BUFFERING MECHANISM AND SENSITIVITY TO ACID DEPOSITION OF SOILS OF AKWA IBOM STATE, NIGERIA

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1 BUFFERING MECHANISM AND SENSITIVITY TO ACID DEPOSITION OF SOILS OF AKWA IBOM STATE, NIGERIA ABSTRACT Akpan *, U. S. and Udoh, B. T. Department of Soil Science and Land Resources Management, University of Uyo,Uyo, Akwa Ibom State, Nigeria The objectives of this study were to identify and classify the buffering mechanisms and sensitivity to acid deposition of soils of Akwa Ibom State. Four parent materials including coastal plain sands, beach ridge sands, river alluvium and sandstone and shale were selected for the study. In each parent material, three profile pits were sunk at representative locations.. Soil samples were collected from each genetic horizon. A total of 12 profile pits were sunk and 62 samples were generated for laboratory analysis. The study revealed that aluminium was the buffering mechanisms in beach ridge sand soils while cation exchange and aluminium were the buffering mechanisms in sandstone and shale and river alluvium soils. Also, aluminium and cation exchange were the major buffering mechanisms in coastal plain sand soils. Coastal plain sand, river alluvium and sandstone and shale soils had higher buffering capacity than beach ridge sand soils. The study also showed that beach ridge sand and sandstone and shale soils were highly sensitive to acidification. Coastal plain soil was moderately sensitive while soils of river alluvium varied from low to high in sensitivity to acid deposition depending on the composition of the depositional materials. Keywords: Buffering mechanism, soil sensitivity, soil buffering INTRODUCTION Buffering mechanisms are materials involved in the neutralization of acid or base when acid or alkaline is added to the soils (Nawaz et al., 2011). It determines the acid or base neutralizing ability of soils or buffering capacity of soil (ability of soil to resist a change in ph). Buffering mechanisms are made up of mineral and organic constituents of soils such as primary minerals (e.g. carbonate), clay and organic matter content. These mixtures of mineral and organic constituents of soils determine the capacity of the soil to resist a change in soil ph. Different soils have different composition of primary minerals, clay and organic matter content due to differences in parent materials, climate, vegetation, cropping history, time etc with attendant differences in buffering mechanisms, and different buffering capacities (Spark, 2003). In alkaline soils (ph above 7), carbonate minerals such as CaCO 3 and Na 2 CO 3 are materials involved in neutralization of acid deposition. The ability of the alkaline soil to buffer depends on the amount of these minerals present in the soil. In acid soils (ph below 5), aluminium take over the buffering role. Aluminum compound (e.g. Al(OH) 3 ) react with hydrogen ions to release aluminium ion. In other words, for every aluminium ion, three hydrogen ions can be consumed, thus ph changes at a very slow rate in comparison to the acidification reactions. In neutral to moderately acid soils (ph 5-7), the ability of the soil to buffer depends on ions exchange reactions associated with clay and organic matter. The added acid cations (H + and Al 3+ ) exchange with base cations from clay or humus exchange sites. The exchangeable base cation buffering mechanism does not actually neutralize the acidity but merely stores it in the soil reserve acidity pool (Merry, 2001). The higher the organic matter and layer silicate clay content, the higher the buffering capacity. Different soils have different degrees of responses to acid deposition (soil sensitivity to acidification) due to variation in materials involved in the neutralization process. This also account for variation in soil vulnerability to acid deposition. The most significant soil properties responsible for soil sensitivity to acidification and neutralization of acid deposition are cation exchange capacity (CEC) and base saturation (Nawaz et al., 2011). Kuylenstiema et al. (2001) used cation exchange capacity and base saturation in the determination of soil sensitivity and vulnerability to acid deposition in selected soils of Northeast region of Thailand. They observed that soils