Impacts of Cultivation and Fallow Length on Soil Carbon and Nitrogen Availability in the Bolivian Andean Highland Region

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Inclusion of periods of unmanaged or natural fallowing is an important soil management practice in the potato-based cropping systems of the resource-poor Andean highland region (Altiplano) of Bolivia. However, in recent years the area in fallow and the fallow length are being reduced due to greater land use intensity and competing land uses. The objectives of this study were to determine the effects of the length of cropping and fallowing periods on soil degradation or soil restoration, and to compare the potential soil carbon and nitrogen mineralization from a range of cropped and fallow lands at different elevations. Four representative indigenous communities of the semiarid central Altiplano were selected, 2 at a relatively high elevation and 2 at a relatively low elevation. Soil samples were collected in 2006 and 2007 from fields at the first, second, and third year of crop rotation and from fields with 1, 10, 20, 30, and 40 years of fallow and analyzed for several soil properties. In general, the upper elevations had significantly higher soil organic carbon, total nitrogen, inorganic nitrogen, soil test phosphorus and potassium, exchangeable calcium and magnesium, and cation exchange capacity than the lower elevations. Cropping significantly decreased total and active soil organic carbon and total, inorganic, and active soil nitrogen. Fallowing was observed to restore total and active soil organic carbon and total and active soil nitrogen more rapidly in the higher communities than in the lower communities; this difference was mainly attributed to differences in initial soil properties, climate, and land management in cropped fields with elevation. Further research may be needed to determine which factor has the most influence on soil degradation and soil fertility restoration in this environment in order to assist farmers to improve soil fertility.