Evaluation of erodibility indices and soil properties affected by land-use types in Mbano, south-east Nigeria

Soil erosion has been at the forefront of the degradation of soils under
 different land-use types in southeast Nigeria. Hence, this research aimed to
 determine the erodibility of the soils under different land-use types in
 Mbano. The four land-use types selected were oil palm plantation (OP),
 cassava farm (CF), pineapple orchard (PO) and plantain plantation (PP).
 However, clay dispersion ratio (CDR), clay dispersion index (CDI) and clay
 flocculation index (CFI) were the erodibility indices used. A total of 5
 composite samples were collected randomly from each type of the land-uses at
 a depth of 0-20 cm using the free survey. The samples were subjected to
 laboratory analysis. Data generated were analyzed statistically using a
 completely randomized design of analyses of variance (ANOVA) and
 correlation. CDR had an increasing order of 47.9% <48.16% <51.49% <56.24%
 for soils under CF, PP, PO and OP, while CDI had 36.44%, 29.06%, 40.96% and
 49.04% for soils under OP, CF, PO and PP land uses. CFI had 63.62%, 71.24%,
 59.14% and 50.96% for soils under OP, CF, PO and PP, respectively. The ANOVA
 indicated that studied erodibility indices had no significant difference
 (p=0.05) among the soils under the different land-uses. The research will
 unite farmers and other land-users in adopting only good conservation
 practices that will aid the sustainability of the area.


Introduction
Soil erosion depends on the erosivity of the rainfall and erodibility of soil. The soil erodibility depends primarily on the nature and amount of soil aggregates, organic matter content, hydraulic conductivity, root abundance, and particle size

Study area
The sites are situated at Mbano in Imo State, south-eastern Nigeria. The area lies between latitude 5°35' N to 5°48' N and longitude 7°02' E to 7°18' E. A tropical wet and dry season prevails in the study area. The wet season extends from March to October with peaks in June/July and September (Iloeje, 2002). In some years, rainfall may be prolonged while the onset may be delayed in some other years. The area has an annual mean rainfall of 2000 mm and an annual mean temperature of 27°C (NIMET, 2015). The area is geomorphologically plain and nearly flat with a gentle slope, while dominant vegetation types are tropical rainforests (Onweremadu and Uhuegbu, 2007). The study sites comprised four land-use types, including oil palm plantation (OP), cassava farm (CF), pineapple orchard (PO) and plantain plantation (PP).

Soil sampling and analysis
Soil samples were randomly collected at a depth of 0-20 cm from surface soils at different sampling points on each of the four land-use types. The collected samples were air-dried, sieved and subjected to routine laboratory analyses. Particle size distribution was determined by the Bouyoucos hydrometer method (Gee and Or, 2002). Soil pH was determined electrometrically using the glass electrode pH meter in a solid-liquid (water) ratio of 1:2.5 (Hendershot et al., 1993). Exchangeable bases were determined by the neutral ammonium acetate procedure buffered at pH 7.0 (Thomas, 1982). Exchangeable acidity was determined by a method described by McLean (1982). Total carbon was determined by the wet digestion method (Nelson and Sommers, 1982). The soil moisture content at saturation was determined by the Klute method (1986). Soil bulk density was determined by the core method (Grossman and Reinsch, 2002). Total porosity (P o ) was obtained from bulk density (ℓp) values with an assumed particle density value of (ℓs) 2.65 gcm -3 as follows, Porosity (Po) = 100 -(ℓp/ℓs) ×100/1. The higher the CDR and CDI, the higher the ability of the soil to disperse while the higher the CFI, the better aggregated the soil. The clay dispersion ratio was used to determine the erodibility of the soils. Hence, soils with a clay dispersion ratio greater than 15% are erodible, and with less than 15% are not erodible.

