ASSESSMENT OF MINED SOILS IN EROSION-DEGRADED FARMLANDS IN SOUTH-EASTERN NIGERIA

This study assessed degradation status of erosion devastated soils as well as ameliorative measures on such soils. Field sampling was aided by morphological landscape changes. Surface samples (0-20 cm) depth were randomly collected from two erosion units and used in conducting a greenhouse experiment using maize (Zea mays L.) as a test crop. The experiment was laid out in a completely randomized design (CRD) and rates of ground periwinkle shells (GPS) and sewage sludge (SS) used as treatment. Treatments were replicated 9 times. Statistical tests of difference in soil properties were performed using t-statistic at p < 0.05. Status of soil degradation was evaluated using Land Degradation Index (LDI). Results showed low organic carbon content, and high bulk density and aggregate instability in eroded soils. The LDI values were high in most of physical parameters. The GPS and SS treatments significantly (p < 0.05) improved yield and soil properties except soil pH and aluminium saturation.


Introduction
Agriculture is a soil-based industry that extracts nutrients from the soil.Because soils are the storehouse of most plant nutrients continual withdrawals from them by continuous cropping practices without equivalent soil regenerating inputs endanger resource sustainability.An alarming concept that African soils are steadily depleted of nutrients due to farming without fertilizers has gained wide credence in the scientific community (1).As these soils become non-productive and vulnerable to natural agents of degradation, farmers still continue to cultivate on them, even expand to available and poorer non-farmlands in order to meet their basic food needs.This situation become worse in African as her population increased from an average of 2.5% in the 1960s to more than 3% in the 1990s (2).
The story of soil degradation by water erosion is topical in southeastern Nigeria (3).In this region, harsh climate, especially high intensity and long duration rainfall, high population density, land tenure constraints, shortened fallow periods, overgrazing and inefficiency of the traditional soil management practices have reduced the productive potentials of soils.The increasing farming population of the area cultivate even on fragile slope soils without appropriate conservation technology.As soils are not yielding to satisfy their basic needs, they resort to cutting down trees for fuelwood for economic gains as well as for domestic use in cooking as petroleum products are unaffordable given their socio-economic status.All these practices do not match soil quality and land use.
Several techniques and approaches are suggested in the evaluation of soil quality particularly as it affects soil degradation.Some authors evaluate decline in agricultural productivity in terms of fertilizer input, water management and tillage methods (4).Others use the soil quality morphological index (SQMI) to assess soil quality of near-surface soil physical properties (5).Again, criteria such as land use, management, prevalent weather, relative or comparative degradation, reversible and irreversible degradation indices have been suggested for the assessment of effects of degradation in terms of agricultural productivity (6).The degradative effects of soil mining due to continuous cultivation was evaluated using Land Degradation Index (7).In this technique, status of soil properties in less degraded soils are compared with values of the same properties in adjacent highly degraded sites.Levels of soil degradation are ascertained by observation of morphological variabilities of farmlands.Such morphological attributes include soil colour, soil drainage soil texture, soil structure, rupture resistance and other properties.The major objective of this study was to investigate the status of soil degradation in nutrient-mined soils of Southeastern Nigeria using Land Degradation Index (LDI).Specifically, the study evaluated the effect of different rates of ground periwinkle shells and sewage sludge in restoring degraded soils of the study area.

Site description
Southeastern Nigeria lies between latitudes 4 o 15 1

Field studies
Four maps, namely geological, geomorphological, hydrological and land use maps were superimposed to create homogenous study units.The study area covers about 13032 km 2 and soils were delineated from the same parent material (Coastal Plain Sands).Four sampling units were identified as Mbaise, Umuahia, Akwette and Oguta and their geographical coordinates are given on Table 1.Geographical coordinates were obtained using Handheld Global Positioning System (GPS) Receiver (Garmin Ltd, Kansas, USA).Macromorphological features are shown on Table 1.In each sampling unit, an eroded farmland and its adjacent uneroded farmland were chosen for the study.These two farmlands were considered similar in many edaphological features but intensity of water erosion.These sampling units were identified assisted by village informants and were randomly chosen as there existed several other similar units in the area.
Fifty topsoil samples were collected from each subsampling unit (eroded or uneroded) and from the four sampling units, giving a total of 200 soil samples for eroded and 200 soil samples for uneroded units.Undisturbed cores (diameter, 15 cm; length, 15-20 cm were excavated from the surface horizon (Ap).The column of soil encased in the polyvinyl chloride pipe (diameter, 15 cm) was trimmed at the base, packed, and transported to the laboratory for relevant determinations.Soil samples were collected at 0-20 cm depth (Ap horizon).These soil samples were air-dried, crushed and sieved using 2 mm sieve.Soils of the sampling units were classified as Ultisols (Acrisols) (8).

