Research paperCarbon losses from prolonged arable cropping of Plinthosols in Southwest Burkina Faso

- Carbon losses occur in both topsoil and subsoil after 29 years of cultivation.
- Carbon losses occur in all particulate organic matter (POM) and also in non POM.
- Fe oxides play a minor role in stabilizing topsoil organic carbon in Plinthosols.

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 (0-10 cm) SOC remaining after Fe oxide removal as well as its distribution among the following particle-size fractions: 2000-250 μm (coarse sand-sized SOC; POM1), 250 μm-53 μm (fine-sand-sized SOC; POM2), 53 μm-20 μm (very fine sand-sized SOC; POM3), and <20 μm (nonPOM). We found that the soils lost 24 t C ha−1 from the upper 10 cm and 49 t C ha−1 from the upper 30 cm, respectively, after 29 years. SOC losses extended also into the subsoil, and, on the average, 0.7-19.5 t C ha−1 was lost from the 30-100 cm 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.