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Variation in the relationship between nitrification and acidification of subtropical soils as affected by the addition of urea or ammonium sulfate
Soil Biology & Biochemistry 41 (2009) 2584–2587
Contents lists available at ScienceDirect
Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio
Short communication
Variation in the relationship between nitrification and acidification of subtropical soils as affected by the addition of urea or ammonium sulfate Xu Zhao a, Guang-xi Xing b, * a b
Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 April 2009 Received in revised form 24 August 2009 Accepted 30 August 2009 Available online 15 September 2009
The effects on nitrification and acidification in three subtropical soils to which (NH4)2SO4 or urea had been added at rate of 250 mg N kg1 was studied using laboratory-based incubations. The results indicated that NHþ 4 input did not stimulate nitrification in a red forest soil, nor was there any soil acidification. Unlike red forest soil, (NH4)2SO4 enhanced nitrification of an upland soil, whilst urea was more effective in stimulating nitrification, and here the soil was slightly acidified. For another upland soil, NHþ 4 input greatly enhanced nitrification and as a result, this soil was significantly acidified. We conclude that the effects of NHþ 4 addition on nitrification and acidification in cultivated soils would be quite different from in forest soils. During the incubation, N isotope fractionation was closely related to the nitrifying capacity of the soils. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Acidification NHþ 4 input Nitrification Subtropics
Studies into the relationships between nitrification and acidification have previously tended to focus on in temperate forest soils (Likens et al., 1969; Van Breemen et al., 1982; De Boer et al., 1992). The effects of NHþ 4 input on nitrification in subtropical soils have not been investigated extensively, and the results to date are inconsistent. For example, elevated NHþ 4 in tropical and subtropical soils has been found to stimulate nitrification and accelerate soil acidification (Moody and Aitken, 1997; Matson et al., 1999; Krusche et al., 2003). However, contradictory results have also been observed (Khalil et al., 2001; Zhao et al., 2007). The purpose of this study was to investigate the effects of the addition of two NHþ 4 sources – urea and (NH4)2SO4 – on nitrification and acidification in two upland soils and a forest soil from a subtropical region with varying properties. We hypothesized that the effects of elevated NHþ 4 on nitrification and acidification of subtropical soils might vary with respect to land-use and soil properties. Kinetic isotope fractionation accompanies N transformations (Shearer et al., 1974). 14N of NHþ 4 is preferentially incorporated into þ 15 NO 3 , resulting in high d N in residual NH4 during nitrification (Mariotti et al., 1981; Choi and Ro, 2003). Therefore, changes in d15N values of NHþ 4 were examined to identify the pattern of N isotope fractionation associated with nitrification.
* Corresponding author. Tel.: þ86 25 8688 1019; fax: þ86 25 8688 1028. E-mail addresses: [email protected] (X. Zhao), [email protected] (G.-x. Xing). 0038-0717/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2009.08.022
Three contrasting soils were studied (Table 1). For each soil, a series of 250 mL plastic bottles were prepared, each with 20 g of soil. (NH4)2SO4 or urea solution was applied at rates of 0 (i.e. controls) and 250 mg N kg1. Deionized water was added to adjust the moisture content of the soils to 65% WHC. All bottles were covered with polyethylene film punctured with needle holes to maintain an aerobic condition, and incubated at 30 C in the dark. The loss of water through evaporation was reimbursed using deionized water every 3 d. Six replications from each treatment were sampled on Days 0, 3, 7, 14, 21, 28, 42, and 56. Three were treated with 100 mL 2 mol L1 KCl and shaken for 1 h to extract NHþ 4 and NO3 ; the other three were used for measurement of soil pH (soil:water ¼ 1:2.5). NHþ 4 and NO3 concentrations were measured using indophenol blue method and colorimetric method. d15N value of NHþ 4 was analyzed using an isotope mass spectrometer (MAT-251, USA, with analytic precision 0.2 d15N) and calculated as follows:
d15 Nð&Þ ¼
i nh ð15 N=14 NÞsample ð15 N=14 NÞstandard o ð15 N=14 NÞstandard 1000
where ‘‘standard’’ refers to the atmospheric N2, which by definition has a d15N value of 0&. The addition of NHþ 4 had no or little effect on nitrification in the RF soil (Fig. 1e, f). Soil pH and d15N values of NHþ 4 showed little change at the end of the incubation (Fig. 2c; Fig. 3c). These results
200
100
100 0
0 3
7
14 21 28 42 56 0
3
7 14 21 28 42 56
Incubation time (day) NH4+ in treatment with controls +
NO3- in treatment with controls -1
NH4 in treatment with 250 mg N kg
NO3- in treatment with 250 mg N kg-1
Fig. 1. Changes of NHþ 4 -N and NO3 -N concentrations over 56-d incubation at 30 C with soil moisture of 65% WHC for soil RU(a, b), YU(c, d) and RF(e, f) after adding ammonium sulfate or urea at the rate of 0 (i.e. controls) and 250 mg N kg1. Standard deviation of replicates (n ¼ 3) is shown by bars.
