Soil fertility of fluvial islands increases with proximity to an Amazonian white-water river

Abstract The nutrient content of soils is considered a productivity key factor. Sediment input from Amazonian rivers is one of the natural sources of soil fertility for fluvial islands and riverbank flooded forests. Despite the importance of soil factors for ecosystems, few edaphic studies along riverine sediment-gradients have been undertaken in tropical areas. The current study provides a step forward by describing soil nutrients in a mixed water (sediment-poor black water with input of sediment-rich whitewater) fluvial archipelago. To investigate how geographic distance from a whitewater river mouth affects island soil properties, soil chemical and physical attributes were determined at 61 sampling sites on 35 islands. The studied Central Amazon fluvial islands showed high variability in our fluvisols soil properties. In general, the fluvial island soils were acid and with low fertility. Islands from Jaú and other sites closer to the sediment source (the river Rio Branco) had higher soil fertility than islands of the Anavilhanas Archipelago, which are further away from the sediment source. Our results show that sediment inputs from Amazonian rivers can play an important role in soil properties, and this research increases our understanding on the origin and evolution of one of the largest freshwater archipelagos in the world. Given that soil fertility is often correlated with forest productivity, the main results reported here may also help improve management plans and conservation policies for fluvial island environments of Central Amazonia.