Statistical analysis
Analysis of variance (ANOVA) for a completely randomized design (CRD) was used to compare the influence of the land-use types. Similarly, the least significant difference (LSD) at the 5% probability level was used for comparison. The correlation was used to determine the relationship between erodibility indices and selected soil properties. Table 1 shows that the textural class of the studied site is predominantly sandy loam and loamy sand. Sand particle dominated among the soil particles of the different land-uses with mean values ranging from 829.60 to 857.56 g/kg. The sandiness of the soils suggests low moisture content and high porosity which indicate high infiltration and erosion. There was no significant difference in sand content among different land-uses, which could be associated with homogeneity in the parent material and climatic conditions. The sandy nature agrees with the findings of Enwezor et al. (1990) and Ahukaemere et al. (2012) that soils reflect the parent material from which they originated. However, soils that have high sand content are prone to erosion due to a low binding agent. There was no difference among soils under OP, CF, PO and PP land-uses in terms of silt particles. The level of silt content is in concurrence with the findings of Osujieke et al. (2017) in soils of south-eastern Nigeria. The OP soils (105.76 g/kg) had the highest clay particle while the PO soils (84.48 g/kg) had the lowest among the studied land-uses. However, the land-uses indicated the low clay content which agrees with the findings of Malgwi et al. (2000) and Wakene (2001) and it is attributed to sorting of soil minerals by biological and/or agricultural activities, clay migration or surface erosion by runoff or combination of these. There was no significant difference (p = 0.05) in clay content among the different land-uses. This conforms to the findings of Onweremadu and Mbah (2009) that suggest the effect of climate on the area under the same agro-ecological zone. The presence of clay material provides the required bondage between the varying soil particles resulting in the formation of more stable aggregates which makes them less susceptible to erosion. However, clay absence reduces the capacity of the soil particles to bind together and form aggregates that can resist the shearing force of flowing water, thus making the soils vulnerable to soil erosion. This is in compliance with the finding of Parfitt and Salt (2001), who have indicated that higher clay content of soils reduces erodibility. Hence, soils with low clay content are more prone to erosion and have lower binding forces and poor cohesion. Bulk density had mean values ranging from 0.96 to 1.14 g/cm 3 among the different land-uses. However, bulk density of soils of CF and PP differed significantly (p= 0.05) from that of OP.

Results and Discussion
while the bulk densities of soils of CF, PO, and PP land-use types showed no significant difference. This was in agreement with the work of Lemenih et al. (2005), who have postulated that the significant difference is due to land management practices and land-use history. Heard et al. (1988) have stated that soil bulk density is an important property in relation to water erosion since it determines the level of compaction, structural development, and cohesiveness of soils. However, the soil of PP land-use type had the highest bulk density, which can lead to losses by surface runoff among the land-uses due to a low infiltration rate. This is in concurrence with the findings of Weil and Brady (2016). Soil moisture content was low in all the studied land-uses, which could be associated with high sand content, low organic matter and high porosity. The result, as indicated in Table 2, shows that the soil pH (H 2 O) had mean values ranging from 4.97 to 5.48 among the different land-uses. The soils under the land-use types were moderately-strongly acidic according to the rating of Chude et al. (2011). The CF soil was the most acidic while the PP soil was the least acidic. The acidic nature of the soils of the various land-use types could be attributed to organic acids released by litter decomposition (Jandl et al., 2004) and parent material. The result also indicated that the pH of PP soils differed significantly (p=0.05) from that of CF soils and had a non-significant difference with OP and PO soils.

