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Ammonification and Nitrification of Urea in Different Egyptian Soils as Affected by its Application at Different Depths
ZbJ. Bald. II. Abt. Bd. 135 (1980),492-500 [Microbiology Dept., Faculty of Agriculture, Ain Shams University, Shoubra El-Khaima , Cairo, Egypt]
Ammonification and Nitrification of Urea in Different Egyptian. Soils as Affected by its Application at Different Depths S.
A. Z.
MAHMOUD,
Y. Z.
ISHAe, S.
H.
SALEM,
and .1\1:. Z.
EL-FOULY
Summary In pot experiments, urea was applied to different Egyptian soils at a standard rate of 150 kgj fed. at different depths, namely at 0, 0.5,1.0, and 2.0 inches below the surface of the ~oil.Moisture content was kept constant in all treatments so that the only variable factor was the depths of application. Representative soil samples were taken at intervals of I, 3, 7, 14, and 28 days. They were periodically analysed for total microbial counts, urease activity, ammoniacal and nitrate nitrogen. Results can be summarized as follows: 1. The highest microbial densities were generally achieved when urea was mixed with the soil or laid at 2 inches depth. In calcareous soil, however, mixing urea with the soil gave the best result in this respect. Spreading urea on the surface of different soils gave the lowest microbial size during the experimental periods. 2. Urease activity was not appreciably affected with the depth of urea application. However, when urea was mixed with the soil, the highest urease activity was generally achieved. 3. NHa-N in loamy, saline, and alkaline soils showed no clear differences due to application of urea in different dephths, In sandy soil, however, mixing urea with the soil gave the highest values of NHa,N during the experimental period, followed by 2 inches depth application. In calcareous soil, either mixing or laying urea in 2 inches depth gave the highest values of NHa-N. In general, when applied on the surface, the lowest values of NHa,N were observed in all soils. 4. NOa,N in alkali and calcareous soils showed the highest values when urea was mixed with the soil. In the rest of the soils, application of urea either at 2 inches depth or mixed with the soil gave the highest values of NOa-N. Application of urea on the soil surface gave the lowest values in all the soils. This may be due to the high volatilization rate of ammonia in such treatment.
Zusammenfassung In GefalJver;;uchen wurde Harnstoff mit einer Menge von 150 kgjFeddan unterschiedlichen agypt.ischen Boden in versohiedenon Tiefen verabreicht, und zwar 0,5, 1,0 und 2,0 Zoll unter del' Oberf'lache. Es wurdenProben am 1.,3.,7.,14. und 28. Tagentnommen. Anihnen wurde bestimmt: Gesamtkeimzahl, Ureaseaktivitat, loslioher Ammon, und Nitratstiekstoff. Hochste Gesamtkeimzahlen wurden ermittelt bei Einmischung von Harnstoff (Kontrolle) in den Boden oder Einbrin . gung in 2 Zoll(5,08 em) Tiefe. Die Ureaseaktivitat war nieht von del' Einbringung;;tiefe des Hamstoffs abhangig, doch war Einmischung in den Boden generell gimst.ig. VOl' allem im Sandboden ergab Einmisehung in den Boden bzw. Einbringung in 2 Zoll Tiefe hohere NHa-N-Gehalte, am niodrigsten waren sie bei oberf'lachliohor Ausbringung. Gleiches gilt fur die NOa,N,Gehalte.
The loss of nitrogen from urea is known to occur in sizeable amounts under some conditions, particularly when the urea is applied as top dressing, which is the normal practice of nitrogen fertilizers application. There is general agreement that volatile ammonia losses are decreased when the urea application is either mixed with the soil or when it is covered with a layer of soil (ERNST and MASSEY 1960, GASSER 1964). KHAN and HAQUE (1966) found that
Ammonification and Nitrification of Urea in Different Soils
493
placing urea two inches below the surface markedly inhibited loss of gaseous N. They also found that placing super with urea at depth further lessened N losses to quite small proportions. OVERREIN and MOE (1967) reported that effect of depth of the urea application on volatile ammonia losses was particularly striking. Losses may be greatly reduced even in light sandy soil by merely incorporating the urea with a few centimeters of moist soil. They added that ammonia volatilization rates decreased more rapidly with depth in wet soil than in moist soil. DAS and KHAN (1968) stated that, if urea was applied a few inches below the surface, losses were negligible. This experiment was conducted, therefore, to test the suitable depth in which urea should be applied to the soil to give the best results and to decrease the nitrogen loss by volatilization under our Egyptian soil conditions.
Materials and Methods Referring to the previous work (l\IAH~[Ol:D et u.l. 1977), the mentioned soils were also used here. Soil samples were collected from various locations in Egypt representing the loamy, saline, alkali, sandy, and calcareous soils. The previous soils were collected from Giza, Dakahliu, Kafr El.Shoikh , Sharkiu governments and Tahrer province, respectively. Soil samples were taken from the surface to a depth of 15 cm. After drying in air, the soils were ground to pass a sieve of 2 mm diameter mesh. The soils were analysed for their physical and chemical properties. From the afore-mentioned soils, 1000 g of each sample were placed in a pot. The best level of urea, resulting from the previous experiment (MAHMOUD et al , 197i), was chosen here to test the suitable depth at which urea should be mixed with the soil in order to give the most beniflcial results. Such was indicated by the low level of nitrogen loss by volatilization. Therefore, urea was applied to each soil at the rate of 150 kg/fed. at different depths, namely at 0, 0.5, 1.0, and 2.0 inches below the surface of the soil, placed in a pot. Moisture content was kept constant. in all treatments (75 % W. H. C.) so that the only variable factor was t.ho depth of application. The pots were incubated at room temperature anrl representative soil samples were taken at intervals of 1, :l, 7, 14,21, and 28 clays. They were periodically analysed for total microbial counts, urease activity, ammoniacal and nitrate nitrogen.
Microbiological analysis Total microbial counts were determined in soil samples by the standard plate count method, using soil extract yeast agar medium (MAHJ\HJl:D 1955).
Chemical determinations Urease activity: Urease activity was assayed according to the technique developed by EL-EsSAWI (197:3) as it has been mentioned in the previous work of MAHMOUD et al. (1977). Soluble nitrogen: Ammoniacal nitrogen was determined according to JACKSON (1958). Nitrate nitrogen was also determined according to PIPER (Hlij5).
Results and Discussion A. Total micro bial flora
Data in Table 1 show that the microbial counts generally increased with time when urea was applied to loamy soil at different depths. The counts increased in most treatments up to the 7th day, followed by progressive decrease, except when urea 33*
494
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et al.
