Sedimental Journey: Soil Fertility of Fluvial Islands Increases with Proximity to An Amazonian White-Water River

The nutrient content of soils is considered a key productivity 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 fluvial islands sediment-gradients have been undertaken in tropical areas. The current study provides a step forward by describing a mixed water (sediment-poor black water with the input of sediment-rich white water) fluvial archipelago with an emphasis on soil characteristics of seasonally inundated forests. To investigate how geographic distance from a white-water river mouth affects island soil properties, soil chemical and physical attributes, we sampled 61 sites on 35 islands at Jaú and Anavilhanas archipelago. The studied Central Amazon fluvial islands showed high variability in hydromorphic soils properties. In general, the fluvial island soils were acid and with low fertility. Islands from Jaú and other sites closer to the sediment source (Branco River) had higher soil fertility than the Anavilhanas Archipelago islands, which are further away from the sediment source. Our results show that sediment inputs from the Branco River can play an important role in soil properties in a relatively nutrient-poor area, and shed light on the role of biogeochemical processes in the largest freshwater archipelago in the world. Given that soil fertility is often correlated with forest productivity, the results reported here may serve as a baseline to support conservation politics with scientific data for Central Amazonian fluvial island environments.