Greenhouse studies
Differences in productivity potentials of different rates of amendments on soil samples collected from uneroded and eroded portions of owner-managed farmlands were assessed by comparing yields obtained using maize (Zea mays L.) as indicator crop.Maize variety used was 8329-22.
In the greenhouse study, bulked soil samples collected from Ap horizon (0-20 cm depth) in each sampling unit of the study site were maintained at field capacity (20% gravimetric moisture content) and planted with maize in a Completely Randomized Design (CRD) with each treatment replicated 9 times.The maize grain yield was harvested and recorded at maturity.

Laboratory analysis
Bulk density was measured by core method of Grossman and Reinsch (2002) (9).In the laboratory, the polyvinyl chloride casing was carefully removed from each column.Therefore, each column was coated with 5-to 8 -mm layer of liquid paraffin to seal the column walls against side flow and creation of crevices between the wall of the casing and the soil.This approach would equally circumvent possible overestimation of saturated hydraulic conductivity (Ksat) in each column.The soil column were slowly welted from the bottom.After the wetting process, two 10-cm stainless steel probes were inserted horizontally to monitor volumetric water content by using time domain reflectometry (10).Saturation hydraulic conductivity (Ksat) was determined using the constant-head method (11) and 0.025 mol L -1 of KNO3 solution was used to minimize clay dispersion.Soil water-holding capacity was measured on undisturbed samples as the difference of water contents at -0.03 MPa, determined by pressure plate, and at -1.5 MPa, determined by pressure membrane (12).Total porosity was estimated from bulk density using an assumed bulk density of 2.65 Mgm -3 while macro-porosity (MP) was calculated as the ratio of volume of water drained at 60 cm tension to the volume of the bulk soil used (13).Aggregate instability was estimated as the number of water droplets required to break a ped.
After equilibrating for 30 minutes, soil pH was determined in 0.1 N KCl solution, with a soilliquid ratio of 1:2.5 using a Beckman Zeromatic pH meter (9).Soil organic carbon, total nitrogen, and available phosphorus were measured by wet oxidation (14,15), micro-kjeldahl ( 16) and bicarbonate techniques (17).Exchangeable basic cations were determined by the method of the Association of Official Analytical Chemists (18) while exchangeable acidity was measured titrimentically.

Calculation
Land degradation index (LDI) was computed as follows: Where D = degraded value of selected parameter ND = non-degraded value of selected parameter 100% = percentage grade 100 = constant representing ideal soil state.
Negative values indicate degradation while otherwise show movement in parameter values.

Statistical analyses
Statistical tests of difference of selected soil properties were performed using t -statistic at 5% level of probability.

Macromorphological
Table 1 shows results of macromorphological studies of the site.Soils were sandy and well-drained.Uneroded soils were very dark to dark brown while eroded soils were dark gray to yellowish red while sandiness is attributable to nature of parent materials, soil colour variability was in response to differential removal of organic materials by runoff water from the Ap horizon.Although soil colour is scarcely used as an indicator of soil fertility reliable extrapolations from colour of these soils can be used to predict soil quality.Grade of soil structure indicates moderate to strong structure (uneroded soils) while structureless to weak soil structures predominated in eroded soils.Similar findings were made in earlier studies in the region (19).They attributed poor soil structure at topsoils to low organic carbon content leading to macro-aggregate instability.

TABLE 1 -Macromorphological features of study site (Ap horizon)
Colour: VDB = very dark brown, DG = dark gray, YB = Yellowish brown, YR = Yellowish red Texture: LS = loamy sand, SL = sandy loam Structure: O = structureless, 1 = weak, 2 = moderate, 3 = strong Consistence: ml = loose, mfr = friable, mfi = firm Drainage: wd = well-drained, PD = poorly drained Geographical coordinates: Z = altitude.consequent reduction in total porosity, macroporosity and saturated hydraulic conductivity has been reported (20).These lead to reduction in infiltration capacity and total available water capacity of soils.Aggregate instability values were higher in eroded soils possibly due to loss of binding agents to the erosive runoff.