Table 1. Effect of urea application at different depths on the total microbial counts in different soils (milljg soil) Treatment
Time in days 1
0
3
7
14
21
28
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
Loamy soil 41.5 123.5 41.5 108.5 41.5 107.0 41.5 103.5 41.5 92.5
137.0 104.0 112.5 119.0 125.5
141.0 96.5 104.0 121.0 149.0
134.5 88.5 92.5 99.0 114.0
105.0 74.0 86.0 101.0 109.5
92.5 72.0 85.0 87.0 92.0
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
Saline soil 4.6 23.5 4.6 19.4 4.6 18.1 4.6 17.7 4.6 14.2
46.0 25.3 33.0 38.5 27.5
38.5 23.3 24.3 28.5 33.0
32.5 19.8 22.6 26.7 34.5
26.2 20.1 19.8 22.4 26.9
22.9 17.3 18.2 20.4 24.7
25.2 16.4 16.9 14.3 15.1
84.0 61.0 54.0 32.5 28.0
66.0 56.0 67.5 62.5 52.5
37.0 32.0 44.5 49.0 48.0
26.0 22.0 28.1 37.5 39.0
28.3 17.4 22.9 20.7 24.3
2.4 1.2 2.2 2.8 1.9
7.4 4.3 6.2 7.6 6.5
8.4 3.6 4.4 8.4 9.6
8.2 2.4 3.7 5.2 8.1
4.5 2.8 2.4 4.1 3.5
3.6 2.3 2.1 3.1 3.2
soil 5.6 4.8 4.7 4.2 4.3
8.2 5.6 6.4 5.9 6.2
19.3 7.2 8.4 8.9 9.6
11.7 4.6 5.1 6.3 10.4
10.1 3.9 4.5 6.2 8.1
8.6 4.2 4.5 4.9 6.2
Alkali soil Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
8.2 8.2 8.2 8.2 8.2 Sandy soil
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
0.159 0.159 0.159 0.159 0.159 Calcareous 2.1 2.1 2.1 2.1 2.1
was applied at the surface and at 0.5" depth. The highest counts were observed after one and three days from urea application for the surface and 0.5" treatments, respectively. The decrease in numbers which occurred early when urea was either applied at the surface or at 0.5" depth, might be due to the high volatization of ammonia, resulting from the decomposition of urea at or near to the surface. The increase in counts generally observed after urea application could be due to the decomposition of urea by ureolytio organisms, liberating free ammonia that acted as a favourable nitrogen source for soil micro-organisms. Moreover, in loamy soil considerable amounts of ammonia are likely to be adsorbed on soil colloids. They serve as a pool of nitrogen for these organisms and also decrease the toxicity that might occur, due to the ammonification of urea. Deorease in counts, observed after the third or seventh day, could also be due to the consumption of a valuable energy source. Although the curves came to regression, they did not reach the level of the initial count in all treatments.
Ammonification and Nitrification of Urea in Different Soils
495
In saline soil, however, the microbial counts were relatively lower than those found in the loamy soil at all depths and during all the experimental periods. This can be due to the physico-chemical conditions of saline soil, being unsuitable for microbial proliferation. The microbial load in saline soil increased gradually in all treatments up to the 3rd day from urea application, then decreased, except when urea was applied at 2" depth, since the increase continued up to the 7th day. In the alkali soil, the microbial counts were found to be nearly the same like in saline soil. The microbial load in sandy soil was generally very low as compared to other soils. Application of urea at 2" depth in sandy soil gave high total counts that were close to those when urea was mixed with the soil. This could be due to the high porosity of the sandy soil. This phenomenon was not observed in case of the other soils. When urea was applied at the surface, the microbial numbers were the lowest of all treatments, being similar to alkali and saline soils. This can be attributed to the high volatilization of ammonia at the surface compared with the depths. The microbial counts in calcareous soil were at the peaks 19.3, 7.2, 8.4, 8.9, and 10.4 milljg when urea was mixed with the soil, laid at surface at 0.5", l.0", and 2.0" depth, respectively. The low microbial densities observed in calcareous soil in all treatments can be attributed to the low fertility of this soil and the high volatilization of ammonia (KHAN and HAQUE 1966). B. Urease activity The urease activity in loamy soil, expressed as mg NH 4-N released from urea/IOO g soiljhr, was initially 13.4 (Table 2). The urease activity showed a slight increase in most treatments up to the 3rd day after urea application, followed by a very low rate of decrease, indicating that urease activity was nearly constant. The persistence of high urease activity indicates that native urease is present in the active form for quite some time. This is evidenced by the presence of high microbial size of ureolytic microflora (EL-:FoULY 1976). The native urease enzyme in soil was found by many investigators to be less susceptible to attack by the protease enzymes, released by some other microorganisms (SKUJINS and McLAREN 1968, PAULSON and KURTZ 1969, and BURNS et al. 1972). Data showed also that, after the slight increase in urease activity due to urea application, the figures decreased thereafter to a level nearly similar to that of the initial. It was reported by some investigators that, although the native soil urease can remain for a very long period in soil without inactivation, the added urease is rapidly inactivated within short time. S'fO,JANOVIO (1959), MOE (1967), ROBERGE (1970), and BURNS et al. (1972) found that the addition of urease to soil increased urea hydrolysis temporarily. They suggested that added urease was either inactivated or destroyed by proteolysis. The initial urease activity in saline soil was lower than that in loamy soil. This could be deduced from low microbial size of ureolytic organisms in this soil (EL-FoULY 1976). Moreover, the salinity in such soil may result in inactivation of native or formed urease. RACHINSKII and PBL'TTSER (1966) suggested that the slower decomposition of urea in saline soil Illay be related to the number of ureolytic bacteria and the presence of inhibitors of urease. The lowest urease activity in saline soil was observed in surface-treated soil, while the highest one was obtained when urea was mixed with the soil. In the last treatment, mixing urea with the soil, increased the contact between urea and soil microorganisms, thus urease induction was expected to be high. Moreover, the liberated ammonia is not concentrated in one place, hut distrihuted in the soil and thus decreases its toxicitv to soil microflora. •
496
s, A. Z. l\[AH;\IO UD
et aI.
