ABSTRACT

Spatial analysis of soil C sequestration, aggregation and aggregate stability under selected land use types and parent materials was carried out in some parts of Southern Nigeria. Experimental design used was a 2 x 5 x 3 factorial in randomized complete block design (RCBD).  Factors were land use [managed tree croplands (MTC) and continuously cultivated croplands (CCC)], parent material [Coastal Plain Sands at the Forestry Research Institute of Nigeria (FRIN), Isieke; Shale at the National Horticultural Research Institute (NIHORT), Okigwe; Imo Clay Shale at the Anambra-Imo River Basin Development Authority (AIRBDA), Agbala; Coastal Plain Sands at the Niger Delta Basin Development Authourity (NDBDA), Kpong and Alluvium at the Niger Delta Basin Development Authority (NDBDA), Isiokolo] and depth (0-19, 20-39 and 40-100cm soil depths). Correlation and regression analyses were calculated. Best fit semivariogram models with ordinary kriging was employed to generate predictive maps of selected soil properties and how they are distributed over the study area. Results showed that higher (p<0.05) MWD and %WSA in the fractions >0.50mm were observed under MTC, whereas higher (p<0.05) %WSA in the fractions <0.50mm were observed in soils under CCC. Oil palm plantation improved (P<0.05) soil aggregation and aggregate stability more than other MTC and CCC land uses. Soils under MTC sequestered higher (p<0.05) amount of C across the soil depths relative to CCC. Among the MTC land utilization types assessed, Pine forest sequestered the highest (575.80MgCha-1) while Citrus orchard had the least (340.30MgCha^-1). Coastal plain sands at the FRIN-Isieke had the highest (p>0.05) amount of SOC pool (575.80MgCha-1). While Alluvium at the NDBDA-Isiokolo gave the least (409.50MgCha-1). Also, soils formed under coastal plan sands and Imo clay shale improved soil aggregation and aggregate stability when compared to other parent materials assessed. SOC significantly accounted for between 35% and 60% of variability in MWD, ASC and %WSA in WSA classes >2.00mm, 1.00-2.00mm and 1.00-0.50mm in soils under CCC. It also accounted for 40% variability in %WSA in >2.00mm and 1.00-2.00mm WSA fractions for soils under MTC. Soil parameters were variable from semi-variogram model. Coefficient of determination (R²) of the predicted soil parameters were >0.50. Low (0.312-0.572) root mean square error (RMSE) showed that the theoretical model was an adequate representation of the spatial properties of the soils. The study revealed that SOC sequestration is a function of land use, parent materials, depth and aggregatie sizes. Whereas, aggregate stability is a function of SOC content and / or Fe and Al oxides in the soil mineralogy. GIS based maps of soil properties in other areas across the country should be generated. This will help in making site-specific decisions and policies relating to land use planning.