21.6 46.2 32.2 10.2 0.36 3.00 4.8 Red pine forest Quaternary red earth 1750
19.3 74.0 6.70 14.8 1.33 11.6 5.8 Wheat/maize
35.7 43.7 11.1 0.50 4.00 4.3 Quaternary red earth
Quaternary Xiashu loess 1106
The fresh soil samples (0–20 cm) were passed though 2-mm sieve before incubation.
200
a
300
18.0
urea
300
0
Oilseed rape/peanut
Silt (v/v, %) Sand (v/v, %) CEC (cmol kg1) Total N (g kg1) Total C (g kg1)
400
f
ammonium sulfate
42
e
Mid-subtropical Jiangxi Province (28 15’2400 N, 116 50’3100 E)
400
RF
0 RF
15.4
0
7
100
North-subtropic Jiangsu Province (31 48’3900 N, 118 51’100 E)
100
YU
200
1750
200
18.0
300
-
300
-1
400 urea
d
ammonium sulfate
Table 1 Site description and soil properties.
+
-1
c
NO 3 concentration (mg N kg )
400
Annual rainfall (mm)
0 YU
43
100
Mid-subtropical Jiangxi Province (28 15’3000 N, 116 50’3000 E)
100
RU
200
Annual temperature ( C)
200
Altitude (m)
300
Location
300
Parent material
urea
0
NH4 concentration (mg N kg )
pH (water)
400
b
ammonium sulfate
Soilsa
a
Vegetation
RU
400
Clay (v/v, %)
implied that high acidity in the subtropical forest soil might have limited enzyme capacity for nitrification rather than availability of NHþ 4 (Zhao et al., 2007). Therefore, it is unlikely that elevated atmospheric NHþ 4 deposition would stimulate nitrification and, thereby, accelerate soil acidification. The RU soil was more acidic than the RF soil, however, nitrification of the RU soil was stimulated by NHþ 4 input (Fig. 1a and b), resulting in slight acidification in the soil (Fig. 2a). d15N values of NHþ 4 in the RU soil increased correspondingly (Fig. 3a), and significantly correlated to NO 3 concentration (Fig. 4a and b). Longterm cultivation in the upland soil RU might have improved the physicochemical and biological properties, thus facilitating microbial activity and enhanced nitrification. In fact, total C, total N, and CEC of the RU soil were consistently greater than those in the RF soil (Table 1). The response of the YU soil to NHþ 4 input was different from the RU soil, even though both were upland soils. Nitrification of the YU soil was greatly stimulated by NHþ 4 input (Fig. 1c and d). As a result, the soil was significantly acidified (Fig. 2b). d15N values of NHþ 4 increased markedly over 56 d of incubation (Fig. 3b), which showed a strong correlation with NO 3 concentration (Fig. 4c and d). The great difference in properties between YU and RU soils probably accounted for the response difference. Indeed, the YU soil had higher soil pH, total C, total N, and CEC than the RU soil (Table 1). Strong et al. (1997)
2585
20.6
X. Zhao, G.-x. Xing / Soil Biology & Biochemistry 41 (2009) 2584–2587
2586
X. Zhao, G.-x. Xing / Soil Biology & Biochemistry 41 (2009) 2584–2587
a
RU
a
6.5
RU 30 25 20 15
5.5
10 5 0
4.5
-5
N value of NH 4 (‰)
b YU
6.5
5.5
15
pH
b
+
3.5
YU 30 25 20 15 10 5 0 -5
c
4.5
RF 30 25 20 15
3.5
c
10
RF 6.5
5 0 -5 0
5.5
3
7
14
21
28
42
56
Incubation time(day) ammonium sulfate 15
4.5
3.5 0
3
7
14
21
28
42
56
urea
NHþ 4
Fig. 3. Changes of d N values of over 56-d incubation at 30 C with soil moisture of 65% WHC for soil RU(a), YU(b) and RF(c) after adding ammonium sulfate or urea at the rate of 250 mg N kg1. Standard deviation of replicates (n ¼ 3) is shown by bars. d15N values of NH4þ were not detected in the soils with addition of urea at day 0 because the amount of NHþ 4 -N was not enough for isotope mass spectrometry analysis.