with high cation exchange capacity and high base saturation were insensitive to basic cations depletion, while soils with low cation exchange capacity and moderate base saturation were sensitive to basic cations depletion. They also observed that basic cations were not depleted in soils with very low base saturation irrespective of CEC. Soils with CEC between 0-3 cmol kg -1, base saturation is less meaningful agronomically. This is because the holding power of the soil is so low that even insignificant amount of basic cations will result in a relatively high saturation (Kuylenstiema et al., 2001). Adverse changes in soil ph can affect crop growth due to variety of reasons. Soil acidification caused a decrease in soil ph of the surface soil by replacing basic cations (Ca 2+, Mg 2+ and K + ) on soil exchange sites with hydrogen ions and the basic cations may be lost through leaching. Soil acidification also brings about upsurge in the concentrations of Al, Cu, Fe, Zn, B, Mn, Cr, and Ni up to the levels that may be phyto-toxic and reduces the availability of P through fixation. NJAFE VOL. 11 No. 3,

2 When acid rain falls on soils that cannot neutralize the acidity, surface waters become more acidic. The acid disrupt the productive cycle of fish, harms aquatic plants and micro-organisms, cause decline and dieback of forest trees and accelerates the rusting of corrugated iron sheet roofs (Stevens et al., 2009). Hence, the objectives of this study were to identify and classify the different buffering mechanisms and soil sensitivity to acid deposition, based on parent materials in Akwa Ibom State, Nigeria. MATERIALS AND METHODS Description of the study area Akwa Ibom State is located in the southeastern Nigeria and enclosed within latitudes 4 30 ' and 5 30 ' N and longitudes 7 30 ' and 8 20 ' E. The State is underlain mainly by coastal plain sands, beach ridge sands, sandstone / shale and alluvial parent materials (Petters et al., 1989). Physiographically, the landscape comprises of a low plain and riverine area with almost no portion of the State exceeding 175 m above sea level. The climate is humid tropical with annual rainfall varying from 3000 mm along the coast to about 2250 mm at the extreme north with 1-3 dry months in the year. Mean annual temperature varies between C with relative humidity of %. Field work Four parent materials were selected for the study. In each parent material, three profile pits were sunk at representative locations. The parent materials were coastal plain sand, river alluvium, beach ridge sands and sandstone/shale. A total of 12 profile pits were sited and soil samples were collected based on genetic horizons for laboratory analysis. Laboratory analysis The samples were air dried, pulverized and sieved with a 2 mm sized sieve. The samples were analyzed in the laboratory for the following properties: Particle size distribution was determined by the hydrometer method as described by Gee and Bauder (1986). Soil ph was determined in water 1:2.5 soils: water ratio using ph meter with glass electrode (Thomas, 1996). Exchangeable cations- the bases were extracted with neutral NH 4 OAC. Calcium and magnesium were determined in the extract by EDTA titration and potassium and sodium by the use of flame photometer (Udo et al., 2009). Exchangeable acidity was extracted with one normal potassium chloride solution. The exchangeable acidity and the exchangeable aluminium were determined by titration as described by Thomas (1996). The exchangeable hydrogen was obtained by subtracting exchangeable aluminium from exchangeable acidity. (Exchangeable acidity (Al +H) Exchangeable Al = Exchangeable H). Organic carbon was determined by the dichromate wet-oxidation method as described by Nelson and Sommers (1996). The values were multiplied by to obtained organic matter content. Available phosphorus was determined by the Bray-1 method as described by Kuo (1996). Total nitrogen was determined by the micro-kjeldahl digestion and distillation method as described by Bremner (1996). Cation exchange capacity was determined by method described by Sumner and Miller (1996). Percentage base saturation was calculated using the formula: summation of exchangeable bases / CEC 100 Rating of Soil Buffering Mechanisms and sensitivity to acid deposition