Different land-use affects the organization of mineral soil particles and soil organic components into aggregates and the consequent arrangement of the aggregates will influence essential ecosystem functions. We investigated a continuous rubber plantation (forested), land fallowed for 10 y (fallow), 10-y continuous arable cropping land and cropped land with top soil removed (TSR) for concentrations of C, N, and P in bulk soil and dry aggregates. Results showed that a high level of soil disturbance decreased the proportion of surface (0-15 cm) soil aggregate stability (low mean weight diameter) in TSR by 149% and arable cropping by 125% compared with the forested. Aggregate associated SOC was higher in aggregate-size fractions of forested land-use when compared with that in 10-y fallow, continuous arable cropping, and TSR. For aggregate associated N, fallow and forested land-use types concentrated higher proportion across aggregate sizes than the arable cropping and TSR. Macro aggregate fractions generally contained higher concentrations of C, N, and P compared with the micro-aggregates. Water transmission indicators like total porosity and saturated hydraulic conductivity recorded higher values with forested and fallow land-use than the others. We can thus conclude that long-term soil disturbance due to cultivation and removal of top soil reduces the accumulation of soil C, N, and P in bulk soil and decreases water transmission properties. On the other hand, aggregate-associated C, N and P accumulations are dependent on the level of soil surface disturbance and aggregate sizes.

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Global emphasis on food security and soil health should consider rehabilitation of degraded lands, especially where oil contamination limits the use of such lands. Three legume plants (Gliricidia sepium, Leucaena leucocephala and Calapoconium caerulean) alone or along with 0.5% (w/w) (equivalent of 10 tons ha -1 ) poultry manure, were used to treat the soil, in which the oil residuals were monitored for three years. Results showed that significantly high levels of residual total petroleum hydrocarbon content (RTPHs) persisted in the non-amended soil after 36 months. At 3 months, 43% of RTPHs was removed by Gliricidia sepium and Leucaena luecocephala along with poultry manure. Net loss of RTPHs after 12 months was 69% for Gliricidia sepium and Leucaena luecocephala combined with poultry manure and only 38% for A 5 , explaining that, degradation of petroleum hydrocarbon by indigenous soil micro-organisms was very low. At 18 months when additional load of oil was applied, the degradation rate increased from 71.7 mg kg -1 day -1 to 142 mg kg day -1 within 6 months for all the legume plants along with poultry manure. This explained increased in number of hydrocarbon-degrading micro-organisms due to increase in oil load. The oil inhibited germination and yield of maize crop completely. Treatments with legume plant along with poultry manure significantly (p < 0.05) increased both germination and yield of maize crop. The effects of legume plants combined with poultry manure were the better treatment than legume or poultry manure alone in improving the soil properties for growth and performance of maize crop.

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Information on the use of plant species and organic nutrients to improve the physical properties of oil-contaminated soil, with a view to making it conducive for crop production, is very important. Three legumes (Gliricidia sepium, Leucenae leucocephala and Calapogonium caeruleam) combined or not with poultry manure were tested for their ability to improve the physical properties of a sandy soil, contaminated with 5% (w/w) spent lubricating oil, each for two years, and its effects on the growth and yield of maize in south eastern Nigeria. Aggregate stability (MWD), saturated hydraulic conductivity, and macro-to micro-porosity ratio improved with time in all the treatments over the 5% oil contamination (A 5 ) and control. At 12 and 18 months, the use of Gliricidia sepium with 0.5% (w/w) poultry manure (A 5 +Gl+PM) gave the highest improvement of 58% and 94% in MWD, corresponding to 136% and 187% improvement in saturated hydraulic conductivity, respectively over the A 5 . The A 5 +Gl+PM also enhanced soil aggregate sizes > 0.25 mm by 63.6% and showed a 3-fold positive modification in soil macro-porosity. Positive relationships, significant at 0.01 level of probability, were observed between crusting hazard (R) and soil organic matter (r = 0.814), microporosity (r = 0.686) and saturated hydraulic conductivity (r = 0.787). The A 5 +Le+PM also increased maize growth and grain yield. Gliricidia sepium and Leucenae leucocephala combined with.0.5% (w/w) poultry manure is recommended for follow-up investigation, as it may offer a viable choice for remediation of oil-contaminated soil.