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Soil macroaggregate stability under different fallow management systems and cropping intensities in southwestern Nigeria
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Evaluation of soil aggregate stability under managed fallow systems is very relevant in the assessment of their agricultural sustainability. It is also important to identify the parameters that are most responsive to the effects of these fallow systems on aggregate stability. This study was carried out in southwestern Nigeria to quantify the effects of various fallow management systems on the macroaggregate stability of surface soil (0–15-cm depth) using a long-term fallow management trial established in 1989 on an Alfisol toposequence. Data were collected between 1994 and 1995. Three fallow systems (bush fallow, Pueraria phaseoloides, and Leucaena leucocephala) and three fallow periods (1–3 years after 1-year cropping) were evaluated. Cropping consisted of maize+cassava intercropping. A split-plot design with fallow systems as main plot and cropping intensities as subplot in four replications was used in the trial. Soil aggregate samples were wet-sieved to evaluate their mean-weight diameter (MWD) and fractal dimension (D). The means of MWD for the fallow systems and cropping intensities ranged from 2.4 to 6.4 mm. The mean D values ranged from 2.29 to 2.72 while the mean intercepts (log k) of the regression ranged from 2.01 to 2.28. Low D values (cohesive and stable aggregates) were associated with fallowing whereas high D values (fragmented aggregates) were associated with cultivation. Fallowing enhanced soil aggregate stability. The Pueraria system also enhanced soil aggregate stability more than the bush fallow and Leucaena systems when continuous cropping was practised. The subplots, which were fallowed for 2 or 3 years after 1 year of cropping, were usually similar in aggregate stability to the secondary forest soil. Fractal analysis showed further that soil aggregates in the dry season and on the upper slope of the toposequence were more cohesive than in the wet season and on the lower slope. Observed D and log k values were also significantly influenced by laboratory method.