Degradation assessment of wetlands under different uses: implications on soil quality and productivity

The assessment of degradation status of wetland soils under five different land use types (LUTs) in Ogun state, Nigeria were studied. The laboratory study was conducted to determine the physical, chemical and biological properties of these wetlands and the results obtained were compared with the food and agriculture organization (FAO) standard indicators and criteria for land degradation assessment. The textural composition of the soil ranged from sandy loam to sandy clay loam. Total porosity was generally low with the mean value of 40.5%. The pH ranged from moderately acidic to slightly alkaline with fallow soil having the highest value. Available phosphorus was low across the LUTs. Total nitrogen was predominantly low in most of the cultivated soils to moderate in the fallow soils. Cation exchange capacity (CEC) was low, while the exchangeable sodium percentage (ESP) was high (>5) in all the LUTs. The organic matter ranged from low to moderate indicating low nutrients status of the soil. The soils were classified and placed in the order Alfisols. The degradation results showed that most of the cultivated wetlands were highly degraded compared to the reference (fallow) soils which were slightly degraded. It is opined that soil conservation practices like the use of inorganic fertilizers, organic manure, and composts should be intensified in these fragile low fertile wetlands. Also, there should be a periodic monitoring of the fertility status of the wetlands from the time it is first open for cultivation to subsequent uses.


INTRODUCTION
Globally, soil degradation is one of the greatest challenges facing the developing countries in the tropics and sub-tropics (IFAD, 2010).Its extent and effect on agriculture and the larger environment is more severe now than ever before and cannot be ignored because most soils cannot continue to support crops production and yield maximally (Lal, 2015).
Some estimates provide that between 1950 and 2010 degradation decreased soil ecosystem services by 60% (Leon and Osorio, 2014).Soil degradation always leads to political and social instability due to its enormous impact.It is associated with increased deforestation rate, intensive and continuous use of marginal and fragile soil, accelerated runoff and erosion, natural waters pollution, and green house gases emission (Adaikwu et al., 2012).
Soil degradation is defined to be the physical and chemical deterioration of soil or reduction in soil quality which has pronounced implication on agricultural productivity by not able to support plant and animal growth optimally due decline in the levels of available moisture, available nutrients and biological activity (Ernst, 1995).It can also mean a reduction of the biological and economic productivity potentials of rain-fed cropland, irrigated cropland or range, pasture and forested land by one or a combination of processes such as displacement of soil materials by wind and water erosion, deterioration of soil physical and chemical properties as a result of long-term loss of natural vegetations due to misuse and lack of proper management (Amalu, 1998;Lal, 2009).
Agriculture and wetlands are closely linked together, and this relation between agriculture and wetlands has developed against wetlands up to date (Gopal, 2000).The conversion of wetlands into agricultural land and intensive agricultural activities around them has caused the degradation and destruction of the wetlands.Presently, productivity levels of soils have remained stagnant and even dropped despite the introduction of new crop varieties and germplasms, and an increase in quantity of agrochemicals application.This situation has been attributed largely to declining soil fertility (Ogbodo et al., 2012).
There is therefore need to have a comprehensive and useful information on the influence of different land uses on wetland degradation and agricultural productivity in Ogun State, Nigeria.Information on the extent to which the wetland has been misused overtime in this area is also required.The aim of this study is to assess the degree of degradation of selected wetlands under different uses in the study area using the food and agriculture organization (FAO) standard indicators and criteria for soil degradation assessment with a view to making modest and practical recommendations on the rehabilitation and proper management of degraded wetland soils.

