DISTRIBUTION OF SOIL ORGANIC CARBON IN SOIL FRACTIONS OF MANGROVE ECOSYSTEM IN ILAJE AREA OF ONDO STATE, NIGERIA

Abstract

Distribution of soil organic carbon in soil fractions of the mangrove ecosystem in Ilaje Area of Ondo State, south-western Nigeria was investigated. Soil samples were obtained from three sites under different land management practices of cultivated land, regenerated land and natural forest land at three soil depths of 0-20 cm, 20-40 cm, and 40-60 cm, and 3 points along the diagonal of each plot measuring 20 m × 20 m earlier laid out on three transects cut 100 m apart in each of the study sites using a 3 cm diameter soil auger. The auger samples were composited separately on a plot and soil depth basis, air dried at room temperature, sieved through a 2mm sieve, and analysed for total, particulate, mineral-associated soil organic carbon as well as other physico-chemical soil properties. The experiment was laid out as a randomized complete block design (RCBD) with the land management practice and soil depth constituting the major sources of variation considered. Results revealed that Soil Organic Matter (SOM) ranged from 9.82±0.89 % for 20-40 cm depth of cultivated land to 21.33±2.58 % for the 20-40 cm depth of natural forest. There is a significant difference (P< 0.05) in the SOM of the soils across the land management practices. However, there is no significant difference (P> 0.05) in the SOM of the soils with respect to the soil depth for all land management practices. Total Organic Carbon (TOC) values ranged from 5.70±0.52 % for 20-40 cm depth of cultivated land to 12.37±1.50 % for 20-40 cm depth of natural forest. There is significant difference (P< 0.05) in the TOC of the soils across the land management practices but there is no significant difference (P> 0.05) in the TOC of the soils with respect to the soil depth for all land management practices. Particulate Organic Carbon (POC) and Mineral-associated Organic Carbon (MOC) on the other hand was influenced by variations in land management practices with the POC ranging from 3.21±0.37 % for 20-40 cm depth of cultivated land to 6.42±0.36 % for 20-40 cm depth of natural forest. Also, MOC ranged from 1.81±1.02 % for 0-20 cm depth of cultivated land to 5.95±1.28 % for 20-40 cm depth of natural forest. There is significant difference (P< 0.05) in the POC and MOC of the soils across the land management practices but there is no significant difference (P> 0.05) in the POC and MOC of the soils with respect to the soil depth for all land management practices. The Total Nitrogen (TN) content also followed a somewhat similar trend as the POC with a range of 0.18±0.00 % for 40-60 cm depth of cultivated land to 0.22±0.02 % for 0-20 cm depth of the natural forest. There is also a significant difference (P< 0.05) in the TN of the soils across the land management practices, but no significant difference (P> 0.05) was observed in the TN of the soils with respect to the soil depth for all land management practices. The pH of the soils was consistently moderately acidic for all soil depths across the land management practices investigated with a range of 4.24±0.10 for the 40-60 cm depth of the natural forest to 5.38±0.06 for the 0-20 cm depth of the cultivated land.