Effects of changes in fallow length on soil organic C due to climate change and socioeconomic factors in potato-based cropping systems in the Bolivian Highlands
Motavalli, Peter P.
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The Bolivian highland plateau region (Altiplano) is a semi-arid region in the Andes Mountains that occupies approximately 27% of the area of Bolivia and has a range in elevation of between 3600 and 4300 m above sea level. The region's climate is characterized by high diurnal temperature variations, frost risks, low and irregular precipitation and high risks of drought during the growing season (Garcia et al., 2007). Potato-based cropping systems and livestock rearing of cows, sheep and camelids (e.g., alpaca and lama) are the primary agricultural activities (Valdivia et al., 2001). Crop rotations are initiated with potato followed by two to three years of cereal crops (e.g., quinoa, barley) and then an extended uncultivated fallow period which can last from 1 to over 40 years. This long fallow period allows for restoration of soil fertility, control of crop disease and pests, grazing of natural vegetation (e.g., 'thola' (Parastrephia lepidophylla)) and fuel for cooking (Bottner et al., 2006; see Fig. 2 A-D)). Increasing human population in the Altiplano and competing land uses have caused a growing reduction in the use and length of fallow (Coûteaux et al., 2008). Moreover, practices, such as use of mechanized disc plowing and cutting native vegetation for fuel, have reduced the regrowth of natural vegetation during the fallow period, possibly diminishing the amount of organic inputs and the rate of soil fertility restoration. The impact of fallowing in this environment on soil properties is unclear. For example, Hervé (1994) in a comparison of soils with up to 20 years of fallow from the Bolivian Altiplano did not observe any clear improvement in soil physical (e.g., bulk density) and chemical properties (e.g., soil total organic C) with increasing fallow. Cabaneiro et al. (2008) in the Andes in Venezuela determined that fallowing only increased labile organic C in soils collected from slopes with a northeast compared to a southwest aspect, possibly due to differences in microclimate.