International Journal of Agriculture and Forestry

p-ISSN: 2165-882X    e-ISSN: 2165-8846

2016;  6(3): 123-131

doi:10.5923/j.ijaf.20160603.04

 

Assessment of Mixed Tree Rehabilitation Impacts on Soil Physicochemical Properties of Degraded Landscapes

Wondwossen Gebretsadik

Central Ethiopia Environment and Forest Research Center, Ethiopia

Correspondence to: Wondwossen Gebretsadik , Central Ethiopia Environment and Forest Research Center, Ethiopia.

Email:

Copyright © 2016 Scientific & Academic Publishing. All Rights Reserved.

This work is licensed under the Creative Commons Attribution International License (CC BY).
http://creativecommons.org/licenses/by/4.0/

Abstract

The present study highlights the effect of slope and tree species on soil properties of degraded semi arid hills at Kuriftu Lake catchment. Five native broad leaves and legumes were planted with conservation structures on slopes: 0-17% and 17-23%. After seven years of rehabilitation soil sampling pits were randomly assigned within a 50 m2 plot on planting space of a given tree species and samples were collected at three distinct depths (0-15, 15-30 and 30-50cm) to be homogenized later by depth. Slope and soil depth changes impacted significant differences in soil properties under canopies of the study trees. TN and OM significantly declined from surface (0-15cm) to inner surface (30-50cm). OM on surface soils of broadleaves was significantly higher than tree legumes. Lower slopes had significantly higher pH than higher slopes and there was significant negative correlation (-0.65) between slope and pH. CEC under evaluated trees was significantly higher on lower slopes than higher slopes. Slope had significantly higher negative correlations with CEC, exchangeable Ca, Mg, Na and pH. OM and TN had no significant correlations with slope. BD was greater on higher slopes than lower slopes though tree species induced no change on texture. Surface, sub surface and inner surface MC in lower slopes were significantly higher than con specifics in higher slopes.

Keywords: Cation exchange capacity, Bulk density, Moisture content, Organic matter

Cite this paper: Wondwossen Gebretsadik , Assessment of Mixed Tree Rehabilitation Impacts on Soil Physicochemical Properties of Degraded Landscapes, International Journal of Agriculture and Forestry, Vol. 6 No. 3, 2016, pp. 123-131. doi: 10.5923/j.ijaf.20160603.04.

1. Introduction

The severity of erosion on hills depends on a combination of many factors, including the amount and intensity of precipitation, the texture of the soil, the steepness of the slope, and the amount of ground cover (Hillel, 1998). Topographic factors such as the orientation of the hill slope and the steepness of the slope affect the microclimate, vegetation establishment and water movement (Rasool et al., 2014). Changes in soil profiles are due to the variations in soil forming factors along the slope. Geomorphic processes such as drainage of ground water, sediment transport, and removal of mobile chemical elements are all important in the variation in soils over the slope (Ritter 1986; Rasool et al., 2014). Accordingly the growth condition and distribution of vegetation types in different slope positions are controlled by the bioavailability of soil nutrients (Kubota et al., 1998; Tsui et al., 1999). The removal of trees without sufficient reforestation has resulted in damage to habitat and biodiversity loss accompanying with increasing soil compaction, erosion and decrease in soil fertility (Geist and Lambin, 2002; Williamson and Neilsen, 2000; Abdu. et al 2013). Deforestation exposes soil to erosion, fertility depletion, compaction and landslides (Grainger, 1993; Abdu. et al 2013).
The most viable countermeasures to reverse degraded hillsides to better productive areas are rehabilitation activities that culminate with plantation establishment. Plantation forest establishment through rehabilitation activities are important tools from a global perspective in terms of wood and ecosystem resources and eco-friendly environment services (Arifin et al., 2008a; 2008b; Cole et al., 1996; McNamara et al., 2006; Akbar. et al 2013). Rehabilitation commonly involves plantation of native and exotic species on degraded land and attempts to return the forest to a stable and productive condition in terms of soil fertility status, but not necessarily the original diversity, structure and function (Abdu et al., 2010). Successes in most works of degraded land rehabilitation programs in Ethiopia mainly focused on evaluating the performance of adaptive species for planting purposes. Several studies have been conducted to characterize physicochemical properties of forest soils in Ethiopia. However assessing changes in soil properties on areas under rehabilitation programs as influenced by tree species, slope, soil depth and related parameters is limited. Empirical data on soil characteristics under rehabilitation program are still lacking to evaluate the successes of rehabilitation programs through quantifiable bio physical data that reflect tree- soil synergies. In this study it was hypothesized that slope and tree species changes impact/induce no change in soil physicochemical properties of rehabilitated landscapes. Therefore the objective of the study was to assess mixed tree rehabilitation impacts by analyzing variabilities of selected soil physicochemical properties as influenced by tree species, landscape position, soil depth and related soil parameters.
This would provide information on changes in soil properties under different tree species along slope gradient and give a clue as to which tree species plays a vital role in soil amelioration while contributing to develop a sound basis to objectively evaluate the success of rehabilitation programs.

