Carbon losses from prolonged arable cropping of Plinthosols in Southwest Burkina Faso

Abstract

The ongoing conversion of natural ecosystems of the tropics into agricultural land usually results in a loss of soil organic carbon (SOC), the extent of which being largely affected by climate and soil order. We assumed that i) SOC losses are not restricted to the surface soil but extend into the subsoil, but that ii) the rate of SOC losses is controlled by interactions of SOC with Fe oxides. Hence, we sampled Plinthosols down to one meter (if feasible) that had been converted from native savannah into cropland 0 to 29 years ago in the Dano district (Southwest Burkina-Faso). Beside the assessment of SOC stocks, we also determined the proportion of surface (0–10cm) SOC remaining after Fe oxide removal as well as its distribution among the following particle-size fractions: 2000–250μm (coarse sand-sized SOC; POM1), 250μm–53μm (fine-sand-sized SOC; POM2), 53μm–20μm (very fine sand-sized SOC; POM3), and <20μm (nonPOM). We found that the soils lost 24tCha−1 from the upper 10cm and 49tCha−1 from the upper 30cm, respectively, after 29 years. SOC losses extended also into the subsoil, and, on the average, 0.7–19.5tCha−1 was lost from the 30–100cm depth interval. The loss rates of SOC were similar to those reported for other soil types. Rapid losses of SOC occurred from all POM fractions as well as from the Fe-oxide pool, which, on average, contained about 16% of total SOC stock. We conclude that even if Fe oxides as well as other silt- and clay-sized organomineral complexes delay losses of SOC relative to POM, they failed to efficiently protect SOC from decomposition in these Plinthosols.