Evaluation of erodibility indices and soil properties affected by land-use types 367
The significant difference could be associated with vegetation type, leaching/plant uptake of basic cations and soil management practices. Organic carbon (OC) had means of 2.92%, 2.87%, 2.88%, and 2.28% for soils under OP, CF, PO and PP land-use types, respectively. They were low when compared to critical values of 3.55% for Eastern Nigeria (Akinrinde and Obigbeson, 2000). The generally low level of OC could be attributable to plant uptake, leaching, and climatic factors. However, soils under OP had high organic carbon when compared to that of other land-uses. This could be attributed to the amount of litter deposit and a high rate of decomposition. The OC had no significant difference (p 0.05) among soils under OP, CF, and PO land-uses, but OC of soil under PP differed significantly (p=0.05) from that of soils under OP, CF, and PO land-uses. The difference could be associated with vegetation cover, runoff, and litter size. Morgan (2001) has reported that soil erodibility decreases linearly with increasing soil organic matter content. However, organic matter and chemical constituents of the soil are important because of their influence on the stability of aggregates which reduces erodibility. The total exchangeable bases of the soil were dominated by Mg for soils under OP, CF and PO land-uses while calcium dominated the exchangeable bases for the soil under PP land-use. However, according to the ratings of Landon, calcium, magnesium, potassium, and sodium were very low in soils under the landuse types except for magnesium that was moderate in the soil under PO. The effective cation exchange capacity was generally low according to the rating of Landon. However, total exchangeable acidity dominated over the total exchangeable bases. Table 3 indicates the result of the clay dispersion ratio (CDR), clay dispersion index (CDI), and clay flocculation index (CFI), under the different land-use types. The clay dispersion ratio has mean values ranging from 47.92 to 56.24% for soils under the different land-uses, while the CDI has mean values ranging from 29.06 to 49.04% for soils under the different land-uses. However, CFI had a mean of 63.62%, 71.24%, 59.14%, and 50.96% for soils under OP, CF, PO, and PP landuses, respectively. The CDR and CFI values were contrary to the finding of Igwe and Udegbunam (2008) in southern Nigeria. The clay dispersion ratio showed no significant difference (p=0.05) among the soils of the different land-use types. Soils of the studied land-use types were found to be sensitive to erosion since they are above 15% as recommended by Middleton (1930). The clay dispersion index for the soil under the PP land-use differed significantly (p=0.05) from the soil under the CF land-use. However, the clay dispersion index had no significant difference among OP, CF, and PO land-uses. The clay flocculation index for the soil under the CF land-use differed significantly (p=0.05) from that for the soil under the PP land-use, but CFI had no significant difference among the soil under OP, PO, and PP land-uses. According to Agassi and Bradford (1999) and Igwe (2012), the variation among the erodibility indices under different land-use types could be associated to soil textures, aggregate stability, shear strength, soil structures, infiltration capacity, soil depth, bulk density, soil organic matter and chemical constituents. The CFI index was greater than the CDR and CDI indices in all the studied land-uses as rated in percentage. However, the CDR was greater than the CDI in all the studied land-uses except for the PP land-use where the CDI (49.04%) was greater than the CDR (48.16%). Soils high in CFI are well aggregated and will not easily disperse in water. Igwe (2005) opined that CFI ranked highest among other micro and macro aggregate indices in predicting potential soil loss in some soils of south-eastern Nigeria. Soils with high CDR and CDI are known to be structurally weak and can easily erode. Also, the high value of CDR with low CFI indicates low resistance of soil aggregates to the breakdown by water. This is in conformity with the findings of Bajracharya et al. (1992). Research findings showed that soil with a relatively low erodibility factor might indicate signs of severe erosion. However, the soil could be highly erodible and suffer little erosion because soil erosion is a function of many factors (Nyakatawa et al., 2001). However, Chakrabarti (1990) has reported that soils susceptibility to erosion is significantly related to the clay dispersion ratio. This result, therefore, indicates that the soils from the studied area are susceptible to erosion. Similarly, CDR, CDI and CFI depend on the aggregation of soil particles.

The relationship between erodibility indices and soil properties
The result, as indicated in Table 4, shows that CDI correlated positively (r=0.25, r=0.10, r=0.38) with silt, porosity and moisture content but had a negative correlation (r=-0.17, r=-0.21, r=-0.094, r=-0.155, r=-0.19) with sand, clay, bulk density, pH(H 2 O) and OC. CFI had a positive correlation (r=0.17, r=0.21, r=0.095, r=0.19) with sand, clay, bulk density and OC while it had a negative relationship with silt, moisture content and pH (H 2 O). CDR had a positive significant relationship (r=0.47, p=0.05) with OC and a negatively relationship (r=-0.286, r=-0.527) with bulk density and pH(H 2 O) while it had a negative significant relationship (r=-0.56, p=0.05) with silt. Igwe and Udegbunam (2008) reported that CFI correlated positively with clay and OC. The sign of the coefficient indicates the direction of the relationship. If both variables tend to increase or decrease together, the coefficient is positive, whereas if one variable tends to increase as the other decreases, the coefficient is negative.  Table 5 shows the relationship between the erodibility indices. The CDR had a positive relationship (r=0.143) with CDI, whereas it had a negative relationship (r=-0.146) with CFI. However, CDI had a highly significant negative relationship (r=-1.000, p=0.01) with CFI.

Conclusion
The study assessed erodibility indices relationship with soil properties affected by land-use types. According to the evaluation made for three erodibility indices (CDR, CDI and CFI), it was found that soils of the study area are susceptible to erosion. The erodibility varied among the land-use types in an increasing order of CF<PP<PO<OP, which is an indication that soils of CF were the least erodible. However, preventive measures against erosion should be taken through modifications of the regular management procedures, which will ameliorate the rate of soil degradation. In order to prevent soil erosion, some preventive measures such as planting of cover crops, application of organic matter and mulching should be put in place as these will help ensure proper aggregate stability, required infiltration rate and soil productivity. Also, activities such as topsoil mining, indiscriminate bush burning, and deforestation should be avoided. The government should conduct enlightenment programmes for the public on sustainable environmental conservation practices.