2. Materials and Methods

2.1. General Description of Study Area

The present study area which is Kuriftu lake catchment is located in Ada woreda, East Shoa zone of Oromia, Ethiopia. The total area of the lake is 4 ha at an elevation of 1,883m a.s.l. being located at 846′28″N and 3900′38″E. Total annual precipitation at the lake is 745.6 mm. The soil on the rehabilitated hills of the study area is characterized as Vertic Andosol, a very fine textured soil of volcanic origin and the most productive soil type of Ethiopia given adequate supply of water (Mesfin 1998). Until June of 2006 the hillsides of the catchment were under tremendous states of land degradation due to open grazing system in the area and human infringement. The native grass and woodland species were highly threatened and it was completely stripped of vegetation cover until the introduction a rehabilitation task launched in July 2006 using mixed tree species and water harvesting structures. Rehabilitation of degraded lands has attracted worldwide attention in view of the shrinking arable land, especially in developing countries (Barrelt Lennerd., etal 1986; Singh. B et al., 1992). The area was divided in to two major slope classes: slope class I (0−17%) and slope class II (18%−27%). Slope class I had thicker soils and better micro-site conditions while the slope class II was relatively degraded with shallow soils and poor micro-site conditions. The rehabilitation plots that were planted with mixed native tree species since 2006 included: Acacia abyssinica, Euclea schimperi, Olea africana, Dodonaea angustifolia and Acacia tortilis. Contour bunds were made by digging a furrow along the contour alignment following a curved path and heaping the soil down slope. This was followed by compaction and stabilization of the structure by planting grasses and construction of infiltration pits of dimension 40cm40cm20cm in each micro catchment. A plot with an area of 50m2 was planted with a given species on both lower and higher slopes. A plot comprised 15 tree seedlings of a species at spacing of 1.5m. Seedlings were planted at the space between the infiltration pit and cross tie of a micro catchment (Fig 1).
Figure 1. D. angustifolia with water harvesting structures (Kuriftu lake catchment)

2.2. Sampling method and data collection

After seven years of rehabilitation a total of 20 soil samples (10 samples from each slope i.e. 0-17% and 17-23%) were collected. Stratified random sampling was used as a sampling technique by taking species types in a given slope category as stratum and random assignment of soil sampling pits in 50 m2 plot. Twenty 50m2 plots of trees (ten from each slope) were considered for soil analysis.
A plot for soil sampling contained 15 survived trees of a given species planted at a spacing of 1.5 m. Sampling pits with in plots of a given species were randomly assigned to the underneath of tree species.
Three soil samples were collected at three distinct depths (0-15, 15-30 and 30-50cm) with in a plot for a given species and were composited by depth prior to analysis. This resulted in 20 homogenized samples (10 samples from each slope) for ultimate analysis. Sampling plot locations were GPS coded and later transferred to a base map (Fig.2). Samples were analyzed for soil physicochemical parameters like pH, CEC, OM, N,K, Ca, Mg, % MC and BD.
Figure 2. Soil sampling locations of the study area