'I'ab le :? Effect of urea application at differen t depths on the ure ase activity in different soils (mg
NH4 ·N/100 g soiljhr) Treatmen t
Tim e in days 0
3
7
14
21
28
Mixed with soil At su rface At 0.5" depth At 1.0" depth At 2.0" depth
Lo amy soil 13.40 15.30 14.25 13.40 14.40 13.40 14.20 13.40 13040 14.50
15.90 14.05 14.60 14.85 14.95
15. 10 13.70 14.15 14.30 14.60
15.30 13.40 13.60 13.70 14.15
15.55 13.30 13.90 13.90 14.00
14. 70 13. 10 13.50 13.80 14.(15
Mixed with soil At su rfa ce At 0.5 " depth At 1.0" depth At 2.0" depth
Saline soil 6.35 H AO 6.35 8.10 8.45 6.35 8.90 6.35 6.35 8.60
11.65 8.20 8.65 9.05 9.90
10.25 7.85 8.05 8.9 5 9.55
8.15 7.60 7.75 8.45 9.20
7.80 7.15 7.30 7.90 8.90
7.90 7.05 7.40 7.70 8.15
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
Alkali soil 3.75 6.00 3.75 5.55 5.90 3.75 5.70 3.75 5.65 3.75
6.60 6.10 5.90 6.00 6.05
5.95 5.30 5.20 5.55 5.20
5.30 5.05 5.30 5. 25 5.40
6.00 4.80 5.05 5. 15 5.55
5.90 4.95 5.30 5.70
1.90 1.95 I.l 5 1.30 1.60
1.60 I.l 5 1.20 1.25 1.35
2.90
2.30
Mixed with soil At sur fa ce At 0.5" depth At 1.0" depth At 2.0 " depth
Sandy soil 0.6 0.6 0.6 0.6 0.6
1.80 1.20 1.40 1.45 J.55
2.05 1.30 1.70 1.85 1.95
2.10 I.l5 1.45 1.40 1.85
2.15 1.20 1.30 1.20 1.55
Mixed wi th soil l\ t surface At 0.5" depth At 1.0" depth At 2.0" depth
Calca reous 1.25 1.25 1.25 1.25 1.25
soil 2.25 2.05 1.95 1.95 1.90
3.80 2.60 2.30 2.35 2.20
4. 10 2. 15 2.45 2.60 2.95
3.45 1.80 1.90 2.10 2.55
lAO 1.55 1.60 2.15
5045
lAS
1.60 1.55 1.95
I n the alka li soil, the urease enzyme behaved nearly the same like in sali ne soil. It seems, however, that alka line conditions somehow deleteriously affected urease activity to a higher extent t han salinity. The initial urease activity of sandy soil was very low (0.6), due t o the low percentage of organic matter and cla y in such soil (PAULSON and KURTZ 1969 an d BURNS et al. 1972). As in other soils, application of ur ea mixed with the soil ga ve high er values of the enz yme activity. I n calcareous soil, the initial urease ac t ivity is relatively higher (1.85) t han in sandy soil. Th e highe st value of urease activity was obtained when urea was mixed wit h t he soil, as observed in the other soils. C. Ammoniaca l n itrog en Th e initial nitrogen in loa my soil was 48.75 ppm (Ta ble 3). H igh in crease in the ammonia was observed one day aft er urea application in all treatments at different dephths. This could be due to t he high ur ease activity in this soil. The ammoniaca l nitrogen gradua lly decreased t hereafter, due to the nitrification p rocess . Th e high est
Ammonification and Nitrification of Urea in Different Soils
497
Table 3. Effect of urea upplieation at different depths on ammoniacal nitrogen in different soils (ppm) Treatment
Time of
da.v~
3
7
14
21
28
11.00 90.00 93.15 83.75 92.50
70.00 51.25 67.50 66.25 72.50
45.00 35.00 38.75 41.25 43.75
30.00 20.00 23.75 30.00 33.75
23.75 18.75 20.00 22.50 26.25
20.00 17.50 23.75 21.25 23.75
85.00 68.75 75.00 81.25 76.25
77.50 65.00 66.25 63.75 70.00
61.25 42.50 47.50 51.25 53.75
41.25 28.75 33.75 38.75 43.75
36.25 25.00 26.25 30.00 28.75
28.75 21.25 25.00 26.25 22.50
76.25 53.75 66.25 77.50 75.00
81.25 42.50 52.50 67.50 71.25
60.00 26.25 28.75 36.25 56.25
50.00 30.00 ;{2.50 38.75 45.00
38.75 22.50 27.50 28.75 30.00
30.00 17.50 20.00 30.00 26.25
47.50 35.00 38.75 42.50 46.25
72.50 31.25 48.75 53.75 60.00
38.75 16.25 18.75 26.25 36.25
20.00 10.00 11.25 15.00 25.00
15.00 7.50 11.25 12.50 17.50
12.50 6.25 7.50 11.75 12.50
57.50 28.75 41.25 51.25 60.00
77.50 26.25 43.75 53.75 76.25
38.75 18.75 22.50 17.50 36.25
26.25 15.00 16.25 17.50 20.00
20.00 13.75 17.50 16.25 18.75
0 Loamy soil Mixed with soil At surface At 0.5'1 depth At 1.0'1 depth At 2.0'1 depth
48.75 48.75 48.75 48.75 48.75 Saline soil
Mixed with soil At sudaee At 0.5" depth At 1.0" depth At 2.0'1 depth
45.00 45.00 45.00 4;;.00 4;;.00 Alkali soil
Mixed with soil At surface At 0.5'1 depth At 1.0'1 depth At 2.0" depth
46.25 46.25 46.25 46.25 46.25 Sandy soil
Mixed with soil At surface At 0.5'1 depth At 1.0" depth At 2.0'1 depth
18.75 18.75 18.75 18.75 18.75
Calcareous soil Mixed with soil At surface At 0.5'1 depth At 1.0" depth At 2.0" depth
:n.25 31.25 31.25 31.25 31.25
36.25 22.50 25.00 28.75 31.25
amount of ammonia during the first period (1, 3, and 7 days) was observed when urea was mixed with soil (110.0 ppm). The lowest value was obtained when urea was applied at the surface, possibly due to the volatilization of ammonia. In saline soil the values of ammoniacal nitrogen behaved almost the same like in loamy soil. Generally, there was no difference between treatments detected. In alkaline soil, like in the other soils, the ammoniacal nitrogen showed the lowest increase when urea was applied to soil surface (53.7). This could be due to the high volatilization of ammonia, especially under high alkali condition. ERNST and l\L-\sSEY (1960) reported that increasing soil pH increased the volatilization of ammonia formed from urea, especially when applied to the surface, and there was a definite trend of decreasing ammonia losses when urea was mixed with soil. Because of the rough texture of the sandy soil and the low organic matter content, the volatilization of ammonia was high. The level of volatilization is expected to be higher at soil surface than at different depths. This is in agreement with OVERREIN
498
S. A. Z.
MAH~lOUD
et al.
