Abstract

Soil organic carbon (SOC) and total nitrogen (TN) play a central role in physico-chemical fertility of a soil, and thus promoting agricultural productivity. Yet little is known about SOC and TN dynamics in tropical ferruginous soils of Sub-Saharan Africa. In this study, thicket and intergrowth soil samples, under both cultivation and perennial grass fallow (Andropogon gayanus), were collected in Bondoukuy, western Burkina Faso. The samples were fractionated and their SOC and TN contents in six organo-mineral fractions were analyzed. Because of the high labile organic matter pools in coarse sand fractions, SOC (~630 µg C g ^–1 soil) associated with these fractions appeared to be more accessible to soil microbes than recalcitrant and occluded pools (~440 µg C g ^–1 soil) within the fine fractions of the fallow soils. The results also indicated that clay fractions are likely to represent a source of the available nitrogen to crop following long fallow periods (~20 years). In contrast, the differences in TN contents were not significant (p>0.05) between ploughed plots and young fallow lands (~10 years). The substantial decrease in C/N ratios from coarse particulate organic matter pools (C/N=68) to fine pools (C/N=10) suggested an increase in the SOC decomposition rate in the fine fractions. This indicates a substantial decrease in microbial activities following a reduction in particulate organic matter sizes. The SOC contents were relatively high in coarse (~930 µg C g ^–1 soil) fractions of the thicket soils compared to those of the adjacent intergrowth soils (~620 µg C g ^–1 soil). A similar SOC distribution pattern was also observed in fine fractions of the thicket and the intergrowth soils. Total nitrogen also exhibited a high distribution pattern in fine sand and very fine sand fractions. The findings of this study demonstrated that SOC and TN restoration in semi-arid tropical savannah soils is a function of particulate organic matter sizes, vegetation type and soil management practices.

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