Particulate macronutrient exports from tropical African montane catchments point to the impoverishment of agricultural soils

Abstract. Catchments in the tropics often generate high concentrations of suspended sediments following the conversion of forests to agriculture. The eroded fine particles are generally enriched with carbon and nutrients originating from the topsoil. Sediment associated carbon and nutrients are an important loss to the terrestrial ecosystem and tightly connected to processes controlling riverine particulate carbon and nutrient export. Soil nutrient depletion can limit crop growth and yields, whereas an excess of nutrients in streams can cause eutrophication in freshwater systems. Streams in East Africa, with widespread land conversion are expected to receive high loads of sediment associated carbon, nitrogen and phosphorus. In this study, we build the knowledge base for particulate carbon, nitrogen and phosphorus for contrasting land uses. Suspended sediments (time integrated, manual event based and automatic event based sediment samples) were analysed for total carbon, nitrogen and phosphorus concentrations collected at the outlet of a natural montane forest, a tea tree plantation and a smallholder agriculture catchment in western Kenya during two sampling campaigns in 2018 and 2019. Particulate carbon and nutrient concentrations were up to three fold higher (p < 0.05) in the natural forest catchment compared to fertilized agricultural catchments. The higher carbon and nutrient ratios in the natural forest suggest that the particulate nutrients are of organic origin due to tighter nutrient cycles, whereas lower ratios in both agricultural catchments suggest mineral sediment sources. The findings of this study imply that with the loss of natural forest, the inherent soil fertility is progressively lost under the current low fertilization rates and soil management strategies.