Description of the study area
The study area is the Odeda Local Government Area, Ogun State located at point Latitudes 07°10'N and 07°30'N and between Longitudes 03°15'E and 03°50'E.It shares boundaries with Oyo State to the north and east, Abeokuta South and North Local Government Areas to the west, and Obafemi-Owode Local Government Area to the south (Figure 1).The area has bimodal rainfall patterns, with peaks between June to July and September to October.This is followed by a short period of dry season that is usually between November and February.It has an annual rainfall of about 1113 mm and it is located in the guinea and derived savanna belt.The mean relative humidity of the area is high (above 70%) with the peak between May and October and the annual Osinuga and Oyegoke 11 mean temperature is 27°C.The major land use types (LUTs) in the study area were arable crop, cash crop production and nonagricultural uses (such as residential, industrial, and roads construction).The wetlands have been ploughed at different intervals over times, and also agrochemicals both pesticides and fertilizers have been applied to the soil.

Field work
Five land use types (LUTs) which are rice (LUT 1), yam (LUT 2), oil palm (LUT 3), built-up sites (LUT 4) and fallow land (LUT 5) were evaluated and within each of the chosen LUTs, an area of 3 ha was demarcated, and bulk samples consisting of ten surface (0 to 15 cm) and ten subsurface (15 to 30 cm) samples were randomly collected and place in a properly labeled bags for physical, chemical and biological analyses.Core samples were also collected with the use of core samplers for bulk density determination.The general site description was described after the FAO guidelines for site and profile descriptions (FAO, 2006).Attributes like the climate, vegetation, land use, slope, drainage type, soil surface form, micro relief and depths to ground water table (GWT) were described and recorded.The soils classification was according to Soil Survey Staff (2014).The land use histories of the study area were obtained through field observations and interviews/interactions with the local farmers.

Laboratory methods
After the soil samples were air-dried for days, they were crushed and sieved using a 2 mm screens.The samples were then analyzed for the following parameters: Particle size analysis was done through hydrometer method (Bouyoucos, 1951), saturated hydraulic conductivity (Ksat) was determined using a constant head method and bulk density by core method.The soil porosity was estimated from the bulk density data at an assumed particle density of 2.65 g/cm 3 .Soil pH in water (1:1) using glass electrodes pH meter (Mclean, 1965).Total nitrogen was determined by the macrokjeldahl digestion method of Jackson (1962), available P was after (Bray and Kurtz, 1945) extraction using Bray-l extract followed by molybdenum blue colorimetry.Exchangeable cations were extracted with 1M NH4OAC (pH 7.0), K and Na were determined using flame photometer while Ca and Mg were by atomic absorption spectrophotometer (Sparks, 1996).Organic carbon was after dichromate wet oxidation method (Walkley and Black, 1934), and the organic matter content was got by multiplying a factor of percent organic carbon by 1.724.Cation exchange capacity (CEC) was determined by neutral, 1N Ammonium acetate method.Base saturation was computed by dividing the sum of exchangeable bases by CEC and multiplying by 100, while Exchangeable Sodium Percentage (ESP) was calculated by dividing the exchangeable sodium by the CEC.

Soil degradation assessment
The degradation status of the wetlands across different land use types (LUTs) was assessed by field observation and the standard indicators, and criteria for land degradation assessment (Table 1) by FAO (1979) using the obtained laboratory results.The four degrees of degradation level identified includes:

Statistical methods
The results were subjected to analysis of variance (ANOVA) to know the effect of different land use types on soil properties/quality and the means were separated with the New Duncan Multiple Range Test (DMRT) at p<0.05 (Table 1).

