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Sodium Chloride induced salinity reduces the microbial processes in soil

Zahir Shah1 and Syed Asif Shah2

1 Department of Soil and Environmental Sciences, Khyber Pakhtunkhwa Agricultural University Peshawar, Pakistan, www.aup.edu.pk; Email zahirshah@aup.edu.pk
2
Department of Soil and Environmental Sciences, Khyber Pakhtunkhwa Agricultural University Peshawar, Pakistan, Email asif_soilscience@yahoo.com

Abstract

Salts accumulation in soil may seriously affect the biological processes and N dynamics in soil. The effect of sodium chloride induced salinity on CO2 evolution and N mineralization was assessed in a silty clay loam soil during laboratory incubations. Soil was amended with NaCl salt to increase the electrical conductivity of soil from 0.65 dSm-1 to 40 dS m-1. Both CO2 evolution and N mineralization decreased significantly with increasing NaCl salinity, and the reduction was proportional to the NaCl levels. The results showed that increasing NaCl salinity from 0.65 to 40 dS m-1 decreased cumulative CO2 production by 56% and N mineralization by 51% during 40 days of incubation. These results demonstrated that soil microorganisms were highly sensitive to NaCl salinity indicating that salinity is a stress factor and can reduce microbial diversity and control microbial abundance, composition and functions.

Key Words

Key Words: NaCl salinity, CO2 evolution, N mineralization

Introduction

Soil salinity is a worldwide problem. More than 40% of soils in the world are faced with salinity problems. It is a serious issue and gradually increasing mostly in arid and semiarid regions of the world. Currently, out of 1.5 billion hectares of cultivated land around the world, nearly 77 million hectares is affected by various types of salinity (Evelin et al. 2009). Salts accumulation in soil may seriously affect the biological processes as well as the N dynamics in soil and exert negative influence on the establishment, growth and development of plants (Sarig and Steinberger 1994). Several studies have reported negative impact of salinity on various soil microbial attributes (e.g., Tripathi et al. 2006). Recently Shah and Shah (2012) found that increasing magnesium chloride salinity beyond 4 dSm-1 reduced N mineralization in soil by almost 80% during 40 days of incubation. This paper report the effect of NaCl induced salinity on CO2 evolution and N mineralization in a silty clay loam soil in north Western Pakistan.

Methods

Experimental Site

Soil sample (0-20 cm) was collected from a fallow field in November, 2011 from the Research Farm of Khyber Pakhtunkhwa Agricultural University Peshawar, Pakistan. The site was situated near the eastern end of the Khyber Pass on the Iranian plateau with geographical coordinates 34 0' 28" North, 71 34' 24" East with an elevation of 329 meters. The area has a warm to hot, semi-arid, sub-tropical, continental climate. The average annual rainfall in the area is 360 mm with winter dominance. The soil was non-saline (EC=0.65 dS m-1), alkaline (pH 8.2) and low in organic fertility (Shah and Shah 2012).

Incubation Experiments

Soil sample was amended with NaCl salt to raise the electrical conductivity (EC) of soil to 0, 4, 8, 12, 16, 20, 30 and 40 dS m-1. After thorough mixing of NaCl solution with soil, two sets of same treatments were prepared. One set was run for the determination of CO2 evolution and the other for N mineralization. The CO2 and N mineralization in soil samples were measured at 28C during 10, 20, 30 and 40 days of incubations.

Statistical Analysis

Analysis of variance was conducted using a Completely Randomized Design. Means significance at the 1% level were tested using LSD test. The reported values are means of three replications.

Results

Rate of Soil Respiration

The NaCl amendments significantly (P<0.01) reduced the rate of CO2 evolution compared with the control treatment (Table 1). The reduction in rate of CO2 evolution ranged from 10.1% at 4.0 dS/m to 89.3% at 40 dS/m during 10 days, from 21.6 to 89.9% during 20 days, from 20.8 to 80.4%, during 30 days and from 17.5to 83.3% during 40 days of incubation period compared with the control treatment.

Table 1. Rate of CO2 evolution (mg/kg soil/ day) as influenced by NaCl induced salinity.

Salinity levels

EC dS/m

Incubation period (days)

10

20

30

40

 

% Reduction

 

% Reduction

 

% Reduction

 

% Reduction

0

126

-

60

-

64

-

77

-

4

113

10

47

22

50

21

64

17

8

88

30

47

22

43

33

47

39

12

65

48

35

42

33

49

36

53

16

54

57

29

52

30

53

37

51

20

47

62

25

59

20

68

29

63

30

41

68

16

73

23

64

26

66

40

13

89

6

90

12

80

13

83

% Reduction in CO2 production = [(control – treatment)/control] x 100

Cumulative CO2 Evolution

The cumulative CO2 evolution decreased significantly (P<0.01) during 40 days of incubation period (Table 2). The maximum cumulative CO2 of 328 mg/kg soil was produced in the control treatment. However, the cumulative CO2 production decreased gradually with increasing salt concentration. The results showed that cumulative CO2 production decreased from 328 mg at EC of < 4 dS/m to only 45 mg/kg soil at EC 40 dS/m during 40 days of incubation period.

