Land use Effects on Soil Properties and Carbon Stocks of Agricultural and Agroforestry Landscapes in a Rainforest Zone of Nigeria

This study examined the impacts of land use on soil physical, chemical and biological properties along agroforestry and agricultural landscapes in a rainforest zone of Nigeria. The land use systems are forest, agroforestry, fallow and ornamental plant field in addition to permanent crop fields constituted by cocoa, oil palm and citrus and annual (arable) crop fields (maize). Profile pits were dug from the land use types from which samples were collected 0-20 cm and 20-50cm for laboratory analysis of soil properties. Undisturbed soil samples were also collected from the pits but opposite sides for soil bulk density and moisture content determination. Standard soil analytical procedures were followed in carrying out soil analysis. Results showed that among the land use types, soil physical properties: sand, clay, soil bulk density and chemical: soil pH, SOC total nitrogen, P, K, Ca, Mg and CEC differed significantly among the land use types. The bulk density of the soils, pH, SOC, total N and stocks of SOC and total N statistically differ along 0-20 and 20-50 cm soil depths. SOC and total N stocks increased downwards along depths sampled. Sandy loam was the dominant soil textural class. Permanent croplands including forest and agroforestry had higher SOC, total N, pH and CEC while arable crop land had relatively low amount of SOC, TN, pH and, P, K, Ca, Mg and CEC. In addition, the arable cropland had significantly lower soil C and N stocks in the top 50 cm (0.50 m) soil layer compared with the permanent crop fields. The lower values of these variables from maize field may be due to the effects of continuous tillage practices by the smallholder farmers in the area, and soil erosion may be responsible to the removal SOC and total N from soil surface (0 – 20 cm depths). Among permanent and annual crop fields, the SOC and total N stocks of the land uses for 0-20 cm depth ranged from 5.75 to 3.12 kg/m2 for 0-20 cm depths and 2.44 to 1.93 kg/m2 for 20-50 cm depth. Relative to forest soil, stocks of SOC in the surface soils (0-20 cm) decreased in the order: agroforestry > ornamental plant field > cocoa> fallow land > citrus > oil palm > annual cropping system. Following this decreasing order, soil deterioration indices are equivalent to 27 > 28 > 30 > 31 > 32 > 34 >38 % compared with forest soil respectively. Strong significant correlations (p < 0.05) were observed between SOC and TN stocks and some soil properties (bulk density, clay contents, pH and CEC) with R2 values ranging from 1.0 to 0.85. It is concluded that land-use and soil depth influenced soil physical, chemical (nutrient fluxes: organic carbon, N, P, K, Ca, Mg and CEC), biological properties and carbon storage potential in the study area.