Soil buffering mechanism and sensitivity to acid deposition of the study area were determined by comparing the soil ph, base saturation and cation exchange capacity of the study area with the soil buffering mechanism and sensitivity to acid deposition rating recommended by Nawaz et al. (2010) as shown in Tables 1 and 2. Table 1: Rating of soil buffering mechanisms Descriptive terms Class Soil ph range Buffering mechanism Extremely acid 1 <4.5 Iron range Very strongly acid Aluminium / iron range Strongly acid Aluminium range Moderately acid 1V Cation exchange Slightly acid to neutral V Silicate buffers Slightly alkaline V Silicate buffers Source: (Nawaz et al., 2010) RESULTS AND DISCUSSION Physical and chemical properties of soils of the study Area Soils of beach ridge sand parent material The minimum, maximum and mean of soil properties of beach ridge sand parent material are presented in Table 3.The sand fraction varies from 89.9 to 95.9 %, silt varies from 0.06 to 3.9 % while clay varies from 3.9 to 8.2 %, indicating sandy texture throughout the profile. The soil ph varied from 3.2 to 5.4, indicating the range of extremely to strongly acid. Organic matter varied from 1.0 to 4.6 % indicating low to high throughout the profile. NJAFE VOL. 11 No. 3,

3 Total N varies from 0.04 to 0.1%, indicating very low to moderate. Available P varied from low to moderate (3.3 to 11.9 mg kg -1 ) throughout the profile. Exchangeable Ca ( cmol kg -1 ), Mg ( cmol kg -1 ) K ( cmokg -1 ) and Na ( cmol kg -1 ) were low throughout the profile. Cation exchange capacity was very low ( cmol kg -1 ) throughout the profile. Base saturation was moderate throughout the profile ( %). Table 2: Rating of soil sensitivity to acid deposition Base saturation Cation Exchange capacity (CEC) (cmol kg -1 ) (%) < > I II II I II III II III IV III IV V V V V Source: (Nawaz et al., 2010) Key: Class I: Very high sensitivity or vulnerability II: high sensitivity or vulnerability sensitivity or vulnerability IV: Low sensitivity or vulnerability V: Very low sensitivity or vulnerability III: moderate Table 3: The mean, minimum and maximum of soil properties of beach ridge sand parent material Sand % Silt % Clay % ph Organic matter % Total nitrogen % Available phosphorus mg kg Exchangeable calcium cmol kg Exchangeable. magnesium cmol kg Exchangeable. sodium cmol kg Exchangeable potassium cmol kg Exchangeable Acidity cmol kg Effective cation exchange capacity cmol kg Base saturation % Soil of coastal plain sand parent material The minimum, maximum and mean of soil properties of coastal plain sand parent material are presented in Table 4. The sand fraction varies from 60.0 to 94.6 %, silt varies from 1.8 to 18.0 % while clay varies from 4.2 to 26.0 %, indicating sandy loam to sand texture. Soil ph varied from 4.9 to 6.5, indicating strongly to slightly acid throughout the profile. Organic matter varied from low to very high ( %). Total N varied from very low to medium ( %). Available P varied from low to high ( mg kg -1 ) throughout the profile. Exchangeable Ca ( cmol kg -1 ), Na ( cmol kg -1 ) and K ( cmol kg -1 ) were low. Exchangeable Mg ( cmol kg -1 ) varied from low to moderate. Cation exchange capacity was low throughout the profile ( cmol kg -1 ). Base saturation varied from moderate to high throughout the profile ( %). Soils of sandstone and shale parent material The minimum, maximum and mean of soil properties of sandstone and shale parent material are presented in Table 5. The sand fraction varies from 50.0 to 90.0 %, silt varies from % while the clay varies from 7.0 to 40.0%, indicating sandy clay to sand. Soil ph varied from 4.0 to 5.6, indicating very strongly acid to moderately acid throughout the profile. Organic matter varied from low to moderate ( %). Total N varied from very low to moderate ( %) throughout the profile. Available P varied from moderate to high ( mg kg -1 ). Exchangeable Ca ( cmol kg -1 ), Na ( cmol kg -1 ) and K ( cmol kg -1 ) were low throughout the profile. Exchangeable Mg varied from low to moderate ( cmol kg -1 ). Cation exchange capacity was low throughout the profile ( cmol kg -1 ). Base saturation varied from low to high ( %). Soils of alluvial parent material The minimum, maximum and mean of soil properties of alluvial parent material are presented in Table 6. The sand fraction varies from %, silt varies from % while clay varies from %, NJAFE VOL. 11 No. 3,

