Abstract

Soil organic carbon (SOC) in relation to aggregate stability, plant biomass accumulation and other properties of a Ferric Acrisol under different fallow management practices was determined to ascertain their potential for sequestering carbon. Three minor season fallow treatments replicated four times were natural and burning (T₁), natural and plough-in (T₂), pigeon pea (T₃), bare land (T₄), cowpea (T₅), mucuna (T₆) and natural and fertilized cropped (T₇).Inherent nutrient status of the soil was low. Generally, dry matter (DM) yield increased for all the treatments in the minor season of 2006 more than in 2005. A high DM yield for T₃ resulted from the shrubby and semi-woody nature of pigeon pea.Moisture stress from low rainfall decrease DM yield in 2007. In 2005, SOC contents of the treatment plots were lower than the initial amount although, T₃ produced relatively the highest SOC accumulation (20,293 ± 326 kg C ha^-1). In 2006, the legume-amended treatments (T₃, T₅ and T₆) had similar SOC contents as the control (T₄); the lower SOC contents in the natural fallow plots confirmed the negative effect of burning, especially in T₁. Soil OC accumulation was greater in 2007 than in 2006 (except for T₄). Water dispersible silt fraction decreased with increasing SOC accumulation (r value = - 0.88**). Dispersion ratios, more related to SOC (R² value of -0.95** in the natural fallows and - 0.76* in the legume fallows), generally decreased from an average of 0.88 in 2006 to 0.50 in 2008 emphasizing the positive role of aggregate stability in SOC accumulation.

Key words:    Organic carbon, crop fallow, dispersion ratio, bulk density, dry matter yield, erodibility, plant biomass, particle size distribution.