Table 2. Cumulative CO2 evolution (mg/kg) during 40 days of incubation periods as influenced by NaCl induced salinity.

Salinity levels

EC dS/m

Incubation period (days)

10

20

30

40

 

% Reduction

 

% Reduction

 

% Reduction

 

% Reduction

0

126

-

188

-

251

-

327

-

4

113

10

161

14

211

16

275

16

8

88

30

135

28

178

29

224

31

12

65

48

100

46

133

47

169

48

16

54

57

83

55

113

55

150

54

20

47

62

72

61

92

63

121

63

30

41

68

57

69

80

68

105

68

40

13

89

20

89

32

87

45

86

% Reduction in CO2 production = [(control – treatment)/control] x 100

Nitrogen Mineralization

The NaCl amendments significantly (P<0.01) reduced the N mineralization in soil during 40 days of incubation period (Table 3). The highest reduction in N mineralization occurred in soil with NaCl at EC > 30 dS/m (Table 6). The results demonstrated that increasing salinity to 4 dS/m reduced N mineralization by 16.2% during 10 days of incubation period. The corresponding decrease in N mineralization with further NaCl salt concentration reached 55% at EC 40 dS/m compared with the control treatment during 10 days of incubation period. The extent of reduction in mineralization due to NaCl salinity during 20, 30 and 40 days of incubation was almost similar to that during the first 10 days of incubation.

Table 3. N-mineralization (mg/kgsoil) during 40 days of incubation periods as influenced by NaCl induced salinity.

Salinity level

(EC Ds/m)

N mineralization during 10 days

N mineralization during 20 days

N mineralization during 30 days

N mineralization during 40 days

-

% reduction

-

% reduction

-

% reduction

-

% reduction

0

38

---

70

---

107

---

129

---

4

32

16

56

20

85

20

115

11

8

35

9

58

18

81

24

111

14

12

31

18

58

17

77

28

94

27

16

21

45

43

39

68

37

86

33

20

21

46

39

45

65

39

78

40

30

15

61

32

55

45

58

58

55

40

17

55

35

50

49

54

63

51

% Reduction in N mineralization = [(control – treatment)/control] x 100

Discussion

The depressive effect of NaCl salt on CO2 evolution could be due to one or more of the several reasons. Microbial activities can be depressed due to the specific ion toxicities of Na+ and Cl- (Zahran, 1999), both ion toxicities and high soil pH under sodic conditions (Rietz and Hayness (2003) or deteriorated physical soil conditions (Minhas et al., 2007). Other reason for the reduced activity of soil microorganisms in salt affected soils could be the osmotic stress caused by high concentrations of salts in soil solution. Our results on CO2 evolution are in line with the findings of Shah and Shah (2012) where MgCl2 salinity significantly reduced the rate of CO2 evolution under similar soil and environmental conditions and also with Tripathi et al. (2006). The reduction in N mineralization with NaCl salinity in our study also demonstrates the detrimental effect of NaCl salt on soil microbial activity and hence reduced rate of N mineralization could be associated with reduced microbial activity in soil caused by NaCl salt.

Conclusion

This experiment has demonstrated that NaCl induced salinity significantly reduced the microbial activity in terms of soil respiration and release of mineral N from soil organic matter during 40 days of incubation period.

References

Evelin H, Kapoor R and Giri B (2009). Arbuscular mycorrhizal fungi in alleviation of salt stress: a review. Annals of Botany 104, 1263-1280.

Minhas PS, Dubey SK and Sharma DR (2007). Comparative effects of blending, intera/inter-seasonal cyclic uses of alkali and good quality water on soil properties and yields of paddy and wheat. Agricultural Water Management 87, 83-90.

Rietz DN and Haynes RJ (2003). Effect of irrigation-induced salinity and sodicity on soil microbial activity. Soil Biology and Biochemistry 35, 845-854.

Sarig S and Steinberger Y (1994). Microbial biomass response to seasonal fluctuation in soil salinity under the canopy of desert halophytes. Soil Biology and Biochemistry 26, 1405–1408.

Shah SA and Shah Z (2012). Effect of MgCl2 induced salinity on CO2 evolution and N mineralization in a silty clay loam soil. Communications in Soil Science and Plant Analysis (In press).

Tripathi S, Kumari S, Chakraborty A, Gupta A, Chakrabarti K and Bandyapadhyay BK (2006). Microbial biomass and its activities in salt-affected coastal soils. Biology and Fertility of Soils 42, 273-277.

Zahran HH (1999). Rhizobium-legume symbiosis and nitrogen fixation under severe conditions and in an arid climate. Microbiology and Molecular Biology Reviews 63, 968–989.

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