4 indicating clay to sandy clay loam. Soil ph varied very strongly acid to moderately acid throughout the profile ( ). Organic matter varied from low to high ( %). Total N varied from low to moderate ( %). Available P varied from low to high ( mg kg -1 ). Exchangeable Ca ( cmol kg -1 ) and Mg ( cmol kg -1 ) varied from low to moderate throughout the profile. Exchangeable Na ( cmol kg -1 ) and K ( cmol kg -1 ) were low throughout the profile. Cation exchange capacity varied from low to high ( cmol kg -1 ). Base saturation varied from very low to high ( %). Table 4: The mean, minimum and maximum of soil properties of coastal plain sand parent material Sand % Silt % Clay % ph Organic matter % Total nitrogen % Available phosphorus mg kg Exchangeable calcium cmol kg Exchangeable. magnesium cmol kg Exchangeable. sodium cmol kg Exchangeable potassium cmol kg Exchangeable Acidity cmol kg Effective cation exchange capacity cmol kg Base saturation % Table 5: The mean, minimum and maximum of soil properties of sandstone and shale parent material Sand % Silt % Clay % ph Organic matter % Total nitrogen % Available phosphorus mg kg Exchangeable calcium cmol kg Exchangeable. magnesium cmol kg Exchangeable. sodium cmol kg Exchangeable potassium cmol kg Exchangeable Acidity cmol kg Effective cation exchange capacity cmol kg Base saturation % Buffering mechanisms of soils of the study area The buffering mechanisms of soils of the study area are presented in Table 7. The ph values of soils developed from sandstone and shale and river alluvium parent materials fall within the ph range of 4.6 to 5.7 (very strongly to moderately acid). This range falls within class II- IV of the buffering mechanism rating (Table 1). This is an indication that aluminium, iron and cation exchange were the major buffering mechanism in these soils. In beach ridge sand parent material, the soil ph falls within the range of 3.2 to 5.4 (extremely to strongly acid). This range falls within class I-II. This is an indication that iron and aluminium were the major buffering mechanisms in these soils. In coastal plain sand soils, the soil ph falls within the range of 5.3 to 6.5 (strongly to slightly acid). This range falls within class III, IV and V. This is an indication that aluminium, cation exchange of kaolinitic silicate clay and organic matter were the major buffering mechanisms in these soils. The implication is that when acid is added to the soils of the study area, there will be an upsurge in the concentrations of Al, Cu, Fe, Zn, B, Mn, Cr, Ni and reduction in the availability of P through fixation. The degree of the upsurge of these elements will be highest in soils of beach ridge sand, followed by soils of alluvium, sandstone and shale and followed by soils of coastal plain sand. The trend is as followed: soils of beach ridge sand soil > soils of river alluvium = soils of sandstone and shale > soils of coastal plain sand. Soil sensitivity to acidification NJAFE VOL. 11 No. 3,

5 Based on the rating of soil sensitivity to acidification of Nawaz et al. (2010) shown in Table 2. In beach ridge sand soils, CEC was low while base saturation was moderate, indicating high sensitivity or high vulnerability to acid deposition. In coastal plain sands soils, CEC was low while base saturation varied from moderate to high, indicating moderate sensitivity or moderate vulnerability to acid deposition. Cation exchange capacity of sandstone and shale soils was low while base saturation was moderate, indicating high sensitivity or high vulnerability. In soils developed from river alluvium, CEC and base saturation varied from low to high, indicating low to high sensitivity or vulnerability depending on the composition of the depositional materials that formed the soil. The trend in soil sensitivity to acidification in the study area based on the rating was as followed: beach ridge sand > sandstone and shale > coastal plain sand. The river alluvium depends on the composition of the depositional materials that formed the soil. The high sensitivity or vulnerability to acid deposition of beach ridge sands could be attributed to the high content of oxides of Fe and Al and low clay and organic matter