2.3. Methods of Analysis

Soil samples were air dried and passed through 2mm sieve prior to determination of physical and chemical properties. Soil pH was measured with glass electrode in a 1:2.5 (W/V) soil water mixture Richards (1954). Organic carbon content was determined by the wet digestion method (Walkely and Black 1934). Exchangeable cations were extracted with neutral normal ammonium acetate. Soil moisture was determined gravimetrically where the samples of fresh soil were and oven dried at 105°C for 24 hours. The percentage of water present in the soil was calculated as the weight difference between field soil and oven dried soil divided by the weight of the oven dried soil, then multiplying the result by hundred. Soil bulk density was determined by using the non destructive core volume method through dividing the weight of oven dried soil in the core to the volume of dry soil.
In order to obtain a display of soil property variations with slope, species and depth SPSS statistical analysis and excel spreadsheet were applied to soil data. Comparison of error bars was used to see whether there were significant differences in soil properties between different slope positions and under canopies of tree species.

3. Results and Discussion

The hillside of kuriftu lake catchment after seven years of rehabilitation revealed significant differences in soil properties under canopies of different tree species. Most soil properties were also significantly influenced by slope positions. Significantly higher negative correlations of slope with pH, CEC, exchangeable Ca, Mg and Na were found. The study results also revealed how soil nutrient, soil moisture and bulk density are significantly influenced by tree species and soil depth. Thus it was justifiable to reject the proposed null hypothesis.

3.1. Soil pH

Soil pH is referred to as the acidity of the soil and it is measured by the number of hydrogen ion concentration in the soil solution. When the pH is excessively acidic or basic nutrient availability will highly be altered. Significant differences in soil pH were observed on plots of trees from different slope classes. pH in the study area generally varied between the ranges 6.75—7.75. This clearly indicates that the soil in the study area is slightly alkaline in lower slopes on plots of almost every species and slightly acidic on higher slopes for all species. (Fig. 3A & B).
Figure 3A. Soil pH under five tree species on different slopes and soil depths (Error bars are SEM, n=3); Figure 3B. Soil pH under different trees, slopes and soil depths in the rehabilitation site. (A):0-15cm. (B):15-30cm. (C):30-50cm
Tree species had more influence on mean pH of soils. Soils under Acacia abyssinica had a significantly higher pH than soils under the canopies of Euclea schimperi, Olea africana, acacia tortilis and Dodonaea angustifolia in lower slopes. This would increase the soil pH and hence reduce the acidy of the soil. These results corroborate the already disclosed high pH values of lower slopes. Soil pH is closely associated with the decomposition of litter that comes from tree biomass. pH was found out to be significantly lower (high concentration of hydrogen) on higher slopes than lower slopes for most tree species (Fig. 3A). Lower slope is relatively gentler which assists the accumulation soluble cations leaching from upslope making the pH higher.
A higher level of organic carbon and hence organic matter was observed in higher slopes where the pH is slightly acidic. Organic matter has a variable exchange capacity that is correlated with soil pH. As the soil in the higher slope is acidic the exchange sites will be largely occupied by acid forming cations like H+ and Al+3. This explains that the exchangeable acidity (the amount of acid cations occupied on the CEC) is high as H+ and Al+3 tend to replace base forming cations. High exchangeable acidity would then result in lower pH as revealed in higher slopes (Fig.3B). The pH from surface to sub-surface soils exhibited an increasing trend towards getting more alkaline for almost every species with the exception of Dodonaea angustifolia that revealed acidic surface and subsurface soil conditions (Fig. 4).
Figure 4. Increasing pattern of pH with soil depth across species (Error bars are SEM, n=3)
Furthermore surface pH value was found to be highest for most nitrogen fixing species like Acacia abyssinica that produce highly decomposable litter biomass. pH value was significantly lower for species with relatively slow decomposing leaves like Dodonaea angustifolia (Fig. 4).

