SOIL ORGANIC CARBON SEQUESTRATION POTENTIALS IN AGGREGATE FRACTIONS OF CULTIVATED AND UNCULTIVATED SOILS OF SOUTHEASTERN NIGERIA

 
A study was carried out on soils sampled at 0-10, 10-20, and 20-30 cm depths from both cultivated and uncultivated soils at four different locations (Awgu, Okigwe, Nsukka I, and Nsukka II), to evaluate the potentials of various aggregate size fractions of varying soil textures and depths to sequester carbon under different land uses. A 4 x 2 x 3 factorial experiment was conducted in a completely randomized design (CRD). Factor A was location at four levels, while factor B (land use) had two levels. Factor C (soil depth) comprised of three levels. Results showed that in both land uses, soil texture varied with depth in each location and included clay, loam, clay loam, sandy loam and sandy clay loam. Generally, all the soil properties varied with soil depth across the locations and land uses. Land use significantly (P = 0.05) affected pH in KCl, Ca2+, Al3+, CEC, 0.50-1.00 mm water stable aggregates (WSA), total soil nitrogen (TSN) in 1.00-2.00 mm WSA, and soil organic carbon (SOC) in 1.00-2.00 mm and < 0.25 mm WSA. Cultivation at 0-30 cm depth significantly reduced SOC in 1.00-2.00 mm WSA by 19.30 %, and TSN in 1.00-2.00 mm WSA by 2.50 %. Land use effects on SOC in WSA at 0-30 cm depth of the various locations followed no consistent trend, except that SOC was higher in cultivated than in uncultivated soils of Nsukka II location. The SOC pool significantly decreased with soil depth. The SOC pool at 0-10 cm, 10-20 cm, and 20-30 cm depths averaged 17.62, 16.40 and 13.05 Mg C ha-1 respectively, in cultivated soils; and 19.59, 17.86 and 12.03 Mg C ha-1 respectively, in uncultivated soils. The SOC pool to the depth of 30 cm differed distinctly amongst the study sites in both land uses; however, cultivation had no significant effect on SOC pool. The effect due to soil texture on SOC pool indicated that C sequestration was significantly greater in clay loam > clay > sandy loam > loam > sandy clay loam. In all, SOC pool was most secluded at 10-20 cm depth, and least at 20-30 cm depth. Whereas SOC pool significantly correlated with dispersion ratio (DR), aggregated silt and clay (ASC), water dispersible clay (WDC), microporosity (Pmi), 0.50-1.00 mm WSA, mean weight diameter (MWD), soil pH, K+, and C/N ratio in cultivated soils; it correlated significantly with ASC, Na+, and CEC in uncultivated soils. Apart from Pmi, whose variability was largely due to the effect of SOC that significantly predicted up to 76 %, SOC significantly accounted between 34 % and 54 % of the variability in MWD, WDC, and WSA classes of > 2.00 mm, 1.00-2.00 mm and 1.00-0.50 mm of the cultivated soils.