil, 70-74% in the St Jude soil, 59-72% in the Chaloupe soil and 5968% in the Tilly soil. However, recovery of exchangeable NH: in AP-treated at day 0 was less rate-dependent, averaging 52% in the Beaurivage soil, 54% in the St Jude soil, 55% in the Chaloupe soil and 47% in the Tilly

soil. Extractable NH: increased with N application rate and was much higher in AS-treated soils than in AP- or PAS-treated soils (Fig. 1). In addition to the effect of N rate on extractable NH:, there was an obvious decrease in recovery of NH: with exposure time in soil (Fig. 1). Nitrification curves differed significantly among soils throughout the entire range of incubation times (Table 2). As shown in Fig. 2, nitrification was more delayed in the St Jude sand soil than in other soils. At the lowest and highest N application rates, the apparent lag for nitrification of AP was ca 56 days in the St Jude soil but was only 28 days in the

Table 3. Effect of N rates as polynomial contrasts on nitrate accumulation in soils treated with AP, AS and PAS Incubation time (day) Treatment

1

14

28

42

56

10

84

98

Significance of F values St Jude sand AP AS PAS

NS§ NS l*.c

NS NS l**c

NS NS lr*c

NS NS +r*c

NS NS r**c

***tL$ NS ***c

. ..L NS l**c

l**L NS r.*c

Beaurivage sandy loam AP SA PAS

***c ***c l**c

rrrc .**c l**c

l+*c l**c rrrc

l**L lr*c ***c

. ..L ***c ***c

***L *.*c l**c

l**L ***c lr*c

l*.L l+*c +.*c

Chaloupe silty loam AP SA PAS

. ..L r**c

. ..L **UC

**+L ***c

l**L l**c

. ..L lr*c

“‘Q

“‘Q

. ..Q

+**c

l**L l**c l**c

***c

. ..L l**c l**c

l**L l**c lr*c

Tilly sandy loam AP SA PAS

. ..L l*rc l**c

l**L l**c l**c

**rL . ..C l**c

r*rL +rrc l**c

. ..L ‘..C . ..C

l**L *.*c . ..C

. ..L l*+c +**c

. ..L +**c **.c

tsignificant at ***P < 0.001 and l*p < 0.01, respectively. ZL, Q and C: linear, quadratic and cubic contrasts. $NS: non significant at P < 0.05.

C. ABB& et al.

1046 1.0

1 4.0>

I

Beaurivage randy loam

St Jude sand

i

2.5

0.6

2.0

“.“I

-

0

I

266

-

I

532

-

1

798

-

I

1064

_

I

1330

-

I

-

1596

0

266

532

790

1064

1330

1596

11.57 Tilly sandy loam

Chaloupe silty loam

6.0

6.0

0

266

532

798

1064

1330

1596

0

266

532

798

1064

1330

1596

Application rate (mg N kg-‘) Fig. 3. Change in initial nitrification rates in soils treated with increasing amounts of AP (O), AS (m) and PAS (0).

Beaurivage soil, and 14 days in the Chaloupe soil and the Tilly soil. However, for AS- and PAS-treated soils, no lag was detected. Nitrification was also significantly different among N sources throughout the entire range of incubation times (Table 2). When considering all N application rates, total NO; accumulation in the Beaurivage soil was not significantly different between AP and either AS over the first 14 days or PAS over the first 28 days. The same result was obtained in the St Jude soil over the first 56 days. As shown in Table 2, total NO; produced in the St Jude soil after 56 days and the Beaurivage soil after 28 days was significantly higher with AP, followed by PAS and AS. In contrast, total NO; accumulation in the Chaloupe soil and the Tilly soil differed significantly between AP and either AS or PAS treatments at almost all incubation periods. For these two fine-textured soils, mean NO; concentration at day 14 was significantly higher with PAS and AS than with AP. After day 14, mean NO;

concentration in these soils became higher with AP followed by PAS and AS (Table 2). The effect of N application rate on NO; production was linear for AP-treated soils at all incubation times except over the 7-28-day period for the Beaurivage soil (Table 3). However, N application rate had a significant curvilinear effect on NO; production in AS- and PAS-treated soils at all incubation times. For AS and PAS treatments, nitrification throughout the entire range of incubation times was highest at application rates of 133 mg N kg-’ in the St Jude soil, 133-399 mg N kg-’ in the Beaurivage soil, and 532 mg N kg-’ in the Chaloupe soil and the Tilly soil (data not shown). Nitrification was thus mitigated at application rates exceeding 399 mg N kg-’ in the coarse-textured (Beaurivage and St Jude) soils and 532mg N kg-’ in the fine-textured (Chaloupe and Tilly) soils (Fig. 3). Despite an increase in NO; production in AP-treated soils, conversion of added N to NO; at day 98 was maximum