Incubation time (day) pH in treatment with controls pH in treatment with ammonium sulfate pH in treatment with urea Fig. 2. Changes of soil pH over 56-d incubation at 30 C with soil moisture of 65% WHC for soil RU(a), YU(b) and RF(c) after adding ammonium sulfate or urea at the rate of 0 (i.e. controls) and 250 mg N kg1. Standard deviation of replicates (n ¼ 3) is shown by bars.
investigated nitrification in an Australian red earth soil, the properties of which are quite different from those in the current study. However, they concluded that nitrification in the soil with KHCO3þbuffer (in order to maintain soil pH at 6.3 for a long period) proceeded faster than in the control soil (pH 5.3) and the soil with KHCO3 (pH 6.3). The results from YU and RU soils suggested that special attention should be given to cultivated soils, which may undergo nitrification and acidification induced by increasing application of ammonium-based chemical fertilisers or atmospheric NHþ 4 deposition to subtropical regions. There is some evidence that a one-unit decline of soil pH has occurred in cultivated soils of the Yangtze River Delta in China over the past two decades (Shao et al., 2006; Wang et al., 2008). They suggested that chemical N fertilisers
especially ammonium-based were likely to be a critical factor influencing this acidification. For RU and YU soils, urea (Fig. 1b and d) was more effective in stimulation of nitrification than (NH4)2SO4 (Fig. 1a and c). This result was consistent with previously reported results (Martikainen, 1985; De Boer et al., 1988). This might be because the hydrolysis of urea leads to increase soil pH locally (Fig. 2a and b). The higher correlation coefficients between d15N values of NHþ 4 and NO 3 concentration in the urea-treated samples than those in the (NH4)2SO4-treated samples hinted that urea had greater stimulatory effect of nitrification than that of (NH4)2SO4 (Fig. 4). The increased d15N values of NHþ 4 were closely related to the nitrifying activity of soils in nitrification process, suggesting that N kinetic isotope fractionation might be an effective tool for investigating the effects of NHþ 4 input on nitrification.
Acknowledgements We thank two anonymous reviewers for their valuable suggestions that have greatly improved the manuscript. This work was supported by National Key Technology R&D Program of China (2007BAD87B07) and National S&T Major Project of China (2008ZX07101-005).
X. Zhao, G.-x. Xing / Soil Biology & Biochemistry 41 (2009) 2584–2587
2587
RU 30
30
a
25
b
urea 25 y = 0.1461x - 1.9644
20
ammonium sulfate
20
R2 = 0.9782 (P<0.0001)
15
15
10
10
5
5
+
N value of NH 4 (‰)
y = 0.1379x + 1.3461 R2 = 0.696 (P=0.0100)
0
0
0 30
30
60
15
120
0
150 YU 30
c
25
90
urea
d
25
20
20
15
15
10
y = 0.1598x - 12.421
10
5
R2 = 0.9908 (P<0.0001)
5
10
5
15
20
25
30
ammonium sulfate
y = 0.0768x - 2.4874 R2 = 0.8506 (P=0.0011)
0
0
0
50
100
150
200
250
300 -
0
40
80
120
160
-1
NO 3 concentration (mg N kg ) 1 Fig. 4. Relationships between d15N values of NHþ 4 -N and NO3 -N concentration in RU and YU soils after adding ammonium sulfate or urea at the rate of 250 mg N kg .
