Short and long-term impact of urban gardening on soil organic carbon fractions in Lixisols (Burkina Faso)

•Urban gardening promotes accumulation of total, labile and stable SOC fractions.•Total SOC content increases asymptotically with the duration of cultivation.•SOC is continuously stabilized in micro-aggregates favored by Fe and Al oxides.•For SOC fractionation in Lixisols, sonication appears preferable to shaking. In sub-Saharan Africa, market gardening relies on the intensive use of organic amendments, but little is known about the impact of these practices on soil organic carbon (SOC) dynamics. Recent studies have demonstrated that a better understanding of carbon dynamics can be achieved by considering different carbon pools. Here we used a simple method of physical SOC fractionation to assess the impact of market gardening practices on the evolution of total SOC and its stable and labile sub-fractions over a period of several decades. The study was conducted at the Kuinima market gardening zone in Bobo-Dioulasso (Burkina Faso). Composite soil samples (0–15 cm depth) were collected from a chronosequence of 69 fields including control plots and plots farmed for more than 50 years. Samples were wet sieved at 20 µm after agitation or sonication to assess carbon content in the fine (i.e. stable) and coarse fractions. Our results show an asymptotic increase in total carbon content, from 9 g C kg−1 for uncultivated control plots to 28 g C kg−1 for plots cultivated for more than 50 years. This increase tends to stabilize after 30 years. A similar trend is observed for C content in the coarse fraction > 20 µm. In contrast, carbon content in the fraction