Nitrification of ammoniated peat Table 4. Conversion Nitrate

of added N to nitrate at day 98 conversion

(%)t

at N application

Treatment

133

266

399

532

798

St Jude sand Ap AS PAS

12 16 23

10

6

Beaurivage AP AS PAS Chaloupe AP AS PAS

1047

rates (mg kg-‘) 1064

1330

1596 5

4

2

3

5

1

1

tS

t

t

t

t

8

5

2

t

t

t

t

49 53 33

35 20 22

28 8 19

24 3 1

21 ;

18 0 0

17 0 0

17 0 0

59 82 89

55 76 84

57 58 66

59 46 52

51 26 30

44 10 13

37 7 8

33 4 6

76 90 83

67 70 77

63 55 71

59 48 63

44 13 19

37 8 11

33 5 7

29 3 6

sandy loam

silty loam

Tilly sandy loam AP AS PAS tNitrate conversion $t = trace <0.5%.

= 100 x [(NO;-N),,,,,

- (NO;-N),,,,,]/added

at the lowest N application rate and decreased to a minimum at the highest application rate (Table 4). A similar trend was found for AS and PAS treatments. Net N mineralization rate ‘i’ estimated from the Nishio and Fujimoto (1990) model decreased toward negative values as the application rate was increased, more so in AS- and PAS-treated soils than in APtreated soils (Table 5). Total N in N-treated soils was lower at day 98 than at day 0 (data not shown), thus explaining the negative value of i observed at higher N application rates. Net mineralization rates in untreated soils were 20, 100, 150 and 14Opg N kg-‘day-’ for the St Jude sand, the Beaurivage sandy loam, the Chaloupe silty loam and the Tilly sandy loam, respectively. The apparent specific growth rate constant ‘p’ decreased generally with N application rate (Table 5). The decrease was more pronounced in the AS- and PAS-treated soils than in the AP-treated soils. All treatments significantly inlluenced soil pH at day 98 (Table 6). At the beginning of the incubation, AP and PAS treatments increased soil pH linearly, whereas AS treatment had generally little intluence on soil pH (Table 6). At the end of incubation, soil pH decreased and was generally higher in PAS- than in AP- and AS-treated soils (Fig. 4). Compared to AS, AP slightly decreased soil pH values at day 98 (Fig. 4).

DISCUSSION

Exchangeable NH: at day 0 differed among soils and treatments. In general, the relative capacity of soils to fix NH: followed clay content (Table 1): St Jude sand-c Beaurivage gravelly sandy loam < Chaloupe silty loam < Tilly sandy loam. Nommik and Vahtras (1982) reported that the NH: fixing capacity of soils depended on amount of 2 : 1 lattice

N.

clay minerals. Sowden (1976) found that about 40% of NH: in dairy cattle manure and ammonium sulfate was fixed by a Brunisolic sandy loam soil containing 20% clay, largely vermiculite. Compared to AS-treated soils, the lower recovery of exchangeable NH: from peat-treated soils (AP and PAS) at day 0 is attributable to fixation of NH: by peat (Abbes et al., 1993) and the soil clay minerals (Drury and Beauchamp, 1991). Young (1964) found that the amount of NH, fixed by soils was correlated with organic C and clay content. AS-treated soils released more exchangeable NH: than AP or PAS-treated soils at all added N concentrations. Abbes et al. (1993) found that NH: could react with peat to produce hydrolyzable and non-hydrolyzable N not extractable by 1 M CaCl,. At 5% ammoniation rate, peat fixed 46% of the added N as acid-hydrolyzable or non-hydrolyzable N, since organic matter contains functional groups contributing to NH: fixation. Accumulation of NO; was higher in fine-textured soils than in coarse-textured soils, probably due to a larger number of nitrifying organisms and higher pH (Schmidt, 1982). Hebert et al. (1991) found a highly significant effect of soil type on net N mineralization in soils amended with cow and sheep manure composts during 16 weeks of incubation. At all N application rates, a lag of nitrification of 14 (fine-textured soils) to 56 (coarse-textured soils) days was observed only in AP-treated soils. The nitrification lag was longer for AP than for poultry and dairy manures (Hadas et al., 1983). Hadas et al. (1983) found that the delay of nitrification was 3 days in two soils treated with poultry manure pellets. Compared with high application rates of nitrogenous materials, nitrification at low N application rates ( < 399 mg N kg-‘) was generally lower in AP-treated soils than in AS- or PAS-treated soils. NH: supply in soils receiving low amounts of AP could be insufficient to activate nittifying microbes.

