Anthropogenic soils in central Amazonia: farmers’ practices, agrobiodiversity and land-use patterns

Keywords: Terra Preta; Amazonian Dark Earths; Shifting cultivation; Homegardens; Intensification; Diversification; Smallholder farming. Andre Braga Junqueira (2015). Anthropogenic soils in central Amazonia: farmers’ practices, agrobiodiversity and land-use patterns. PhD thesis, Wageningen University, The Netherlands, with summary in English, 163 pp. Rural Amazonia is increasingly experiencing environmental and socio-economic changes that directly affect smallholder farmers, with potential negative effects for environmental quality, agrobiodiversity and livelihoods. In this dynamic context, there is an urgent need to support pathways for smallholder agriculture that guarantee farmers’ economic and food security while maintaining and enhancing ecosystem functions. Amazonian Dark Earths (ADE, or Terra Preta) are anthropogenic soils created by pre-Columbian populations. Due to their high carbon content and enhanced fertility, ADE have been considered models for sustainable agriculture, based on the idea that transforming soils by mimicking some of the properties of ADE would benefit farmers, sequester carbon and reduce pressure on forests. Investigating the current use of ADE and surrounding soils by smallholder farmers allows us to evaluate the relevance of anthropogenic soils and of soil heterogeneity for smallholder farming in Amazonia, and to identify opportunities and constraints associated with the cultivation of fertile soils. The main objective of this thesis is to understand how ADE are understood and cultivated by smallholder farmers in Central Amazonia, and how these soils influence cultivation systems, agrobiodiversity and land-use patterns. Ethnographic data indicated that farmers’ understanding of ADE – and of soils in general – is based on their historical and shared knowledge about soil variation across the landscape, on physical attributes of the soil, and mainly on the recognition of different soil-vegetation interactions. A widespread perception about ADE is that these soils are suitable for the cultivation of ‘almost everything’ and always produce decent yields, but they require much more weeding during cultivation. Farmers’ decision-making in shifting cultivation is grounded in this differential understanding of soil-vegetation relationships, and weighed against the labor demands. Soil and vegetation inventories in swiddens used for shifting cultivation showed that the soil fertility gradient between surrounding soils and ADE was associated with more intensive cultivation (shorter fallow periods, shorter and more frequent cultivation cycles, higher labor requirements) and with changes in the crop assemblages, but with similar or larger numbers of species cultivated. In homegardens, vegetation structure and crop diversity were mainly influenced by natural variation in soil texture (homegardens on sandier soils being denser and more diverse), while the soil fertility gradient between ADE and adjacent soils influenced mainly the crop assemblages. At the farm level, the relationship between farmers’ use of ADE and the need to open areas for shifting cultivation was strongly dependent on the labor availability of the household. Instead of driving specific trends in land use, fertile soils are incorporated into local livelihoods as part of an extensive repertoire of resource management activities; most often, farmers with enough available labor manage multiple plots, combining more intensive cultivation on ADE with typical long-fallow shifting cultivation on poorer soils. Farmers’ access to increased soil fertility, therefore, does not necessarily lead to reduced pressure on forests. This thesis has shown that cultivation systems on ADE are associated with specific knowledge, practices and agrobiodiversity, providing increased opportunities for farmers to diversify their cultivation systems and grow a greater diversity of crops. Despite these advantages, ADE can also be associated with conventional intensification practices that can lead to environmental degradation and pose threats to local livelihoods. It cannot be assumed, therefore, that the use of more fertile soils will be associated with sustainable cultivation, neither that it will reduce pressure on forests. Initiatives aiming to promote sustainable pathways for agriculture in Amazonia should promote (and make use of) the heterogeneity of soils and of cultivation strategies, and should aim at increasing and not narrowing farmers’ opportunities for resource use and management.