content (exchange sites). This is in agreement with Kuylenstiema et al, (2001) that soils with low cation exchange capacity and moderate base saturation were sensitive to base cation depletion. The moderate sensitivity to acidification of coastal plain sand and river alluvium could also be attributed to cation exchange of kaolinitic silicate clay and organic matter being the major buffering mechanisms. But the higher the organic matter and layer silicate clay content, the higher the buffering capacity and the lower the sensitivity to acidification. Table 6: The mean, minimum and maximum of soil properties of alluvial parent material Sand % Silt % Clay % ph Organic matter % Total nitrogen % Available phosphorus mg kg Exchangeable calcium cmol kg Exchangeable. magnesium cmol kg Exchangeable. sodium cmol kg Exchangeable potassium cmol kg Exchangeable Acidity cmol kg Effective cation exchange capacity cmol kg Base saturation % Table 7: Buffering mechanisms of soils of the study area Parent materials Soil ph Buffering mechanism Beach ridge sand Iron / aluminium Coastal plain sand Aluminium/cation exchange of silicate clay and organic matter Sandstone and shale Aluminium /cation exchange of silicate clay Alluvium Aluminium /cation exchange of silicate clay CONCLUSION The results of the rating of buffering mechanisms showed that iron and aluminium were the major buffering mechanisms in beach ridge sands while cation exchange and aluminium were the major buffering mechanisms in sandstone and shale and river alluvium soils. Also, aluminium and cation exchange were the major buffering mechanisms in coastal plain sand soils. The study also showed that beach ridge sand and sandstone and shale soils were highly sensitive to acidification. Coastal plain sands was moderately sensitive while soils of river alluvium varied from low to high in sensitivity to acid deposition depending on the composition of the depositional materials. REFERENCES Bremner, J. M Total Nitrogen. In: Spark, D. L. (eds.). Methods of Soil Analysis Part 3-Chemical Methods, SSSA Book Series 5, Madison, Wisconsin, USA. pp FAO Guidelines for Soil Descriptions, 3 rd edition. FAO Rome. Gee, G. W. and Bauder, J. W Particle size analysis. In: Methods of Soil Analysis, Part 1-Physical and Mineralogical Methods, SSSA Book Seria 5, Madison, Wisconsin, USA. pp NJAFE VOL. 11 No. 3,

6 Kuo, S Phosphorus. In: Spark, D. L. (eds.). Methods of Soil Analysis Part 3. Chemical Methods, SSSA Book Series 5, Madison, Wisconsin, USA. 869pp Kuylenstiema, J. C. I., Rode, H., Cinderby, S. and Hicks, K Acidification in developing countries; ecosystem sensitivity and the critical load approach on a global scale. Ambio, 30(1): Merry, R. H Acidity and alkalinity of soils. Environmental and Ecological Chemistry, Vol. 11. Encyclopaedia of Life Support Systems. pp Nawaz, R., Tripathi, N. K. and Parkpian, P Soil acidity ranking, soil sensitivity and vulnerability to acid deposition in the Northeast region of Thailand. Biology, Environment and Chemistry. Vol.1, IACSIT Press, Singapore. Nelson, D. M. and Sommers, L. E Total Carbon, Organic Carbon and Organic Matter. In:.Spark, D. L (eds.). Methods of Soil Analysis Part 3- Chemical Methods, SSSA Book Series 5, Madison, Wisconsin, USA. pp Sparks, D. L Environmental Soil Chemistry, 2 nd edition. Academic Press, Elsevier Science, Amsterdam. Petters, S. W., Usoro, E. J., Udo, E. J., Obot, U. W. and Obot, S. N Akwa Ibom State. Physical Background, Soils and Landuse and Ecological Problems.Technical Report of the Taskforce on Soils and Landuse. Govt. Printer, Uyo. 602pp. Stevens, C. J., Dis, N. B. and Gowing, D. J Regional trends in soil acidification and exchangeable metal concentration in relation to acid deposition rates. Enviro. Pollu. Vol.157. pp Sumner, M. E. and Miller, W. P Cation Exchange Capacity and Exchangeable Coefficients. In: Spark, D. L. (eds.). Methods of Soil Analysis Part 3-Chemical Methods, SSSA Book Series 5, Madison, Wisconsin, USA. pp Thomas, G. W Soil ph and Soil Acidity. In: Sparks, D. L. (eds.). Methods of Soil Analysis Part 3- Chemical Methods, SSSA Book Series 5, Madison, Wisconsin, USA. pp Udo, E. J., Ibia, T. O., Ogunwale, J. A., Ano, A. O. and Esu, I. E Manual of Soil, Plant and Water Analysis. Sibon Books Limited, Lagos.178pp. NJAFE VOL. 11 No. 3,