Table 4. Effect of urea application at different depths on nitrate nitrogen in different soils (ppm) Treatment
Time in day" 0
3
7
14
21
28
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
Loamy soil 25.00 16.25 25.00 18.75 25.00 17.50 25.00 21.25 20.00 25.00
68.75 50.00 40.00 46.25 45.00
108.75 95.00 87.50 93.75 105.00
127.50 106.25 111.25 115.00 130.00
138.75 112.50 123.75 132.50 141.25
140.00 116.25 127.50 133.75 146.25
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
Saline soil 20.00 30.00 26.25 30.00 23.75 30.00 22.50 30.00 25.00 30.00
53.75 42.50 45.00 50.00 47.50
86.35 75.00 77.50 82.50 83.75
117.50 103.75 108.75 111.25 123.75
128.75 113.75 118.75 131.25 146.25
138.75 120.00 123.75 136.25 160.00
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
Alkali soil 36.25 18.75 36.25 36.25 26.25 36.25 36.25 20.00 36.25 30.00
42.50 55.00 48.75 41.25 51.25
90.00 67.50 72.50 77.50 93.75
102.00 72.50 83.75 82.50 102.50
121.25 77.50 92.50 102.50 112.75
133.75 85.00 100.00 106.25 121.25
Mixed with soil At surface At 0.5" depth At 1.,0" depth At 2.0" depth
Sandy soil 28.75 33.75 33.75 30.00 28.75 :~3. 75 32.50 33.75 26.25 33.75
32.50 32.50 37.50 36.25 47.50
60.00 45.00 50.00 61.25 67.50
86.25 56.25 61.25 72.50 93.75
95.00 57.50 58.75 73.75 102.50
102.50 61.25 66.25 77.50 103.75
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
Calcareous soil 25.00 10.00 18.75 25.00 17.50 25.00 15.00 25.00 16.25 25.0J
22.50 30.00 28.75 25.00 26.25
33.75 38.75 40.00 36.25 :32.50
67.50 55.00 58.75 60.00 65.00
82.50 61.25 65.00 68.75 80.00
91.25 63.75 67.50 76.25 85.25
and MOE (1967) who found that loss of ammonia was greatly reduced even in a light sandy soil, by merely incorporating the urea with a few centimeter of wet soil. The highest values of ammoniacal nitrogen in calcareous soil in the first period were obtained when urea was either mixed with the soil (77.5) or applied at 2.0" depth (76.2). This could be ascribed to the high volatilization rate, occurring on the surface of calcareous soils and also to the high pH of the soil. These finding were in harmony with those of MITSUI et al. (1954) and ERNST and MASSEY (1960) who found that liming of moderately acid sails increased losses of ammonia from applied urea, and losses increased with increasing soil pH. D. Nitrate nitrogen The initial nitrate content in loamy soil was low, being 25.0 ppm (Table 4). After the first day from urea application, the nitrate showed lower values than the initial value in all treatments. This could be due to the assimilation activities of soil microorganisms after the urea amendments. On the 7th day higher values were obtained, then gradual increase was observed up to the end of the experiment. This is attributed to the nitrification process.
Ammonification ami Nitrification of Urea in Different Soils
499
It is worth stating that the rate of nitrification process appears to be higher than that of the ammonification process. This is because of the high values of NOa-, compared with the corresponding values of NHa-N. These findings confirm those stated by ALEXANDER (1961) and WAKSMAN (1963). Regarding application of urea at different depths, it appears that urea mixed with soil or applied at 2/1 gave the highest NOa-N content. Each amendment evades the liability of NH a to volatilization, as can be seen in surface and 0.5/1 treatment. In saline soil, the initial nitrate content was 30.0 ppm. There was a slight decrease one day after urea application, followed by gradual increase up to the end of the experimental period, except in 2/1 depth. The highest nitrate content was achieved when urea was applied to 2/1 depth, and that was on the 21st day from application. The lower NOa-N level, however, was found when urea was applied to the surface soil (120.0). In alkali soil, there appears also a drop in the figures of NOa-N for one day in all experiments. Following that, the accumulation of nitrate started after a lag period of one day. The nitrate accumulation increased up to the end of the experiment, where the values were 85.0, 100.0, 106.0, and 121.25 for 0,0.5/1, 1.0/1, and 2/1 depth, respectively. Urea mixed with the soil showed the highest NOa-N among all treatments (140.0 ppm). Comparing the different treatments with each other, the deeper the urea applied the more the NOa-N increases. However, in all treatments the nitrate content did not reach the same level, and such is due to the high ammonia volatilization and low nitrification in alkali soils. WAHAB et al. (1960) and MAYER et al. (1961) found that the high rate of volatilization would happen in alkali soils, especially when urea was applied at the surface. Low and PIPER (1970) also reported that the high pH did not affect the rate of nitrification of ammonia formed from urea. The figures in sandy soil were lower than with other soils, due to the high rate of ammonia volatilization in sandy soil. This conclusion was also obtained by OVERREIN and MOE (1967). The trend in calcareous soil was similar to that observed in the other soils, i.e., the deeper the application of urea, the high ell' the accumulation of NOa-N. The figures were, however, lower than those of other soils, due to high ammonia volatilization rate and the inadequate conditions for nitrification (KHAN and HAQUE 1966). It C3-n be concluded that, using urea a'S a fertilizer, it should be mixed with soil or applied at 2/1 depth, since these two treatments gave the best results in all the investigated soils.
References ALEXANDER, }VI.: Introduction to soil microbiology. Jolin \Viley & Sons, Inc., Now York 1961 BUR~S, R. G., Pr;KITE, A. H., and l\IcLARE~, A. D.: Concerning tho location and persistence of soil urease. Soil Sci. Soc. Amor. I'roc. 36 (197Z), 308-311. DAS, A. C., and KHAN, J. A.: Ureolyt ic bacteria in soil. III. Volutilizat ion loss of ammonia from urea on soil application. Soils and Fert, 31 (1968), ill!. EL-EssAwI, T. M,: Studies on soil enzymes with special reference to soil phosphatase. Ph. D., College of Agriculture, University of Alexandria (197B). EL-FoULY, l\L Z.: Studies on mea decomposition organisms in Egyptian soils. Ph. D, Thesis, Fac. of Agric.. Ain Shams University, Egypt (1976). ERNST, ,T. W., ana ;\,IASREY, H. F.: The effects of several factors on volatilization of ammonia formed from urea in the soil. Soil Sci. Soc. Amer. Proc. 24 (1960), 87 ~ 90. GASSER, J. K. R: Urea as a fert.il izer. Soils and F8rt. 27 (1964) (il), 175-180. JACKSON, M. L.: Soil Chemical An,dysis. Constable and Co., London 19:58. KHA);", D. H., and HAQGE, 2\I. Z.: Volutil izat.ion loss of nitrogen from urea added to some soil of East Pakistan. Soils and For-t. 29 (1966), Z9;~.