References Choi, W.J., Ro, H.M., 2003. Differences in isotopic fractionation of nitrogen in watersaturated and unsaturated soil. Soil Biology & Biochemistry 35, 483–486. De Boer, W., Duyts, H., Laanbroek, H.J., 1988. Autotrophic nitrification in a fertilized acid heath soil. Soil Biology & Biochemistry 20, 845–850. De Boer, W., Tietema, A., Klein Gunnewiek, P.J.A., Laanbroek, H.J., 1992. The autotrophic ammonium-oxidizing community in a nitrogen-saturated acid forest soil in relation to pH-dependent nitrifying activity. Soil Biology & Biochemistry 24, 229–234. Khalil, M.I., Van Cleemput, O., Boeckx, P., Rosenani, A.B., 2001. Nitrogen transformations and emission of greenhouse gases from three acid soils of humid tropics amended with N sources and moisture regime. I. nitrogen transformations. Communications in Soil Science and Plant Analysis 32, 2893–2907. Krusche, A.V., De Camargo, P.B., Cerri, C.E., Ballester, M.V., Lara, L.B.L.S., Victoria, R.L., Martinelli, L.A., 2003. Acid rain and nitrogen deposition in a sub-tropical watershed (Piracicaba): ecosystem consequences. Environmental Pollution 121, 389–399. Likens, G.E., Borman, F.H., Johnson, N.M., 1969. Nitrification: importance to nutrient losses in cutover forested ecosystems. Science 163, 1205–1206. Mariotti, A., Germon, J.C., Hubert, P., Kaiser, P., Letolle, R., Tardieux, A., Tardieux, P., 1981. Experimental determination of nitrogen kinetic isotope fractionation some principles illustration for the denitrification and nitrification processes. Plant and Soil 62, 413–430. Martikainen, P.J., 1985. Nitrification in forest soil of different pH as affected by urea, ammonium sulphate and potassium sulphate. Soil Biology & Biochemistry 17, 363–367.
Matson, P.A., McDowell, W.H., Townsend, A.R., Vitousek, P.M., 1999. The globalization of N deposition: ecosystem consequences in tropical environments. Biogeochemistry 46, 67–83. Moody, P.W., Aitken, R.L., 1997. Soil acidification under some tropical agricultural systems. 1.Rates of acidification and contributing factors. Australian Journal of Soil Research 35, 163–173. Shao, X.X., Huang, B., Gu, Z.Q., Qian, W.F., Jin, Y., Bi, K.S., Yan, L.X., 2006. Spatial-temporal variation of pH values of soils in a rapid economic developing area in the Yangtze River Delta Region and their causing factors. Bulletin of Mineralogy, Petrology and Geochemistry 25, 143–149 (in Chinese). Shearer, G., Duffy, J., Kohl, D.H., Commoner, B., 1974. A steady-state model of isotopic fractionation accompanying nitrogen transformations in soil. Soil Science Society of America Journal 38, 315–322. Strong, D.T., Sale, P.W.G., Helyar, H.R., 1997. Initial soil pH affects the pH at which nitrification ceases due to self-induced acidification of microbial microsites. Australian Journal of Soil Research 35, 565–570. Van Breemen, N., Burrough, P.A., Velthorst, E.J., Van Dobben, H., De Wit, T., Ridder, T.R., Reijnder, H.F.R., 1982. Soil acidification from atmospheric ammonium sulphate in forest canopy throughfall. Nature 299, 548–550. Wang, Z.G., Zhao, Y.C., Liao, Q.L., Huang, B., Sun, W.X., Qi, Y.B., Shi, X.Z., Yu, D.S., 2008. Spatio-temporal variation and associated affecting factors of soil pH in the past 20 years of Jiangsu Province, China. Acta Ecologica Sinica 28, 720–727 (in Chinese). Zhao, W., Cai, Z.C., Xu, Z.H., 2007. Does ammonium-based N addition influence nitrification and acidification in humid subtropical soils of China? Plant and Soil 297, 213–221.
