Carbon Sequestration As Influenced By Cropping Systems, Tillage and Organic Amendments in a Sandy Loam Ultisol in Southeastern Nigeria

Abstract

Climate change has been a global issue in the recent years, as a result of increase in the atmospheric concentration of carbon iv oxide (CO2) and other green house gases (GHGs), emanating from land use changes and other human activities. To date, field-based measurements of SOC changes in agronomic production systems of Tropical Africa especially Nigeria are scarce. The study of SOC sequestration under the local farmer’s real situation in Nigeria especially the southeastern region is of importance due to the alarming rate of land misuse and soil mismanagement (the precursors of land degradation and climate change) in the region. The main objective of this study, therefore, was to investigate the influence of cropping systems, tillage and some organic amendments on soil carbon sequestration in an Ultisol at Nsukka, Southeastern Nigeria. A land area of 0.1125 ha was planted to sole cowpea, sole maize, and maize-cowpea intercrop in no-till (NT) and conventionally tilled (CT) plots, amended with poultry droppings (PD), pig waste (PW) and cassava peels (CP) at the rates of 20, 90, and 100 t/ha respectively. The experimental design was split-split plot in randomized complete block design with three replications. The cropping systems were assigned to the main plots, tillage system to split plot while the organic amendments and a control were assigned to split-split plot measuring 7.5m2. The same test crops, treatments and replications were maintained for the two planting seasons, 2011 and 2012; the residual effects of the organic amendments were studied in 2013. Plant population of 53,333 plant/ha was maintained for maize and 20,000 plants/ha for cowpea in both the sole and intercrop. Weed control was done manually at intervals. Data on crop growth were collected at intervals. At the end of each year’s harvest, the yield of both maize and cowpea were measured and soil samples collected at 0-30cm depth and analyzed in the laboratory for physical and chemical properties using standard analytical procedures. Data collected were subjected to analysis of variance (ANOVA) using GENSTAT release 7.22DE. Correlation and biplot analyses were carried out where appropriate. The study revealed soil chemical properties like Potassium, Sodium, Aluminum, pH, and Hydrogen and physical properties like Mean Weight Diameter (MWD), Water stable Aggregate (WSA), porosity and aggregate stability were significantly (P < 0.05) affected by cropping systems. Sole maize significantly (P < 0.05) increased pH and potassium; sole cowpea significantly (P < 0.05) increased hydrogen, aggregate stability, MWD and WSA. WSA, Soil Organic Carbon (SOC) and total nitrogen (TN) were significantly (P < 0.05) improved by conventional tillage (CT) and pig waste (PW) amendment. Combination of CT and PW were most effective in increasing MWD, WSA, aggregate stability and carbon stock in both the sole crops and the intercrop, followed by the combination of either CT or NT with PD. CT hastened the crop growth parameters. PW and PD were found to significantly (P < 0.05) increase SOC and significantly (P < 0.05) improve most soil chemical (pH, SOC, Phosphorus CEC, Magnesium and total Nitrogen) and physical (MWD, Bulk Density (BD), WSA, aggregate stability) properties as well as the crop yield. Residual effect of PD and PW were manifest in maize-cowpea intercrop as shown in the carbon stock chart. Residual effect of PD was higher than that of PW and CP. Treatment combinations having PW sequestered more carbon than those having other amendments. PW significantly (P < 0.05) improved macroaggregate-associated carbon. CT improved aggregate-associated SOC at fractions > 0.5mm. The highest relative contribution (RC) of aggregate-associated SOC to the SOC of whole soil shifted from the microaggregates (< 0.25mm) to the macroaggregates (1.0 – 0.25mm) for as long as the organic amendments lasted but gradually returned to the microaggregates when the amendments were withdrawn. SOC had linear relationship with MWD, aggregated silt and clay (ASC), clay flocculation index (CFI) and aggregate-associated SOC at all aggregate fractions (r = 0.548**, 0.386 to 0.507**, 0.289*, 0.360* to 0.767** respectively) and an inverse relationship with WSA < 0.25mm and dispersion ratio (DR) (r = -0.610** and -0.388 to 556** respectively). ASC had a linear relationship with CFI (r = 0.395** to 0.694**) and an inverse relationship with DR (-0.915**). CT with PW and PD amendments resulted to significant increase in the yield of sole and intercropped cowpea and maize. Cropping system had no significant (P < 0.05) effect on the major soil nutrient requirements (nitrogen, phosphorus, potassium and carbon). However at the residual year (2013), potassium was significantly (P < 0.05) improved by cropping system. CT significantly affected nitrogen and organic carbon. Combination of CT and PW significantly improved carbon stock irrespective of the cropping system. PD had a higher residual effect than the other amendments. SOC had a linear relationship with aggregate associated carbon indicating the ability of the soil to sequester carbon at all the aggregate sized fractions.