Abstract

This study aimed to assess soil organic carbon (SOC) and total nitrogen (TN) dynamics under fallow lands influenced by the perennial grass Andropogon gayanus and to show how the biological activity is improved during the Sudanian tillage system in the area of Bondoukuy (Western Burkina Faso). Soil samplings were done through cultivated plots (CP), ten (F10) and twenty (F20) years old fallow lands. Measurements were done in thickets and intergrowth areas of the perennial grass in two horizons: the topsoil (0-10 cm) and the subsoil (10-20 cm). Results showed that SOC concentrations are generally higher in the old (0.35%) than in the young fallow lands (0.29%) and in the cultivated plots (0.23%). TN concentrations followed the same pattern (0.022%, 0.017% for the old and young fallows lands and 0.013% for the cultivated plots). The C:N ratio observed (15~20) suggests an important soil organic matter (SOC and TN) maturation state in the fallow lands (F10 and F20) than in the cultivated plots (CP). Soil mineralization is also more important in the two fallow lands than in fields. For the total nitrogen mineralization, we have an important production of mineral nitrogen always in old fallow lands and a positive effect of the thicket on the net mineral nitrogen accumulation (p<0.05). The transition from thicket to intergrowth area permits obtaining positive variations which are relatively significant (p<0.05). A. gayanus fallow lands play an active role in managing SOC and TN dynamics. The most SOC and TN accumulated was found in the topsoil of thickets, where the maximum plant debris is located. Old fallow lands are best conditions for the recovery of SOC and TN from their steady states. Then, when clearing the vegetation for cultivation after the old fallow lands, there is an important input of fresh OM available for plants in the soil for 3 or 4 years. It is recommended to observe the old fallow phase prior to clearing for cropping.

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