Relative Contribution of Aggregated Soil Carbon to Soil Organic Carbon Pool in an Ultisol, Southeast Nigeria

Abstract

Soil organic matter (SOM) maintenance is crucial for mitigating soil degradation, reducing atmospheric CO₂ concentrations, and enhancing the soil organic carbon (SOC) pool, thereby alleviating the impacts of greenhouse gases on the environment. In the humid tropics, soil fertility depletion due to erosion and leaching from intense rainfall poses significant challenges. This study aimed to evaluate the relative contributions (RC) of various water stable aggregated soil carbon (CWSA) to the SOC pool in soil samples collected from plots planted to sole cowpea, sole maize, and a maize-cowpea intercrop, employing both no-till (NT) and conventional tillage (CT) methods. Organic amendments, including poultry droppings, pig waste, and cassava peels at 20 t/ha, were applied alongside a control in a split-split plot arrangement within a Randomized Complete Block Design (RCBD). Soil samples were collected at a depth of 0-30 cm at the end of the planting season, and SOC was quantified using the Walkley and Black wet oxidation method. Results indicated that aggregate size fractions of 1-0.25 mm contributed more carbon to the SOC pool than those larger than 2-1 mm, irrespective of cropping system, tillage method, or organic amendment applied. Notably, the highest relative contribution of aggregated soil carbon transitioned from micro-aggregates (<0.25 mm) to macro-aggregates (1.0-0.25 mm) during the application of organic amendments, but reverted to micro-aggregates upon their withdrawal. These findings underscore the importance of organic amendments in enhancing soil aggregation, improving the SOC pool, and ultimately fostering soil fertility and productivity. Thus, the use of organic amendments is recommended to sustain soil health and agricultural resilience.