3.2. Soil Organic Matter

Soil organic matter is an accumulation of partially disintegrated and decomposed plant and animal residues and other organic compounds synthesized by the soil microbes as decay occurs. It estimates the humus or carbonates present in the soil. In the study area OM generally varied between 12.2 – 31.45 g/kg for lower and higher slopes respectively. Slope changes resulted with no significant differences in OM to a depth of 50cm for most tree species except for tree legume like Acacia abyssinica and Euclea schimperi (Fig.5A). The correlation between slope and OM (0.27) was statistically non significant (Table 1).
A significant decline in OM was observed from surface to subsurface under the canopies of all the study trees. OM on surface soils of broadleaved trees i.e. Dodonaea angustifolia, Euclea schimperi and Olea africana were significantly higher than tree legumes. This would be an indication that in terms of surface soil organic matter accumulation the studied broad leaf trees are equally important and would be preferred over the legumes. However no statistically significant difference in surface soil organic matter was observed among the broad leaved species (Fig.5B). OM was found out to have no significant correlation with most soil chemical properties (Table 1).
Figure 5A. Organic matter under different species and slope categories. (Error bars are SEM, n=5); Figure 5B. Soil organic matter under different species and soil depths. (Error bars are SEM, n=5)

3.3. CEC

The clay mineral and organic matter components of soil have negatively charged sites on their surfaces which adsorb and hold positively charged ions (cations) by electro static force. This electrical charge is critical to the supply of nutrients to plants because many nutrients exist as cations (e.g. Magnesium, potassium and calcium). The main ions associated with CEC in the soil are the exchangeable cations Na+ , Ca+2, Mg+2, K+.
Figure 6. Cation exchange capacity under different species slope categories and soil depths. (Error bars are SEM, n=10)
Summing the analyzed base cations gives adequate measure of CEC (CEC by bases) (Rayment and Higginson 1992). In the study area lower slope soils revealed significantly higher CEC than higher slopes for all tree species considered. CEC was negatively correlated (-0.75) with slope. As lower slopes were characterized by significantly higher pH than higher slopes this would explain the occurrence of high CEC by bases down to a depth of 50 cm.
CEC under soils of Dodonaea angustifolia was significantly higher than soils under the rest of the species in lower slopes. The highest CEC was recorded under Dodonaea angustifolia while the least was under Olea africana in lower slopes. The occurrence of high CEC for Dodonaea angustifolia could be explained by the significantly higher surface soil OM as indicated earlier in soils sampled under its canopies in lower slopes when compared to the rest of the species.
This would facilitate faster decomposition of its litter and nutrient availability which would make it possess high CEC. The surface 15 cm also has the highest CEC for Dodonaea angustifolia. High OM content recorded under its canopies would explain this high surface CEC.

3.4. Nitrogen

Nitrogen (N) is important for plant growth and development, and of the macronutrients it is often the one that is most limiting (Rasool et al., 2014). Planting with nitrogen fixing trees can profoundly influence soil properties such as organic matter, N, nutrient cycling and resultant growth of associated trees (Wang et al., 2010). In the study area total nitrogen varied from 0.7-1.65 g/kg. Slope changes resulted with no significant differences in total nitrogen for most tree species except for tree legume Acacia abyssinica and Euclea schimperi. This could be related to their soil improving quality. The correlation between slope and total nitrogen (0.15) was also not statistically significant. Total nitrogen to 50 cm depth was found out to be significantly higher for Acacia abyssinica in higher slopes followed by Euclea schimperi and Dodonaea angustifolia that revealed a comparative result in lower slope. There was a general trend of decline in soil total nitrogen from surface (0-15cm) to inner surface (30-50cm) under the canopies of all study trees (Fig.8).
Figure 7. Total nitrogen under different species slopes. (Error bars are SEM, n=5)
Figure 8. Total nitrogen under different species and soil depths. (Error bars are SEM, n=5)
However significant decline in total nitrogen from surface to sub surface (15-30cm) was only for Dodonaea angustifolia and Euclea schimperi.

3.5. Correlations between Slope and Soil Properties

In the study area the correlation matrices for soils in 0-50 cm depths show several sets of significant relationships for most soil properties with slope (Table 1). Significantly higher negative correlations of slope with pH, CEC, exchangeable Ca, Mg and Na were found. OM and total N gave no significant correlation with slope.
The correlation between OM and most soil properties was found out to be non significant. In contrary the correlation between slope and pH was found out to be the highest negative correlation value observed. A similar correlation trend between slope and CEC was discovered also.
This result is in close agreement with what has been disclosed as decreasing trend of pH in the study area with increasing slope, indicating the relatively acidic condition of higher slopes .Slope has been regarded as one of the most important a biotic factors that control the pedogenic process on a local scale (McDaniel et al., 1992; Buol et al., 1997; Rasool et al., 2014). Steeper slopes contribute to great runoff, as well as to greater translocation of surface materials down slope through surface erosion and movement of soil mass (Lal, 2015). The findings of this study follow the general principle that the concentration of basic cat ions should increase (high CEC) with increasing pH which was found out to be the characteristic of lower slopes in the study area as previously disclosed.
Table 1. Relationships between selected soil properties across slopes on the rehabilitated hill side
     

