Soil fertility gradients shape the agrobiodiversity of Amazonian homegardens

The importance of homegardens for the conservation of agrobiodiversity, the maintenance of farm ecosystem processes, and the economic and food security of rural populations worldwide is increasingly recognized. While biophysical and socio-economic conditions are considered to influence homegarden management, and affect their ecological and societal relevance, little is known about how variation in soil properties affects these agroecosystems. By combining soil data with extensive botanical inventories, we investigated how farmers' use and management of soil variation results in differences in the structure, diversity and the floristic composition of homegardens in Central Amazonia. We sampled 70 homegardens located along the gradient from low-fertility Ferralsols to Amazonian Dark Earths (ADE), i.e., fertile anthropogenic soils created by pre-Columbian populations at least 500 years ago. Our results show that several characteristics of homegardens are significantly influenced by variation in soil texture and fertility. While differences in soil texture are due to natural soil variation, observed heterogeneity in soil fertility was largely the result of pre-Columbian and modern soil transformations. Homegardens on sandier soils tended to be more diverse in plant species and to have more individual plants; homegardens on more fertile soils tended to have fewer trees and palms, more herbs, shrubs and climbers, and a higher total number of species and landraces; variation in soil fertility was significantly related to differences in the composition of species and landraces. Our results show that farmers' use of natural and anthropogenic variation in soil properties influences agrobiodiversity patterns in homegardens. Pre-Columbian and modern soil enrichment increases soil heterogeneity in the landscape, resulting in strong soil fertility gradients that shape the agrobiodiversity of current Amazonian homegardens.