Soil physical properties
The physical properties of the top and sub soils of the different land use types were showed in Table 2.The particle size distribution result indicates different textural composition of the wetlands.The textural classes are the intrinsic soil properties that are sufficiently permanent and are often used to characterize the physical make up of soils (Hillel, 1980).The highest mean for clay content (302 g/kg) was recorded in built up land and was significantly different (p<0.05) from others, while the oil palm has highest sand content (604 g/kg).The sand content dominates the textural class, and this might be due to erosion from the upland into the wetland and turning of the soil overtime.The lowest bulk density (1.44 g/cm 3 ) was recorded on the fallow soil followed by 1.46 g/cm 3 in oil palm.The high soil bulk density in cultivated soils is due to intensive agricultural practices, low organic matter content and compaction of top soil as a result of overgrazing (Lal, 1986;Ceyhun, 2009).Soil bulk density increase depicts an increasing loss of soil binder materials, reduced soil biological activity, especially earthworms and plant roots, and due to the land use change and significant reduction of clay and silt and instead of increasing the amount of sand in the soil texture (Gholami et al., 2014).Saturated hydraulic conductivity result correlates with that of the bulk density.The effect of land use types on total porosity (Table 2) showed significant difference (p<0.05).Total porosity was low with mean value of 40.5%, highest in fallow soil (45.7%) and lowest in built up soil (35.1%).The low value in built up soil is attributed to the high bulk density.The results of the study agrees with the findings of Senjobi and Ogunkunle (2011) and Adaikwu et al. (2012) who affirmed that the use to which a land is put influences the soil physical quality indicators which are used for soil degradation assessment (Table 2).

Soil chemical and biological properties
The chemical and biological properties of the soils across the land use types were presented in Table 3.The laboratory analyses showed that pH varies from moderately acid (5.6) to slightly alkaline (7.4), and are significantly different at p<0.05.Decrease in soil pH from the cultivated land might be due to exhaustion of basic cations or higher microbial oxidation that creates organic acids causing soil pH reduction (Chauhan et al., 2014).The total nitrogen content also varies from low (0.08%) to moderate (0.25%) with the fallow having the higher content than the rest LUTs.The trend is a pointer to nutrient loss in the farms due to continuous cultivation as Available phosphorus was generally low in all the LUTs been less than 15 mg/kg despite application of inorganic fertilizer (NPK) in the cultivated soils.The fallow soil had the highest value but result was not significantly different (p<0.05)across the LUTs.The exchangeable potassium (K) and calcium (Ca) were low in all the LUTs, but were higher in oil palm and in fallow soils.This could be due to the level of soil organic matter (SOM) and nutrient recycling respectively.Sodium (Na) and Magnesium (Mg) contents were also low across the LUTs except for built up soil which has higher values.This indicated the decrease in the Ca content and increased content of Mg.The exchangeable bases were significantly different at p<0.05 in all the LUTs.
The CEC of the soil was generally low but higher under fallow compared to other LUTs.The CEC increased in the sub soil under fallow which indicates better nutrient recycling.The CEC values obtained depends on the pH and SOM contents.The base saturation values obtained were high in all the LUTs and were not significant different from one another.The exchangeable sodium percentage (ESP) values were high been greater than 5%, and the results were differs significantly (p<0.05)across the LUTs.The implication of this is decline in Ca which can creates a collapse in soil structure and decrease in permeability (CUCE, 2007;Hazelton and Murphy, 2007).
The organic matter (OM) content of the soil ranged from low to moderate (1.22 to 3.41%).The OM decreases with depth in all the LUTs.The result corroborates the findings of Bhunia et al. (2016).Increased long-term cultivation significantly (p>0.05)decreased soil organic matter content in the cultivated soils and this has crucial implication on soil physical and chemical properties.The organic matter content of the soils was significantly higher in fallow soil (3.41%) than the cultivated soils.The low organic matter obtained may be partly due the effect of high temperature and relative humidity in the area which haste rapid mineralization of organic matter (Table 3).