4. Granule Composition, Bulk Density and Soil Moisture Content

The textural class of soils in the study area was mainly clay. The granule composition (% sand, silt and clay content) revealed no significant difference between surface (0-15cm) and sub surface (15-30 cm & 30-50 cm) soils at P<0.05. Tree species revealed no significant effect on granule composition. Soil texture is not usually changed by management practices. Soil texture is inherited from the parent materials and it originates through weathering and pedogenic processes (Osman, 2013). Bulk density describes the relative properties of solid and void in a soil. It is the ratio of soil mass to its volume. Increase in bulk density in the soil could result from soil erosion, soil structure, soil texture, reduction in organic carbon and biomass carbon, and reduced microbial and earthworm activity. The bulk density in the study area generally varied from 1.74-1.99g/cu.cm. From the study results higher slopes were found to posses significantly higher BD values as compared to lower slopes. This is an indication of low level of soil compactness and associated improvement in root penetration in lower slopes. Surface and sub surface bulk densities in lower slopes were significantly lower than con specifics in higher slopes (Fig.9A). However with in slope differences of surface to sub surface bulk densities were statistically non significant.
In the study area change in surface (0-10cm) bulk density among species was statistically non significant. BD varied with soil depth showing a general increasing trend from surface to sub surfaces under the canopies of all trees (Fig.9B). Bulk density tends to increase with depth primarily due to the lack of organic matter and soil aggregation (Osman K.T 2013).
Figure 9A. Soil bulk density as influenced by slope and soil depth. (Error bars are SEM, n=6); Figure 9B. Bulk density being influenced by tree species and soil depth. (Error bars are SEM, n=6)
There was a significant difference in surface soil (0-10cm) moisture content under canopies of tree species considered for evaluation. Soil moisture content in the study area generally ranged from 8% 14.8%. The highest surface soil moisture content was observed under Euclea schimperi while the least was under the canopy of Dodonaea angustifolia. This could be because of the dense and evergreen canopy nature of Euclea schimperi that reduces evapotranspiration loss as compared to relatively sparse and deciduous canopy nature of Dodonaea angustifolia.
Moisture scavenging nature of Dodonaea angustifolia roots could also be the possible reasons for the reduced surface moisture content under its canopy.
Statistically significant surface moisture differences were observed among the study trees with the highest for Euclea schimperi and the lowest for Dodonaea angustifolia. This difference suggests that there is species effect on soil moisture content (Fig.10A). Comparison of Soil moisture content revealed a significant decline from lower to higher slopes. Surface, sub surface and inner surface soil moisture contents in lower slopes were significantly higher than con specifics in higher slopes (Fig.10B).
Figure 10A. Soil moisture being influenced by tree species and soil depth. (Error bars are SEM, n=6); Figure 10B. Soil moisture being influenced by slope and soil depth. (Error bars are SEM, n=6)