soC)
S. A. Z. MAHMOUD et al., Ammonification and Nitrification of Urea in Different Soils
Low, A. J., and PIPER, F. J.: The ammonification and nitrification in soil of urea with and without biuret. J. Agric. Sci. 75 (1970), 301-309. MOHAMOUD, S. A. Z.: A study of spore formers occurring in soils. Germination and biochemical activity. Ph. D. Thesis, Leads Univ., England, U. K. (1955). ISHIAHc, Y. Z., SOLEM, S. H., and EL-FoULY, M. Z.: Effect of urea application in different rates on the microbial activity and mineralization of urea in Egyptian soils. GTAMS, Bangkok, Tajland , Nov. 1977. :JIAYER,R. D., OLSON, R. A., and RHOADES, H. F.: Ammonia losses from fertilized Nebraska soils. Agron. J. 53 (1961), 241- 244. MITSUI, S., OZAKI, K., and l\loRIYA:IIA, M.: The volatilization of ammonia transformed from urea. J. Sci. Soil Manure (Japan) 25 (1954), 17 -19 [c. f. MOE (1967)]. MOE, P. G.: Nitrogen losses from urea as affected by altering soil urease activity. Soil Sci. Soc. Amer. Proc. 31 (1967), 380-382. OVERREIN, L. N., and MOE, P. G.: Factors affecting urea hydrolysis and ammonia volatilization in soil. Soil Sci. Soc. Amor. Proc. 31 (1967), 57-61. PAULSON, K. N., and KURTZ, L. T.: Locus of urease activity in soil. Soil Sci. Soc. Arner, Proc. 33 (1969),897-901. PIPER, C. S.: Soil and Plant Analysis. University of Adelaide 1950, Reprinted 1955. RACHINSKII, V. V., and PEL'TTSER, A. S.: Kinetics of decomposition of urea in soil. Soils and Fert. 29 (1966),293. ROBERGE, M. R.: Behaviour of urease, added to unsterilized, steam sterilized, and gamma radiation sterilized Black Spruce humus. Can. J. Microbiol. 16 (1970),865-870. SKUJINS, J. J., and McLAREN, A. D.: Persistence of enzymatic activities in stored and geologically preserved soils. Enzymologica 34 (1968), 213-225. STOJANOVIC, B. J.: Hydrolysis of urea in soil as affected by season and by added urease. Soil Sci. 88 (1959),251-256. WAHAB, A., KHON, M., and ISHAQ, M.: Nitrification of urea and its losses throughout volatilizat.ion under different soil condition. J. Agron. Sci. 55 (1960), 47-51. \VAKSMAN, S. A.: Soil Microbiology. John Wiley and Sons, Inc., New York 1963. Authors' address: Prof. Dr. S. A. Z. MAHMOUD, Agricultural Microbiological Department, Faculty of Science, Ain Shams University, Shoubra, Cairo, Egypt.
Ammonification and Nitrification of Urea in Different Egyptian. Soils as Affected by its Application at Different Depths S.
A. Z.
MAHMOUD,
Y. Z.
ISHAe, S.
H.
SALEM,
and .1\1:. Z.
EL-FOULY
Summary In pot experiments, urea was applied to different Egyptian soils at a standard rate of 150 kgj fed. at different depths, namely at 0, 0.5,1.0, and 2.0 inches below the surface of the ~oil.Moisture content was kept constant in all treatments so that the only variable factor was the depths of application. Representative soil samples were taken at intervals of I, 3, 7, 14, and 28 days. They were periodically analysed for total microbial counts, urease activity, ammoniacal and nitrate nitrogen. Results can be summarized as follows: 1. The highest microbial densities were generally achieved when urea was mixed with the soil or laid at 2 inches depth. In calcareous soil, however, mixing urea with the soil gave the best result in this respect. Spreading urea on the surface of different soils gave the lowest microbial size during the experimental periods. 2. Urease activity was not appreciably affected with the depth of urea application. However, when urea was mixed with the soil, the highest urease activity was generally achieved. 3. NHa-N in loamy, saline, and alkaline soils showed no clear differences due to application of urea in different dephths, In sandy soil, however, mixing urea with the soil gave the highest values of NHa,N during the experimental period, followed by 2 inches depth application. In calcareous soil, either mixing or laying urea in 2 inches depth gave the highest values of NHa-N. In general, when applied on the surface, the lowest values of NHa,N were observed in all soils. 4. NOa,N in alkali and calcareous soils showed the highest values when urea was mixed with the soil. In the rest of the soils, application of urea either at 2 inches depth or mixed with the soil gave the highest values of NOa-N. Application of urea on the soil surface gave the lowest values in all the soils. This may be due to the high volatilization rate of ammonia in such treatment.
Zusammenfassung In GefalJver;;uchen wurde Harnstoff mit einer Menge von 150 kgjFeddan unterschiedlichen agypt.ischen Boden in versohiedenon Tiefen verabreicht, und zwar 0,5, 1,0 und 2,0 Zoll unter del' Oberf'lache. Es wurdenProben am 1.,3.,7.,14. und 28. Tagentnommen. Anihnen wurde bestimmt: Gesamtkeimzahl, Ureaseaktivitat, loslioher Ammon, und Nitratstiekstoff. Hochste Gesamtkeimzahlen wurden ermittelt bei Einmischung von Harnstoff (Kontrolle) in den Boden oder Einbrin . gung in 2 Zoll(5,08 em) Tiefe. Die Ureaseaktivitat war nieht von del' Einbringung;;tiefe des Hamstoffs abhangig, doch war Einmischung in den Boden generell gimst.ig. VOl' allem im Sandboden ergab Einmisehung in den Boden bzw. Einbringung in 2 Zoll Tiefe hohere NHa-N-Gehalte, am niodrigsten waren sie bei oberf'lachliohor Ausbringung. Gleiches gilt fur die NOa,N,Gehalte.
The loss of nitrogen from urea is known to occur in sizeable amounts under some conditions, particularly when the urea is applied as top dressing, which is the normal practice of nitrogen fertilizers application. There is general agreement that volatile ammonia losses are decreased when the urea application is either mixed with the soil or when it is covered with a layer of soil (ERNST and MASSEY 1960, GASSER 1964). KHAN and HAQUE (1966) found that
Ammonification and Nitrification of Urea in Different Soils
493
placing urea two inches below the surface markedly inhibited loss of gaseous N. They also found that placing super with urea at depth further lessened N losses to quite small proportions. OVERREIN and MOE (1967) reported that effect of depth of the urea application on volatile ammonia losses was particularly striking. Losses may be greatly reduced even in light sandy soil by merely incorporating the urea with a few centimeters of moist soil. They added that ammonia volatilization rates decreased more rapidly with depth in wet soil than in moist soil. DAS and KHAN (1968) stated that, if urea was applied a few inches below the surface, losses were negligible. This experiment was conducted, therefore, to test the suitable depth in which urea should be applied to the soil to give the best results and to decrease the nitrogen loss by volatilization under our Egyptian soil conditions.