Contents lists available at ScienceDirect
Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio
Short communication
Variation in the relationship between nitrification and acidification of subtropical soils as affected by the addition of urea or ammonium sulfate Xu Zhao a, Guang-xi Xing b, * a b
Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 April 2009 Received in revised form 24 August 2009 Accepted 30 August 2009 Available online 15 September 2009
The effects on nitrification and acidification in three subtropical soils to which (NH4)2SO4 or urea had been added at rate of 250 mg N kg1 was studied using laboratory-based incubations. The results indicated that NHþ 4 input did not stimulate nitrification in a red forest soil, nor was there any soil acidification. Unlike red forest soil, (NH4)2SO4 enhanced nitrification of an upland soil, whilst urea was more effective in stimulating nitrification, and here the soil was slightly acidified. For another upland soil, NHþ 4 input greatly enhanced nitrification and as a result, this soil was significantly acidified. We conclude that the effects of NHþ 4 addition on nitrification and acidification in cultivated soils would be quite different from in forest soils. During the incubation, N isotope fractionation was closely related to the nitrifying capacity of the soils. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Acidification NHþ 4 input Nitrification Subtropics
Studies into the relationships between nitrification and acidification have previously tended to focus on in temperate forest soils (Likens et al., 1969; Van Breemen et al., 1982; De Boer et al., 1992). The effects of NHþ 4 input on nitrification in subtropical soils have not been investigated extensively, and the results to date are inconsistent. For example, elevated NHþ 4 in tropical and subtropical soils has been found to stimulate nitrification and accelerate soil acidification (Moody and Aitken, 1997; Matson et al., 1999; Krusche et al., 2003). However, contradictory results have also been observed (Khalil et al., 2001; Zhao et al., 2007). The purpose of this study was to investigate the effects of the addition of two NHþ 4 sources – urea and (NH4)2SO4 – on nitrification and acidification in two upland soils and a forest soil from a subtropical region with varying properties. We hypothesized that the effects of elevated NHþ 4 on nitrification and acidification of subtropical soils might vary with respect to land-use and soil properties. Kinetic isotope fractionation accompanies N transformations (Shearer et al., 1974). 14N of NHþ 4 is preferentially incorporated into þ 15 NO 3 , resulting in high d N in residual NH4 during nitrification (Mariotti et al., 1981; Choi and Ro, 2003). Therefore, changes in d15N values of NHþ 4 were examined to identify the pattern of N isotope fractionation associated with nitrification.
* Corresponding author. Tel.: þ86 25 8688 1019; fax: þ86 25 8688 1028. E-mail addresses: [email protected] (X. Zhao), [email protected] (G.-x. Xing). 0038-0717/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2009.08.022
Three contrasting soils were studied (Table 1). For each soil, a series of 250 mL plastic bottles were prepared, each with 20 g of soil. (NH4)2SO4 or urea solution was applied at rates of 0 (i.e. controls) and 250 mg N kg1. Deionized water was added to adjust the moisture content of the soils to 65% WHC. All bottles were covered with polyethylene film punctured with needle holes to maintain an aerobic condition, and incubated at 30 C in the dark. The loss of water through evaporation was reimbursed using deionized water every 3 d. Six replications from each treatment were sampled on Days 0, 3, 7, 14, 21, 28, 42, and 56. Three were treated with 100 mL 2 mol L1 KCl and shaken for 1 h to extract NHþ 4 and NO3 ; the other three were used for measurement of soil pH (soil:water ¼ 1:2.5). NHþ 4 and NO3 concentrations were measured using indophenol blue method and colorimetric method. d15N value of NHþ 4 was analyzed using an isotope mass spectrometer (MAT-251, USA, with analytic precision 0.2 d15N) and calculated as follows:
d15 Nð&Þ ¼
i nh ð15 N=14 NÞsample ð15 N=14 NÞstandard o ð15 N=14 NÞstandard 1000
where ‘‘standard’’ refers to the atmospheric N2, which by definition has a d15N value of 0&. The addition of NHþ 4 had no or little effect on nitrification in the RF soil (Fig. 1e, f). Soil pH and d15N values of NHþ 4 showed little change at the end of the incubation (Fig. 2c; Fig. 3c). These results
200
100
100 0
0 3
7
14 21 28 42 56 0
3
7 14 21 28 42 56
Incubation time (day) NH4+ in treatment with controls +
NO3- in treatment with controls -1
NH4 in treatment with 250 mg N kg
NO3- in treatment with 250 mg N kg-1
Fig. 1. Changes of NHþ 4 -N and NO3 -N concentrations over 56-d incubation at 30 C with soil moisture of 65% WHC for soil RU(a, b), YU(c, d) and RF(e, f) after adding ammonium sulfate or urea at the rate of 0 (i.e. controls) and 250 mg N kg1. Standard deviation of replicates (n ¼ 3) is shown by bars.