5. Conclusions

Based on the findings of the study on mixed tree rehabilitation impacts on soil properties it can be concluded that (i) tree species studied had no effect on soil texture (ii) tree species, slope and soil depth differences significantly affected the distribution of soil solutions that led to variations in soil properties. Nitrogen fixing Acacia abyssinica in higher slopes revealed the highest surface (0-15cm) total nitrogen followed by Euclea schimperi with a comparative result in lower slope. Soil total nitrogen and OM declined from surface (0-15cm) to inner surface (30-50cm) under the canopies of all study trees. CEC was significantly higher on lower slopes than higher slopes to 50cm depth under canopies of the evaluated trees. This result agrees with the observed significant negative correlation of CEC (-0.75) with slope. This could be because lower slopes had significantly higher pH (alkaline range) than higher slopes that are relatively acidic. This result is further corroborated by the observed significant negative correlation (-0.65) between slope and pH. The highest CEC was recorded under Dodonaea angustifolia while the least was under Olea africana in lower slopes. The occurrence of high CEC for Dodonaea angustifolia could be explained by the significantly higher surface soil OM observed under its canopy. Surface pH value was found to be highest for most nitrogen fixing species like Acacia abyssinica that produce highly decomposable litter biomass thus contributing to increased CEC. pH value was significantly lower for species with relatively slow decomposing leaves of Dodonaea angustifolia.
Surface pH value was found to be highest for most nitrogen fixing species like Acacia abyssinica that produce highly decomposable litter biomass. pH value was significantly lower for species with relatively slow decomposing leaves of Dodonaea angustifolia iii/ Species effect on soil moisture content was observed. Euclea schimperi by virtue of its dense and evergreen canopy that reduces evapotranspiration loss revealed the highest surface soils moisture content while Dodonaea angustifolia had the least surface moisture content. This could be attributed to its relatively deep growing and moisture scavenging roots. iv/ slope has strong correlations with some soil properties. Statistically significant negative correlations of slope with pH, CEC, exchangeable Ca, Mg and Na were found. OM and total N had no statistically significant correlation with slope.
Knowledge of how mixed tree rehabilitation impacts on soil properties would give a clue as to which tree plays a vital role in soil amelioration process. Mixed tree rehabilitation of degraded lands have influence on soil properties as different tree species add and maintain different essential nutrients to the soil through OM and its microbial decomposition. The primary source of OM in forests is litter fall from trees which is more on surface soils as it is also true for the findings of the present study. OM on surface soils of broadleaved trees i.e. Dodonaea angustifolia, Euclea schimperi and Olea africana was significantly higher than tree legumes. This would indicate that in terms of surface soil organic matter accumulation the studied broad leaf trees are equally important and would be preferred over the legumes. The presence of fully and partially decomposed litter on surface soils helps to hold moisture on surface soils. The study results particularly indicated this for Euclea schimperi that had comparatively higher surface moisture content by virtue of its higher surface OM and dense evergreen canopy that reduces evapotranspiration loss from surface soils. Surface total nitrogen of Euclea schimperi was also one of the highest. This would give a clue to the advantage of integrating broadleaves to enhance soil fertility of degraded lands. Tree legumes like Acacia abyssinica had one of the highest surface total nitrogen.
Surface pH value was also found to be highest for Acacia abyssinica that produces highly decomposable litter biomass and contributes to increased CEC. In terms of OM tree legumes possessed equally comparable results that would make them preferred next to broadleaves. Tree legumes are also reported to assist nutrient cycling and resultant growth of trees through addition of OM to the soil. Based on the assessment of selected soil properties like OM, Total nitrogen, CEC, and %MC, it would be recommendable to use species mixes (mixes of legumes and non legumes) for rehabilitation since the two species groups have their own tree -soil synergy that assists soil fertility and long term potentiality of the resulting plantation. Yet to undertake a comprehensive assessment of rehabilitation programs in relation to soil fertility status there is a further need to evaluate fertility by using soil indices.