Materials and Methods Referring to the previous work (l\IAH~[Ol:D et u.l. 1977), the mentioned soils were also used here. Soil samples were collected from various locations in Egypt representing the loamy, saline, alkali, sandy, and calcareous soils. The previous soils were collected from Giza, Dakahliu, Kafr El.Shoikh , Sharkiu governments and Tahrer province, respectively. Soil samples were taken from the surface to a depth of 15 cm. After drying in air, the soils were ground to pass a sieve of 2 mm diameter mesh. The soils were analysed for their physical and chemical properties. From the afore-mentioned soils, 1000 g of each sample were placed in a pot. The best level of urea, resulting from the previous experiment (MAHMOUD et al , 197i), was chosen here to test the suitable depth at which urea should be mixed with the soil in order to give the most beniflcial results. Such was indicated by the low level of nitrogen loss by volatilization. Therefore, urea was applied to each soil at the rate of 150 kg/fed. at different depths, namely at 0, 0.5, 1.0, and 2.0 inches below the surface of the soil, placed in a pot. Moisture content was kept constant. in all treatments (75 % W. H. C.) so that the only variable factor was t.ho depth of application. The pots were incubated at room temperature anrl representative soil samples were taken at intervals of 1, :l, 7, 14,21, and 28 clays. They were periodically analysed for total microbial counts, urease activity, ammoniacal and nitrate nitrogen.
Microbiological analysis Total microbial counts were determined in soil samples by the standard plate count method, using soil extract yeast agar medium (MAHJ\HJl:D 1955).
Chemical determinations Urease activity: Urease activity was assayed according to the technique developed by EL-EsSAWI (197:3) as it has been mentioned in the previous work of MAHMOUD et al. (1977). Soluble nitrogen: Ammoniacal nitrogen was determined according to JACKSON (1958). Nitrate nitrogen was also determined according to PIPER (Hlij5).
Results and Discussion A. Total micro bial flora
Data in Table 1 show that the microbial counts generally increased with time when urea was applied to loamy soil at different depths. The counts increased in most treatments up to the 7th day, followed by progressive decrease, except when urea 33*
494
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MAHMOUD
et al.
Table 1. Effect of urea application at different depths on the total microbial counts in different soils (milljg soil) Treatment
Time in days 1
0
3
7
14
21
28
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
Loamy soil 41.5 123.5 41.5 108.5 41.5 107.0 41.5 103.5 41.5 92.5
137.0 104.0 112.5 119.0 125.5
141.0 96.5 104.0 121.0 149.0
134.5 88.5 92.5 99.0 114.0
105.0 74.0 86.0 101.0 109.5
92.5 72.0 85.0 87.0 92.0
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
Saline soil 4.6 23.5 4.6 19.4 4.6 18.1 4.6 17.7 4.6 14.2
46.0 25.3 33.0 38.5 27.5
38.5 23.3 24.3 28.5 33.0
32.5 19.8 22.6 26.7 34.5
26.2 20.1 19.8 22.4 26.9
22.9 17.3 18.2 20.4 24.7
25.2 16.4 16.9 14.3 15.1
84.0 61.0 54.0 32.5 28.0
66.0 56.0 67.5 62.5 52.5
37.0 32.0 44.5 49.0 48.0
26.0 22.0 28.1 37.5 39.0
28.3 17.4 22.9 20.7 24.3
2.4 1.2 2.2 2.8 1.9
7.4 4.3 6.2 7.6 6.5
8.4 3.6 4.4 8.4 9.6
8.2 2.4 3.7 5.2 8.1
4.5 2.8 2.4 4.1 3.5
3.6 2.3 2.1 3.1 3.2
soil 5.6 4.8 4.7 4.2 4.3
8.2 5.6 6.4 5.9 6.2
19.3 7.2 8.4 8.9 9.6
11.7 4.6 5.1 6.3 10.4
10.1 3.9 4.5 6.2 8.1
8.6 4.2 4.5 4.9 6.2
Alkali soil Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
8.2 8.2 8.2 8.2 8.2 Sandy soil
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
0.159 0.159 0.159 0.159 0.159 Calcareous 2.1 2.1 2.1 2.1 2.1
was applied at the surface and at 0.5" depth. The highest counts were observed after one and three days from urea application for the surface and 0.5" treatments, respectively. The decrease in numbers which occurred early when urea was either applied at the surface or at 0.5" depth, might be due to the high volatization of ammonia, resulting from the decomposition of urea at or near to the surface. The increase in counts generally observed after urea application could be due to the decomposition of urea by ureolytio organisms, liberating free ammonia that acted as a favourable nitrogen source for soil micro-organisms. Moreover, in loamy soil considerable amounts of ammonia are likely to be adsorbed on soil colloids. They serve as a pool of nitrogen for these organisms and also decrease the toxicity that might occur, due to the ammonification of urea. Deorease in counts, observed after the third or seventh day, could also be due to the consumption of a valuable energy source. Although the curves came to regression, they did not reach the level of the initial count in all treatments.
Ammonification and Nitrification of Urea in Different Soils
495
In saline soil, however, the microbial counts were relatively lower than those found in the loamy soil at all depths and during all the experimental periods. This can be due to the physico-chemical conditions of saline soil, being unsuitable for microbial proliferation. The microbial load in saline soil increased gradually in all treatments up to the 3rd day from urea application, then decreased, except when urea was applied at 2" depth, since the increase continued up to the 7th day. In the alkali soil, the microbial counts were found to be nearly the same like in saline soil. The microbial load in sandy soil was generally very low as compared to other soils. Application of urea at 2" depth in sandy soil gave high total counts that were close to those when urea was mixed with the soil. This could be due to the high porosity of the sandy soil. This phenomenon was not observed in case of the other soils. When urea was applied at the surface, the microbial numbers were the lowest of all treatments, being similar to alkali and saline soils. This can be attributed to the high volatilization of ammonia at the surface compared with the depths. The microbial counts in calcareous soil were at the peaks 19.3, 7.2, 8.4, 8.9, and 10.4 milljg when urea was mixed with the soil, laid at surface at 0.5", l.0", and 2.0" depth, respectively. The low microbial densities observed in calcareous soil in all treatments can be attributed to the low fertility of this soil and the high volatilization of ammonia (KHAN and HAQUE 1966). B. Urease activity The urease activity in loamy soil, expressed as mg NH 4-N released from urea/IOO g soiljhr, was initially 13.4 (Table 2). The urease activity showed a slight increase in most treatments up to the 3rd day after urea application, followed by a very low rate of decrease, indicating that urease activity was nearly constant. The persistence of high urease activity indicates that native urease is present in the active form for quite some time. This is evidenced by the presence of high microbial size of ureolytic microflora (EL-:FoULY 1976). The native urease enzyme in soil was found by many investigators to be less susceptible to attack by the protease enzymes, released by some other microorganisms (SKUJINS and McLAREN 1968, PAULSON and KURTZ 1969, and BURNS et al. 1972). Data showed also that, after the slight increase in urease activity due to urea application, the figures decreased thereafter to a level nearly similar to that of the initial. It was reported by some investigators that, although the native soil urease can remain for a very long period in soil without inactivation, the added urease is rapidly inactivated within short time. S'fO,JANOVIO (1959), MOE (1967), ROBERGE (1970), and BURNS et al. (1972) found that the addition of urease to soil increased urea hydrolysis temporarily. They suggested that added urease was either inactivated or destroyed by proteolysis. The initial urease activity in saline soil was lower than that in loamy soil. This could be deduced from low microbial size of ureolytic organisms in this soil (EL-FoULY 1976). Moreover, the salinity in such soil may result in inactivation of native or formed urease. RACHINSKII and PBL'TTSER (1966) suggested that the slower decomposition of urea in saline soil Illay be related to the number of ureolytic bacteria and the presence of inhibitors of urease. The lowest urease activity in saline soil was observed in surface-treated soil, while the highest one was obtained when urea was mixed with the soil. In the last treatment, mixing urea with the soil, increased the contact between urea and soil microorganisms, thus urease induction was expected to be high. Moreover, the liberated ammonia is not concentrated in one place, hut distrihuted in the soil and thus decreases its toxicitv to soil microflora. •
496
s, A. Z. l\[AH;\IO UD
et aI.