AP AS PAS

AP AS PAS

14.4 -2.6 0.8

11.9 -0.4 -3.7

0.18 4.88 3.13

0.57 3.57 6.30

0.21 6.77 4.26

5.0 - 10.2 -9.4

AP AS PAS

ot

0.06 0.39 3.14

102

2.3 -0.9 -10.1

jltx

133

AP AS PAS

N source

0.38 0.37 0.28

0.12 0.12 0.30

0.07 0.11 0.12

0.01 0.01 0.02

9

3.7 -4.1 -0.3

6.4 -6.0 -2.9

0.6 - 20.0 -8.7

1.3 1.2 - 12.7

pxloz

** **I

NSt ***

l**L NS l**L

l**Q **L . ..C

NS l**L

NS ***

r*rLf

l

** ttt

*** ***

***% ***

l

0 davs

0.83 0.22 0.27

-0.06 0.11 0.21

0.07 0.11 0.10

0.01 0.01 0.01

l

**c

lQ

*‘Q

** **

‘..L

. ..L

l**L

NS tt

0 davs of F values l ** l*

4.8 -3.8 -2.0

5.6 -6.7 -5.3

0.2 -1.7 2.3

2.4 1.0 2.5

j~xlti

. ..L l**L l**L

*** ***

** NS l

98 davs

Chaloupe silty loam

98 davs

l

i

2.3 10.1 11.9

2.7 17.0 16.4

1.70 0.37 0.04

0.07 0.03 0.04

(I

532

0.72 0.22 0.25

-0.06 0.09 0.10

0.07 0.07 0.09

0.03 -0.01 0.07

NS lL

l**L

NS **

*** t.

** **

. ..C ‘..C

l**Q

l

l

** +* l

98 days

Tilly sandy loam 0 davs

i

0.01 -0.30 -0.36

0.24 -0.30 -0.60

0.19 -0.30 -0.20

1.30 0.10 0.10

px102

10.9 1.9 2.9

11.5 2.5 3.3

4.05 0.92 1.05

0.75 0.89 0.83

a

1596

data for AP, AS and PAS in four soils

(0 days) and the end (98 days) of incubation

I .24 9.97 8.83

2.2 15.5 13.1

1.38 5.98 6.54

0.13 0.03 2.68

a

399

Significance l ** l **

Beaurivage sandy loam

98 davs

0 davs

0.52 0.20 0.27

0.01 0.07 0.48

0.08 0.12 0.13

0.01 0.01 0.03

i

on soil pH at the beginning

0.14 9.32 6.11

0.45 7.47 7.05

0.88 3.22 7.93

0.15 0.04 2.00

a

to nitrification

rate (mg N kg-‘)

(1990) model adiusted N application

tNS: non significant at P < 0.05. $L, Q and C: linear, quadratic and cubic contrasts. @Significant at ***P < 0.001, l*P < 0.01 and lP < 0.05, respectively.

N rate AP AS PAS

N source AP vs AS AP vs PAS AS vs PAS

Treatment

St Jude sand

Table 6. Effect of treatments

18.1 -3.8 0.1

16.8 -1.1 1.3

1.7 -6.0 - 14.0

0.9 0.1 -11.0

~~102

266

of the best-fit curves for the Nishio and Fuiimoto

tr: apparent specific growth rate constant (day-‘). ia: proportionality constant (mg N kgg’ day-‘). §i: netmineralization rate (mg N kg-’ day-‘).