References

[1]  Abdu, A., Akbar, M.H., Ahmed, O.H., Jamaluddin, A.S., Nik Ab. Majid, N.M., Abdul-Hamid, H., Jusop, S., Hassan, A., Yusof, K.H. and Abdu, A.(2010) Differences in Soil Physical and Chemical Properties of Rehabilitated and Secondary Forests. American Journal of Applied Sciences 7 (9): 1200-1209.
[2]  Abdu, A., Jamaluddin, A.S., Abdul-Hamid, H., Hadi Akbar, M.H., Banga, T.S., Jusop, S. and Majid, N.M. (2013) Assessing soil fertility status of rehabilitated degraded tropical forest. American Journal of Environmental Science 9 (3): 280-291.
[3]  Arifin, A., S. Tanaka, S. Jusop, N.M. Majid and Z. Ibrahim et al., 2008b. Rehabilitation of degraded tropical rainforest in Peninsular Malaysia with a multi-storied plantation technique of indigenous dipterocarp species. J. Environ., 50: 141-152.
[4]  Barret-Lennard, E. G., Malcolm, C. V., Stern, W., Stern, W. R. and Wilkins, S. M. (1986) Forage and fuel production from salt affected wasteland, Elsevier, Amsterdam.
[5]  Buol, S.W., Hole, F.D., McCracken, R.J., Southard, R.J. (1997) Soil Genesis and Classification, 4th edition. Iowa State Univ. Press, Ames, IA.
[6]  Cole, T.G., R.S. Yost, R. Kablan and T. Olsen, 1996. Growth potential of twelve Acacia species on acid soils in Hawaii. Forest Ecol. Manage., 80: 175-186. DOI: 10.1016/0378-1127(95)03610-5.
[7]  Geist, H.J. and Lambin E.F. (2002) Proximate causes and underlying driving forces of tropical deforestation. Bio Science, 52:143-150.http://www.jstor.org/stable/1314248.
[8]  Grainger, A, (1993). Controlling Tropical Deforestation. 1st Edn., Earthscan, London, ISBN-10: 1853831425, pp: 310.
[9]  Kubota, D., Masunaga, T., Hermansah, Rasyidin, A., Hotta, M., Shinmura, Y., Wakatsuki, T. (1998) Soil environment and tree species diversity in tropical rain forest, West Sumatra, Indonesia. In: Schulte, A., Ruhiyat, D. (Eds.), Soil of Tropical Forest Ecosystems. Springer, Heidelberg, Germany, pp. 159 – 167.
[10]  Lal, R, (2015) Effects of slope length, slope gradient tillage methods and cropping systems on runoff and soil n a tropical alfisol: Preliminary results. Proceedings of the Porto Alegre Symposium (174)79-88.
[11]  MacNamara, S., D.V. P.D., Erskine, D. Lamb and D. Yates et al., (2006) Rehabilitating degraded forest land in central Vietnam with mixed native species plantings. Forest Ecol. `Manage., 233:358-365. DOI:10.1016/j.foreco.2006.05.033.
[12]  McDaniel, P.A., Bathke, G.R., Boul, S.W., Cassel, D.K., Falen, A.L. (1992) Secondary manganese/iron ratios as pedochemical indicators of field-scale through flow water movement. Soil Sci. Soc. Am. J. 56, 1211 – 1217.
[13]  Mesfin, A. (1998) Nature and Management of Ethiopian Soils (1stedition). Alemaya: Alemaya University of Agriculture, p.43.
[14]  Osman, K.T, (2013) Forest Soils. Properties and Management. Springer- XI.217P.http://www.springer.com/978-3-319-02540-7.
[15]  Rasool, S.N., Gaikwad, S.W. and Talat, M.A. (2014) Relationships between soil properties and slope segments of Sallar Wullarhama watershed in the Liddar catchment of Jammu and Kashmir. Asian Journal of Engineering Research, 2(2): 1-10.
[16]  Rayment, G.E and Higginson F.R. (1992) Electrical Conductivity. Australian Laboratory Handbook of Soil and Water Chemical Methods Inkata Press: Melbourne.
[17]  Singh, B. and Garg V. K. (2007) Phytoremidiation of sodic forest ecosystem: Community response to restoration processes. Not. Bot. Hort. Agrobot. Cluj, 35(1):77-85.
[18]  Tsui, Chun-Chih., Chen, Zueng-Sang. and Hsieh, Chang- Fu. (2004) Relationships between soil properties and slope position in a lowland rain forest of southern Taiwan. Geoderma 123 (2004) 131–142.
[19]  doi:10.1016/j.geoderma.2004.01.031.
[20]  Wang, F., Li, Z., Xia, H., Zou, B., Li, N., Liu, J., Zhu, W. (2010) Effects of nitrogen fixing and non nitrogen fixing tree species on soil properties and nitrogen transformation during forest restoration in Southern China. Journal of soil sciences and plantation nutrition, 56(2):297-306.
[21]  doi:10.1111/j.1747-0765,2010.00454.x.
[22]  Williamson, J.R. and Neilsen W.A. (2000) The influence of forest site on rate and extent of soil compaction and profile disturbance of skid trails during ground-based harvesting. American. J. Res. For., 30: 1196-1205.