'I'ab le :? Effect of urea application at differen t depths on the ure ase activity in different soils (mg
NH4 ·N/100 g soiljhr) Treatmen t
Tim e in days 0
3
7
14
21
28
Mixed with soil At su rface At 0.5" depth At 1.0" depth At 2.0" depth
Lo amy soil 13.40 15.30 14.25 13.40 14.40 13.40 14.20 13.40 13040 14.50
15.90 14.05 14.60 14.85 14.95
15. 10 13.70 14.15 14.30 14.60
15.30 13.40 13.60 13.70 14.15
15.55 13.30 13.90 13.90 14.00
14. 70 13. 10 13.50 13.80 14.(15
Mixed with soil At su rfa ce At 0.5 " depth At 1.0" depth At 2.0" depth
Saline soil 6.35 H AO 6.35 8.10 8.45 6.35 8.90 6.35 6.35 8.60
11.65 8.20 8.65 9.05 9.90
10.25 7.85 8.05 8.9 5 9.55
8.15 7.60 7.75 8.45 9.20
7.80 7.15 7.30 7.90 8.90
7.90 7.05 7.40 7.70 8.15
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
Alkali soil 3.75 6.00 3.75 5.55 5.90 3.75 5.70 3.75 5.65 3.75
6.60 6.10 5.90 6.00 6.05
5.95 5.30 5.20 5.55 5.20
5.30 5.05 5.30 5. 25 5.40
6.00 4.80 5.05 5. 15 5.55
5.90 4.95 5.30 5.70
1.90 1.95 I.l 5 1.30 1.60
1.60 I.l 5 1.20 1.25 1.35
2.90
2.30
Mixed with soil At sur fa ce At 0.5" depth At 1.0" depth At 2.0 " depth
Sandy soil 0.6 0.6 0.6 0.6 0.6
1.80 1.20 1.40 1.45 J.55
2.05 1.30 1.70 1.85 1.95
2.10 I.l5 1.45 1.40 1.85
2.15 1.20 1.30 1.20 1.55
Mixed wi th soil l\ t surface At 0.5" depth At 1.0" depth At 2.0" depth
Calca reous 1.25 1.25 1.25 1.25 1.25
soil 2.25 2.05 1.95 1.95 1.90
3.80 2.60 2.30 2.35 2.20
4. 10 2. 15 2.45 2.60 2.95
3.45 1.80 1.90 2.10 2.55
lAO 1.55 1.60 2.15
5045
lAS
1.60 1.55 1.95
I n the alka li soil, the urease enzyme behaved nearly the same like in sali ne soil. It seems, however, that alka line conditions somehow deleteriously affected urease activity to a higher extent t han salinity. The initial urease activity of sandy soil was very low (0.6), due t o the low percentage of organic matter and cla y in such soil (PAULSON and KURTZ 1969 an d BURNS et al. 1972). As in other soils, application of ur ea mixed with the soil ga ve high er values of the enz yme activity. I n calcareous soil, the initial urease ac t ivity is relatively higher (1.85) t han in sandy soil. Th e highe st value of urease activity was obtained when urea was mixed wit h t he soil, as observed in the other soils. C. Ammoniaca l n itrog en Th e initial nitrogen in loa my soil was 48.75 ppm (Ta ble 3). H igh in crease in the ammonia was observed one day aft er urea application in all treatments at different dephths. This could be due to t he high ur ease activity in this soil. The ammoniaca l nitrogen gradua lly decreased t hereafter, due to the nitrification p rocess . Th e high est
Ammonification and Nitrification of Urea in Different Soils
497
Table 3. Effect of urea upplieation at different depths on ammoniacal nitrogen in different soils (ppm) Treatment
Time of
da.v~
3
7
14
21
28
11.00 90.00 93.15 83.75 92.50
70.00 51.25 67.50 66.25 72.50
45.00 35.00 38.75 41.25 43.75
30.00 20.00 23.75 30.00 33.75
23.75 18.75 20.00 22.50 26.25
20.00 17.50 23.75 21.25 23.75
85.00 68.75 75.00 81.25 76.25
77.50 65.00 66.25 63.75 70.00
61.25 42.50 47.50 51.25 53.75
41.25 28.75 33.75 38.75 43.75
36.25 25.00 26.25 30.00 28.75
28.75 21.25 25.00 26.25 22.50
76.25 53.75 66.25 77.50 75.00
81.25 42.50 52.50 67.50 71.25
60.00 26.25 28.75 36.25 56.25
50.00 30.00 ;{2.50 38.75 45.00
38.75 22.50 27.50 28.75 30.00
30.00 17.50 20.00 30.00 26.25
47.50 35.00 38.75 42.50 46.25
72.50 31.25 48.75 53.75 60.00
38.75 16.25 18.75 26.25 36.25
20.00 10.00 11.25 15.00 25.00
15.00 7.50 11.25 12.50 17.50
12.50 6.25 7.50 11.75 12.50
57.50 28.75 41.25 51.25 60.00
77.50 26.25 43.75 53.75 76.25
38.75 18.75 22.50 17.50 36.25
26.25 15.00 16.25 17.50 20.00
20.00 13.75 17.50 16.25 18.75
0 Loamy soil Mixed with soil At surface At 0.5'1 depth At 1.0'1 depth At 2.0'1 depth
48.75 48.75 48.75 48.75 48.75 Saline soil
Mixed with soil At sudaee At 0.5" depth At 1.0" depth At 2.0'1 depth
45.00 45.00 45.00 4;;.00 4;;.00 Alkali soil
Mixed with soil At surface At 0.5'1 depth At 1.0'1 depth At 2.0" depth
46.25 46.25 46.25 46.25 46.25 Sandy soil
Mixed with soil At surface At 0.5'1 depth At 1.0" depth At 2.0'1 depth
18.75 18.75 18.75 18.75 18.75
Calcareous soil Mixed with soil At surface At 0.5'1 depth At 1.0" depth At 2.0" depth
:n.25 31.25 31.25 31.25 31.25
36.25 22.50 25.00 28.75 31.25
amount of ammonia during the first period (1, 3, and 7 days) was observed when urea was mixed with soil (110.0 ppm). The lowest value was obtained when urea was applied at the surface, possibly due to the volatilization of ammonia. In saline soil the values of ammoniacal nitrogen behaved almost the same like in loamy soil. Generally, there was no difference between treatments detected. In alkaline soil, like in the other soils, the ammoniacal nitrogen showed the lowest increase when urea was applied to soil surface (53.7). This could be due to the high volatilization of ammonia, especially under high alkali condition. ERNST and l\L-\sSEY (1960) reported that increasing soil pH increased the volatilization of ammonia formed from urea, especially when applied to the surface, and there was a definite trend of decreasing ammonia losses when urea was mixed with soil. Because of the rough texture of the sandy soil and the low organic matter content, the volatilization of ammonia was high. The level of volatilization is expected to be higher at soil surface than at different depths. This is in agreement with OVERREIN
