ABSTRACT
Soil Organic Carbon (SOC) is generally partitioned into Labile Organic Carbon (LOC) and Mineral Associated Organic Carbon (MAOC) forms without taking cognizance of the Recalcitrant Organic Carbon (ROC) form. This study was conducted in order to gain more insight into the forms and distribution of soil organic carbon associated with different land use practice. Representative soil samples were obtained from the land of different uses in a 5x5km grid size in the three ecological zone of Edo State i.e. the semi-derived savannah, derived savannah and Benin rain forest zones respectively representing Esan South East, Etsako East and Orhionmwon LGAs, Edo State. The soils were collected from 0 – 15cm and 15 – 30cm depths which represented the top soil and sub soil respectively. The soil samples were physically fractionated into labile organic matter and recalcitrant organic matter following standard methods in the laboratory while Walkley and Black method was used to determine the organic carbon in the two fractions. Selected physical and chemical properties of the soils were also determined and the results obtained were subjected to statistical analyses. The results show that the soils were generally acidic with the mean values of pH as 5.3, 5.17 and 4.7 for Esan South East, Etsako East and Orhionmwon Local Government Areas respectively. The soils were generally sandy loamy in texture with percentage mean values of 5.91, 4.47 and 88.49 for clay, silt and sand respectively for Esan South East Local Government Area; 4.6, 4.5 and 91.05 respectively for clay, silt and sand respectively for Etsako East Local Government Area and 7.6,3.9 and 86.6 respectively for Orhionmwon Local Government Area. It was observed that the labile organic carbon was low with percentage mean values of 0.74, 0.79 and 0.71 for Esan South East, Etsako East and Orhionmwon Local Government Area soils respectively while the percentage mean values for recalcitrant organic carbon was found to be xvii 4.40, 3.38 and 4.01 for Esan South East, Etsako East and Orhionmwon Local Government Area respectively. The percentage mean values of total organic carbon for the three soil types were1.59, 1.11 and 0.95respectivelyfor Esan South East, Etsako East and Orhionmwon Local Government Area respectively. Labile organic carbon accounted for 14, 13 and 15 percentages of the total organic carbon for Esan South East, Etsako East and Orhionmwon Local Government Area soils respectively while the recalcitrant organic carbon accounted for the percentages of 86, 87 and 85 respectively for Esan South East, Etsako East and Orhionmwon Local Government Area soils respectively indicating that the recalcitrant organic carbon is of a greater proportion than the labile organic carbon. Furthermore, the labile organic carbon fraction in Orhionmwon Local Government Area soils had the highest value. This could be attributed to the relatively high level of mineralization of the organic materials in the soils. Geospatially, the distribution of the various forms of organic carbon in the soils are as follows; 24.7%, 11.4%, 68.0%; 57.2%, 40.7%, 174.2% and 8.1%, 6.1%, 34.3% for Esan South East, Etsako East and Orhionmwon Local Government Areas respectively. The entire results from the study is suggestive of the following: poor soil fertility, wind erosion, deep leaching and perhaps the adverse impact of global warming in the study areas. The present study has contributed to generating baseline data regarding the different forms and distribution of organic carbon in soils from semi-derived savannah, derived savannah and Benin rainforest representing Esan South East, Etsako East and Orhionmwon Local Government Areas in Edo State, South-South, Nigeria.
