Land use types can of considerable interest in efforts to quantify soil organic C and nitrogen (N) cycling, both of which are important for soil stability and fertility. A study was carried out on a tropical rainforest soils in southern Nigeria to quantify soil organic carbon and nitrogen pools and other indices under forested, 5-year fallow, cocoa plantation and a 5-year continuous cassava land uses. Soil samples were collected at 0-15 cm and 15-30 cm depths for measurement of these indices. Results revealed that total organic carbon (TOC and total nitrogen (TN) varied significantly among the different land uses. TOC of 30.2 g kg-1 was higher (p<0.05) in forested soils at in the 0-15 cm top soil and 22.3 g kg-1 in the subsoil. Total N was statistically significant (p<0.05) at 2.61 and 2.01 in forested and 5-year fallowed soils, respectively. The carbon pool index (CPI) was significant at 1.63 in forested soils, while nitrogen pool index (NPI) 1.30 in similar soil (p<0.05) in 0-15 cm topsoil. In 15-30 cm subsoil, CPI and NPI values were in the order of forested > cocoa plantation > 5-year cassava cultivation, with the least values of 0.44 CPI and 0.35 NPI found in 5-year cassava plots. It was concluded that CPI and NPI are valuable indicators for analyzing changes in soil quality induced by maintaining a 5-year fallow. It was found that forested and 5-year fallow land uses can be used to improved C and N stock, as well as the structural and hydraulic properties of the soils.
The effects of different cultivated crops and their growth periods on some hydraulic and chemical properties of a sandy clay loam soil at the University of Port Harcourt, Teaching and Research Farm, in Choba, Rivers State, Nigeria was studied. The objective was to understand the changes in soil physical and chemical properties near the plant environment during the growing period for optimum management. Four (4) crops: Maize (Zea mays L.), Okra (Abelmoschus esculentus), Pepper (Capsicum annuum), and Garden egg (Solanum melongena F.) were planted to native fallowed soil and the soil properties measured at three (3) growth stages of the crops viz: Establishment, Flowering, and Maturity. Results revealed that the plants did not modify the soil textural class. Significant changes in bulk densities and saturated hydraulic conductivity were found during the flowering stage of the crops. Bulk density of 1.36 and 1.34 g cm-3 were significantly (p<0.05) low at during flowering and maturity stages, respectively, in Garden egg soils. The highest saturated hydraulic conductivity (Ksat) values of 22.9 and 27 cm h-1, respectively, were significant (p<0.05) during the same periods for garden egg, followed by Maize and Okra. Results also revealed that Okra, Pepper and Garden egg significantly (p<0.05) reduced the soil acidity at flowering growth period. Maize contributed significant additions of soil organic matter to the soil at flowering and maturity periods. Maximum removal of available plant nutrients was at flowering stage for maize when more than 80% of the plant had shown tasselling and cob formation. Therefore, monitoring the growth stages of specific plant could help in nutrient and soil management and changes in soil physical and chemical properties.
Deterioration of soil physical properties and organic matter depletion resulting from continuous cultivation of tropical sandy loam soils, even with the regular applications of inorganic fertilizers is common. To regenerate the soil organic matter and maintain soil structural and hydraulic properties for crop production, the effects of different legumes green manures and NPK chemical fertilizer were evaluated after a 3-year field experiments at the University of Port Harcourt, Teaching and Research Farm (2018 – 2021). Treatments were: 5 tons ha-1Centrocema (Ce5), 5 tons ha-1 Gliricidia (Gl5), 5 tons ha-1Calapogonium (Ca5), NPK 20:10:10 (NPK) and the Control, arranged in a Randomized Complete Block Design (RCBD), in 5 replications. Results showed that three annual applications of Centrocema, Gliricidia, and Calapogonium increased soil organic carbon pool at the 0-30 cm topsoil, and reduced the bulk densities by 8.5%, 4.1%, and 7.0%, respectively, compared to the NPK fertilizer and control. The increased in bulk density in untreated plots led to decrease in water stable aggregates, low water holding capacity and low yield of cucumber crop. Green manures applications increased soil organic carbon pool, improved topsoil physical properties and cucumber yield, while NPK chemical fertilizer had negative effects on these properties.
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