498
S. A. Z.
MAH~lOUD
et al.
Table 4. Effect of urea application at different depths on nitrate nitrogen in different soils (ppm) Treatment
Time in day" 0
3
7
14
21
28
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
Loamy soil 25.00 16.25 25.00 18.75 25.00 17.50 25.00 21.25 20.00 25.00
68.75 50.00 40.00 46.25 45.00
108.75 95.00 87.50 93.75 105.00
127.50 106.25 111.25 115.00 130.00
138.75 112.50 123.75 132.50 141.25
140.00 116.25 127.50 133.75 146.25
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
Saline soil 20.00 30.00 26.25 30.00 23.75 30.00 22.50 30.00 25.00 30.00
53.75 42.50 45.00 50.00 47.50
86.35 75.00 77.50 82.50 83.75
117.50 103.75 108.75 111.25 123.75
128.75 113.75 118.75 131.25 146.25
138.75 120.00 123.75 136.25 160.00
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
Alkali soil 36.25 18.75 36.25 36.25 26.25 36.25 36.25 20.00 36.25 30.00
42.50 55.00 48.75 41.25 51.25
90.00 67.50 72.50 77.50 93.75
102.00 72.50 83.75 82.50 102.50
121.25 77.50 92.50 102.50 112.75
133.75 85.00 100.00 106.25 121.25
Mixed with soil At surface At 0.5" depth At 1.,0" depth At 2.0" depth
Sandy soil 28.75 33.75 33.75 30.00 28.75 :~3. 75 32.50 33.75 26.25 33.75
32.50 32.50 37.50 36.25 47.50
60.00 45.00 50.00 61.25 67.50
86.25 56.25 61.25 72.50 93.75
95.00 57.50 58.75 73.75 102.50
102.50 61.25 66.25 77.50 103.75
Mixed with soil At surface At 0.5" depth At 1.0" depth At 2.0" depth
Calcareous soil 25.00 10.00 18.75 25.00 17.50 25.00 15.00 25.00 16.25 25.0J
22.50 30.00 28.75 25.00 26.25
33.75 38.75 40.00 36.25 :32.50
67.50 55.00 58.75 60.00 65.00
82.50 61.25 65.00 68.75 80.00
91.25 63.75 67.50 76.25 85.25
and MOE (1967) who found that loss of ammonia was greatly reduced even in a light sandy soil, by merely incorporating the urea with a few centimeter of wet soil. The highest values of ammoniacal nitrogen in calcareous soil in the first period were obtained when urea was either mixed with the soil (77.5) or applied at 2.0" depth (76.2). This could be ascribed to the high volatilization rate, occurring on the surface of calcareous soils and also to the high pH of the soil. These finding were in harmony with those of MITSUI et al. (1954) and ERNST and MASSEY (1960) who found that liming of moderately acid sails increased losses of ammonia from applied urea, and losses increased with increasing soil pH. D. Nitrate nitrogen The initial nitrate content in loamy soil was low, being 25.0 ppm (Table 4). After the first day from urea application, the nitrate showed lower values than the initial value in all treatments. This could be due to the assimilation activities of soil microorganisms after the urea amendments. On the 7th day higher values were obtained, then gradual increase was observed up to the end of the experiment. This is attributed to the nitrification process.
Ammonification ami Nitrification of Urea in Different Soils
499
It is worth stating that the rate of nitrification process appears to be higher than that of the ammonification process. This is because of the high values of NOa-, compared with the corresponding values of NHa-N. These findings confirm those stated by ALEXANDER (1961) and WAKSMAN (1963). Regarding application of urea at different depths, it appears that urea mixed with soil or applied at 2/1 gave the highest NOa-N content. Each amendment evades the liability of NH a to volatilization, as can be seen in surface and 0.5/1 treatment. In saline soil, the initial nitrate content was 30.0 ppm. There was a slight decrease one day after urea application, followed by gradual increase up to the end of the experimental period, except in 2/1 depth. The highest nitrate content was achieved when urea was applied to 2/1 depth, and that was on the 21st day from application. The lower NOa-N level, however, was found when urea was applied to the surface soil (120.0). In alkali soil, there appears also a drop in the figures of NOa-N for one day in all experiments. Following that, the accumulation of nitrate started after a lag period of one day. The nitrate accumulation increased up to the end of the experiment, where the values were 85.0, 100.0, 106.0, and 121.25 for 0,0.5/1, 1.0/1, and 2/1 depth, respectively. Urea mixed with the soil showed the highest NOa-N among all treatments (140.0 ppm). Comparing the different treatments with each other, the deeper the urea applied the more the NOa-N increases. However, in all treatments the nitrate content did not reach the same level, and such is due to the high ammonia volatilization and low nitrification in alkali soils. WAHAB et al. (1960) and MAYER et al. (1961) found that the high rate of volatilization would happen in alkali soils, especially when urea was applied at the surface. Low and PIPER (1970) also reported that the high pH did not affect the rate of nitrification of ammonia formed from urea. The figures in sandy soil were lower than with other soils, due to the high rate of ammonia volatilization in sandy soil. This conclusion was also obtained by OVERREIN and MOE (1967). The trend in calcareous soil was similar to that observed in the other soils, i.e., the deeper the application of urea, the high ell' the accumulation of NOa-N. The figures were, however, lower than those of other soils, due to high ammonia volatilization rate and the inadequate conditions for nitrification (KHAN and HAQUE 1966). It C3-n be concluded that, using urea a'S a fertilizer, it should be mixed with soil or applied at 2/1 depth, since these two treatments gave the best results in all the investigated soils.
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S. A. Z. MAHMOUD et al., Ammonification and Nitrification of Urea in Different Soils
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