21.6 46.2 32.2 10.2 0.36 3.00 4.8 Red pine forest Quaternary red earth 1750
19.3 74.0 6.70 14.8 1.33 11.6 5.8 Wheat/maize
35.7 43.7 11.1 0.50 4.00 4.3 Quaternary red earth
Quaternary Xiashu loess 1106
The fresh soil samples (0–20 cm) were passed though 2-mm sieve before incubation.
200
a
300
18.0
urea
300
0
Oilseed rape/peanut
Silt (v/v, %) Sand (v/v, %) CEC (cmol kg1) Total N (g kg1) Total C (g kg1)
400
f
ammonium sulfate
42
e
Mid-subtropical Jiangxi Province (28 15’2400 N, 116 50’3100 E)
400
RF
0 RF
15.4
0
7
100
North-subtropic Jiangsu Province (31 48’3900 N, 118 51’100 E)
100
YU
200
1750
200
18.0
300
-
300
-1
400 urea
d
ammonium sulfate
Table 1 Site description and soil properties.
+
-1
c
NO 3 concentration (mg N kg )
400
Annual rainfall (mm)
0 YU
43
100
Mid-subtropical Jiangxi Province (28 15’3000 N, 116 50’3000 E)
100
RU
200
Annual temperature ( C)
200
Altitude (m)
300
Location
300
Parent material
urea
0
NH4 concentration (mg N kg )
pH (water)
400
b
ammonium sulfate
Soilsa
a
Vegetation
RU
400
Clay (v/v, %)
implied that high acidity in the subtropical forest soil might have limited enzyme capacity for nitrification rather than availability of NHþ 4 (Zhao et al., 2007). Therefore, it is unlikely that elevated atmospheric NHþ 4 deposition would stimulate nitrification and, thereby, accelerate soil acidification. The RU soil was more acidic than the RF soil, however, nitrification of the RU soil was stimulated by NHþ 4 input (Fig. 1a and b), resulting in slight acidification in the soil (Fig. 2a). d15N values of NHþ 4 in the RU soil increased correspondingly (Fig. 3a), and significantly correlated to NO 3 concentration (Fig. 4a and b). Longterm cultivation in the upland soil RU might have improved the physicochemical and biological properties, thus facilitating microbial activity and enhanced nitrification. In fact, total C, total N, and CEC of the RU soil were consistently greater than those in the RF soil (Table 1). The response of the YU soil to NHþ 4 input was different from the RU soil, even though both were upland soils. Nitrification of the YU soil was greatly stimulated by NHþ 4 input (Fig. 1c and d). As a result, the soil was significantly acidified (Fig. 2b). d15N values of NHþ 4 increased markedly over 56 d of incubation (Fig. 3b), which showed a strong correlation with NO 3 concentration (Fig. 4c and d). The great difference in properties between YU and RU soils probably accounted for the response difference. Indeed, the YU soil had higher soil pH, total C, total N, and CEC than the RU soil (Table 1). Strong et al. (1997)
2585
20.6
X. Zhao, G.-x. Xing / Soil Biology & Biochemistry 41 (2009) 2584–2587
2586
X. Zhao, G.-x. Xing / Soil Biology & Biochemistry 41 (2009) 2584–2587
a
RU
a
6.5
RU 30 25 20 15
5.5
10 5 0
4.5
-5
N value of NH 4 (‰)
b YU
6.5
5.5
15
pH
b
+
3.5
YU 30 25 20 15 10 5 0 -5
c
4.5
RF 30 25 20 15
3.5
c
10
RF 6.5
5 0 -5 0
5.5
3
7
14
21
28
42
56
Incubation time(day) ammonium sulfate 15
4.5
3.5 0
3
7
14
21
28
42
56
urea
NHþ 4
Fig. 3. Changes of d N values of over 56-d incubation at 30 C with soil moisture of 65% WHC for soil RU(a), YU(b) and RF(c) after adding ammonium sulfate or urea at the rate of 250 mg N kg1. Standard deviation of replicates (n ¼ 3) is shown by bars. d15N values of NH4þ were not detected in the soils with addition of urea at day 0 because the amount of NHþ 4 -N was not enough for isotope mass spectrometry analysis.