Soil degradation assessment
The soil parameters used for the physical degradation assessment of the soils of the LUTs studied indicated that the soils were predominantly moderately degraded (MD) with respect to BD in all the LUTs (Table 4), except for surface (0 to 15 cm) soil of oil palm and fallow soil which were slightly degraded (SD), FAO (1979).The BD of soil is greatly influenced by the OM content.The correlation between BD, clay and OM was significant.This connotes that the lower BD in the cultivated soils compared with the fallow were indications of lower clay content and OM in the former.The continuous cultivation of the soils can modify the soil BD and the pore size distribution since the operation loosens, granulates and crushes the soil particles.On the other hand, the saturated hydraulic conductivity (Ksat.)rating showed that the soils were highly degraded (HD) in LUT 2, LUT 3 and LUT 5, while LUT 1 and LUT 4 were very highly degraded (VHD).The difference in the result could be from differences in the BD of each LUT soil.
The soils chemical degradation of the studied area revealed different degrees of degradation with respect to the parameters assessed.For instance, with respect to nitrogen content, the degradation status of the soils ranged from SD to HD, FAO (1979).The soils under oil palm and fallow conditions were SD, while the soils under rice and yam (surface) were MD.The yam subsoil as well as the built up were all HD with respect to N content.
Nitrogen is a key nutrient which was used as a good soil quality indicator and listed as the one of the most important of all the 16 essential plant nutrient elements needed for plant growth, development and reproduction and also the most easily limiting or deficient throughout the world particularly in the tropics (Agbede, 2009).Available phosphorus was VHD in all the studied LUTs despite fertilizers use in the cultivated soils.The degradation degree with respect to potassium showed that all the LUTs were SD.The fallow soil recorded high percentage when compared with other LUTs.The degradation rating of base saturation (BS) showed that the soils were VHD.The soils were HD to VHD with respect to exchangeable sodium percentage (ESP).The soils were HD at the depth of 0 -15 cm in LUT 2, but HD at both depths in LUT 3 and LUT 5, while the rest LUTs were VHD.
The biological degradation of the surface and subsurface soils ranged from SD to HD with respect to OM content (Table 4) FAO, (1979).The fallow soil was SD, the rice and oil palm soils were MD.The yam and built up soils were HD.This is an indication of very high biological degradation, which is typical of tropical soils.It can also be as a result of top soil removal during clearing for building construction and agricultural purposes.The OM depletion may rise from crop uptake exacerbated by continuous cropping of the wetlands without adequate measures of nutrient replacement either through the use of inorganic fertilizer or other forms of soil conservation measures.Harpstead (1973) reported the low OM content is a phenomenon associated with the tropical soils due to high temperatures that rapidly breakdown OM and inhibit nitrogen fixation by rhizo-bacteria (Table 4).

Conclusion
An investigation study was conducted to assess the degree of degradation of the wetlands in selected land use types of Odeda Local Government Area of Ogun State, Nigeria.The main objective of the study was to assess the degradation degree of the soils in the study area, using the standard indicators and criteria for land degradation assessment of FAO (1979).The results of the study showed the different textural composition of the wetlands.The soils were low in terms of major soil nutrients.The soils were classified as Alfisols according to the provisions of Soil Survey Staff (2014) on the basis of the physical, chemical and biological properties of the soils.The result of soil degradation assessment in these wetlands ranged from SD to VHD soils.On a comparative basis, most of the soils that were under cultivation (LUT 1 -LUT 4) showed higher degree of degradation compared to the fallow soil (LUT 5).The study revealed that 22.5% of the soils were slightly degraded, 17.5% were moderately degraded, and 23.8% were highly degraded while 36.2% were very highly degraded.Thus, this in turn affects the productivity of the soils negatively thereby leading to food Base on the findings of this study, it is recommended that application of mineral fertilizer nutrients especially nitrogen and phosphorus is necessary, the use of organic manure such as cow dung and poultry dropping should be adopted to improve the productivity of these degraded soils.Farmers should be encouraged to leave crop residues on their farms and incorporate same during tillage rather than burning them.Also, monitoring the fertility status of the wetlands at regular intervals is very paramount.

Figure 1 .
Figure 1.Map of Ogun State Nigeria showing the study area.

Table 1 .
Indicators and criteria for land degradation assessment.

Table 2 .
Soil physicalproperties in the study area.

Table 3 .
Soil chemical and biologicalproperties in the study area.

Table 4 .
Soil quality indicators degradation assessment.