Soil carbon stock and Plinthosol fertility in smallholder land-use systems in the eastern Amazon, Brazil

The soil organic carbon (SOC) stock is an important attribute in the maintenance of ecosystem services by natural and agricultural ecosystems. In the humid tropics, slash-and-burn cultivation for food production and establishment of pastures has frequently led to soil degradation. In the eastern Brazilian Amazonia, we assessed impacts of different land-use systems on the SOC stock and some chemical properties of soils. Four land-use systems were studied: shifting cultivation (SC), pasture (PA), mixed fallow (MF) and secondary forest (SF). In the soils studied, most nutrients were not significantly influenced by land-use changes, and the chemical fertility was low. Similarly, SOC concentration was low, ranging, in the 0-10 cm layer, from 9.54 g kg⁻¹ in PA to 12.73 g kg⁻¹ in MF. In the 0-100 cm layer, the SOC stock varied from 42.1 Mg ha-1 in SC to 53.3 Mg ha⁻¹ in MF, without significant differences between the land-use systems. The SOC stock in the 0-30 cm layer represented 50-56% of that in the 0-100 cm layer. The soils studied have the lowest SOC stocks measured in the Amazonia region. The low organic matter content, associated with soil acidity, a small sum of exchangeable bases and low extractable P, can constrain the productivity and the sustainability of these cropping systems.