Incubation time (day) pH in treatment with controls pH in treatment with ammonium sulfate pH in treatment with urea Fig. 2. Changes of soil pH over 56-d incubation at 30 C with soil moisture of 65% WHC for soil RU(a), YU(b) and RF(c) after adding ammonium sulfate or urea at the rate of 0 (i.e. controls) and 250 mg N kg1. Standard deviation of replicates (n ¼ 3) is shown by bars.
investigated nitrification in an Australian red earth soil, the properties of which are quite different from those in the current study. However, they concluded that nitrification in the soil with KHCO3þbuffer (in order to maintain soil pH at 6.3 for a long period) proceeded faster than in the control soil (pH 5.3) and the soil with KHCO3 (pH 6.3). The results from YU and RU soils suggested that special attention should be given to cultivated soils, which may undergo nitrification and acidification induced by increasing application of ammonium-based chemical fertilisers or atmospheric NHþ 4 deposition to subtropical regions. There is some evidence that a one-unit decline of soil pH has occurred in cultivated soils of the Yangtze River Delta in China over the past two decades (Shao et al., 2006; Wang et al., 2008). They suggested that chemical N fertilisers
especially ammonium-based were likely to be a critical factor influencing this acidification. For RU and YU soils, urea (Fig. 1b and d) was more effective in stimulation of nitrification than (NH4)2SO4 (Fig. 1a and c). This result was consistent with previously reported results (Martikainen, 1985; De Boer et al., 1988). This might be because the hydrolysis of urea leads to increase soil pH locally (Fig. 2a and b). The higher correlation coefficients between d15N values of NHþ 4 and NO 3 concentration in the urea-treated samples than those in the (NH4)2SO4-treated samples hinted that urea had greater stimulatory effect of nitrification than that of (NH4)2SO4 (Fig. 4). The increased d15N values of NHþ 4 were closely related to the nitrifying activity of soils in nitrification process, suggesting that N kinetic isotope fractionation might be an effective tool for investigating the effects of NHþ 4 input on nitrification.
Acknowledgements We thank two anonymous reviewers for their valuable suggestions that have greatly improved the manuscript. This work was supported by National Key Technology R&D Program of China (2007BAD87B07) and National S&T Major Project of China (2008ZX07101-005).
X. Zhao, G.-x. Xing / Soil Biology & Biochemistry 41 (2009) 2584–2587
2587
RU 30
30
a
25
b
urea 25 y = 0.1461x - 1.9644
20
ammonium sulfate
20
R2 = 0.9782 (P<0.0001)
15
15
10
10
5
5
+
N value of NH 4 (‰)
y = 0.1379x + 1.3461 R2 = 0.696 (P=0.0100)
0
0
0 30
30
60
15
120
0
150 YU 30
c
25
90
urea
d
25
20
20
15
15
10
y = 0.1598x - 12.421
10
5
R2 = 0.9908 (P<0.0001)
5
10
5
15
20
25
30
ammonium sulfate
y = 0.0768x - 2.4874 R2 = 0.8506 (P=0.0011)
0
0
0
50
100
150
200
250
300 -
0
40
80
120
160
-1
NO 3 concentration (mg N kg ) 1 Fig. 4. Relationships between d15N values of NHþ 4 -N and NO3 -N concentration in RU and YU soils after adding ammonium sulfate or urea at the rate of 250 mg N kg .
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