TABLE 2 -Changes in some physical properties of an ultisol (Acrisol) induced by water erosion (AP horizon)
* = Number of water drops required to break a ped.
Adverse changes in soil chemical properties were observed (Table 3) following water erosion of farmlands.Reduced pH and increased aluminium saturation of studies eroded soils suggest unfavourable chemical changes which impair the over all fertility status of farmlands.With substantial proportion of its occupation of cation exchange site, high Al (Aluminium saturation = 54 -68%) in eroded soils at low soil pH it suggests unavailability of plant essential nutrients at optimal levels.The pH range (pH = 3.7 -4.9) is limiting to plant performance and may cause aluminium toxicity although this depends on the species of aluminium (21).Aluminium in solution

TABLE 3 -Changes in chemical properties in an ultisol Acrisol in southeastern Nigeria induced by water erosion (Ap horizon) (mean values)
forms hydroxyl -Al polymers, ion pairs with anions, and complexes with organic substances and such complexation renders aluminium ions non-toxic.However, low organic matter content of eroded soils (OM = 1.1 -1.4%) may be suggesting poor influence of organic matter in detoxifying aluminium in these highly weathered soils of the tropics.These low pH and organic matter values could be responsible for the lower available phosphorus in eroded soils (Available P = 8.3 -10.1 mg kg -1 ).Again, soil P is lost through surface runoff, erosion of sediment, leaching and plant uptake (22).These losses endanger the environment to non-point-source pollution, such as eutrophication.

Soil degradation
Table 4 shows land degradation index values of these mined and eroded soils as they affect soil physical properties.Water retained at 1.5 MPa and aggregate instability increased in value which implies loss of available water in the rhizosphore and soil structural breakdown respectively.There were outstanding degradation in bulk density (LDI mean = -60), total porosity (LDI mean = -71), macroporosity (LDI mean = -56) and water retained at 0.1 MPa (LDI mean = -75) and these reduced saturated hydraulic conductivity (LDI mean = -25) and total available water (LDI mean = -45).
Soil chemical properties were also degraded (

-Land degradation index values of mined and eroded sites (physical properties)
Agg. Inst.= aggregate instability.
These degradations in both physical and chemical properties of soils influence overall productivity of soils and responses of even inorganic fertilizers when applied on eroded lands.This is worst under soil physical infertility thus the call for integration of organic and inorganic fertilization as a low-cost technology for restoring and sustaining mined and eroded soils of southeastern Nigeria (23).It is also implied that these degradations eventually affect microand macro-organisms as their activities reduced following soil loss (24), suggesting negative effects on the elemental transformations within the soil system.

Effects of amendments on maize yield
Table 6 indicates yield responses of maize (Zea mays L.) following application of different rates of ground periwinkle shells (GPS) and sewage sludge (SS).A mixture of GPS and SS significantly (p < 0.05) improved yield probably due to liming effect and nutrient increase in treated soils.Ground periwinkle shells released high amount of calcium which deacidified soils, and consequent release of fixed nutrients while transformation of sewage sludge resulted in additions of nutrients.in most soil properties with the exception of pH and aluminium saturation (Table 9).Insignificant changes in soil pH could be attributed to dissociation of weakly bonded hydrogen ions of amino and phenolic groups present in sewage-sourced organic matter and their consequent release into the soil system.

TABLE 6 -Effectiveness of ground periwinkle shell/sewage sludgle mixture in restoring productivity of mined and eroded soils in southeastern Nigeria using maize test as crop (t ha -1 )
t-cal ** significant at P<0.01, * significant at P<0.05, NS not significant.numerical subscripts under rate of amendment refer to rates of ground periwinkle shell (GPS) and sewage sludge (SS) measured in kg ha -1 .
Assessment of mined soils in erosion-degraded farmlands in south-eastern Nigeria

TABLE 7 -Post-harvest changes in soil physical properties in ultisols (Acrisols) Nigeria induced by soil amendment TABLE 8 -Post-harvest changes in soil chemical properties in ultisols (Acrisols) in southeastern Nigeria induced by soil amendment
Organic matter (OM) content increased in sewage-treated soils in line with earlier findings (25) that sewage application increased soil OM.The OM increases at different rates may be responsible for improvements in other soil properties, such as total available water bulk density, aggregation, hydraulic conductivity (saturation), total nitrogen, available phosphorus and effective cation exchange capacity (ECEC).Macroporosity increased with reducing aggregate instability in the amended soils leading to increase in saturated hydraulic conductivity.As longterm intensive cultivation without replenishment deteriorates organic matter and loss of productivity (26), addition of sewage sludge which is a major waste products in the study site will be beneficial in arable farming.

Table 2 .
With the exception of bulk density and aggregate instability, values of all physical parameter measured were higher in uneroded units of farmland.Increases bulk in density with