Tilly

Chaloupe

St Jude

Soil series

Table 5. Constants

-1.01 -1.11 -1.21

-1.02 -1.09 -1.22

-0.39 -0.78 -0.80

-0.50 -0.87 -0.81

i

Nitrification of ammoniated peat

0

F

0 t

?i

8 w

1050

C. Aads et al.

Percentage conversion of added N to NO; in all soils decreased with increasing application rates of nitrogenous materials. Maximum conversion was obtained at lowest application rates. According to Broadbent et al. (1957), nitrification is rapid and complete at low NH: concentration. Since 6.4% of total N in the ammoniated sapric peat was found water-soluble and 47.5% was found Ca-extractable (Abbes et al., 1993), 8.5 (at lowest rate of added N) to 102 (at highest rate of added N) mg N kg-’ in AP treatments was water-soluble, and 63-757 mg N kg-’ was Ca-extractable. Assuming that water-soluble N is first nitrified (Goldberg and Gainey, 1955), it follows that water-soluble N of AP-treated soils could reach 100% conversion in the Beaurivage, the Chaloupe and the Tilly soils. At low N application rates (c399 mg N kg-‘), all water-soluble N in the AP-treated St Jude sandy soil could be converted to NO;, but this conversion would be incomplete at high application rates (> 399 mg N kg-‘). In fine-textured soils and for an equivalent rate of application, 42256% of water-insoluble N would be nitrified. A mitigating effect on the nitrification process at high application rates of AS and PAS could have been caused by: (1) excessive NH: concentration in soils (Broadbent et al., 1957) (2) an increase in salt content (Harada and Kai, 1968), (3) an increase in osmotic pressure (Darrah et al., 1986) or (4) a decrease in soil pH (Schmidt, 1982). For the AS and PAS treatments, NH: concentrations were excessive above 133-399 mg N kg-’ in coarse-textured soils and 532 mg N kg-’ in fine-textured soils since nitrification rate was mitigated at concentrations exceeding 399 mg N kg-’ for coarse-textured soils and 532 mg N kg-’ for fine-textured soils. NH: was found to be toxic to nitrifiers when the NH: concentration exceeded 400mg N kg-’ (McIntosh and Frederick, 1958) or 800mg N kg-’ (Broadbent et al., 1957). The AP-treatment did not mitigate nitrification in all soils at high N application rates, due to non-detrimental conditions to nitrifiers growth. This observation may indicate: (1) a slow release of NH: from AP into soil solution, and (2) a stimulating effect of humic substances on the activity of nitrifiers. Although peat ammoniation increased the amount of water-soluble organic matter (Abbes et al., 1993), N loss by denitrification or by volatilization appeared lower with AP than with AS or PAS, due to peat buffer capacity toward NH3. Fenn and Kissel (1976) noted that higher CEC of calcareous soils decreased NH, losses. According to Avnimelech and Laher (1977), adsorption of NH: leads to a decrease in NH: concentration and usually reduces NH3 loss. Our peat material had a high ammonia-retention capacity. In general, N loss was higher in PAS treatment than in AS or PA treatments, due to the presence of lime in PAS. The effect of lime on NH3 volatilization is well documented (Kissel et al., 1985).

Practical considerations

The process of peat ammoniation plays a major role in the nitrification patterns of peat-based fertilizers. Nitrification curves indicate that peat-based fertilizers could provide an initially slow, thereafter sustained, N release in soils, possibly more synchronized with plant requirement than NH: salts. However, AP may be nitrifled faster in most fertile soils. Peat-based fertilizers could provide bio-stimulating substances to the soil-plant system (Tishkovitch et al., 1983) and, presumably, be more acceptable environmentally, since less N would be lost by leaching, volatilization or denitrification. Slow or controlled release fertilizers have environmental, agronomic and physiological advantages (Shaviv and Mikkelsen, 1993). The physical properties of AP granules would be a determinant for controlling nitrate leaching (Richards et al., 1993). A lag of nitrification in AP-treated soils would give an agronomic advantage to broadcast applied AP over AS especially in the spring, where the soil is susceptible to NO; leaching (MacLean, 1977). Acknowledgements-The

research was supported by the

Natural Sciences and Engineering Research Council of Canada. We thank R. Boukchina for providing computer assistance.

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