Agricultural catchments in the tropics often generate high concentrations of suspended sediments following the conversion of natural ecosystems. The eroded fine particles are generally enriched with carbon (TC) and nutrients (TN and TP) originating from the topsoil of agricultural land. Sediment-associated TC, TN and TP are an important loss to the terrestrial ecosystem and tightly connected to an increase in riverine particulate TC 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 from forests to agriculture, are expected to receive high loads of sediment-associated TC, TN and TP. In this study, we assess the effect of land use on particulate TC, TN and TP concentrations. Suspended sediments (time-integrated, manual-event-based and automatic-event-based sediment samples) were analysed for TC, TN and TP concentrations collected at the outlet of a natural montane forest (35.9), a tea-tree plantation (33.3) and a smallholder agriculture (27.2 km²) catchment in western Kenya during a wet sampling period in 2018 and a drier sampling period in 2019. Particulate TC, TN and TP concentrations were up to 3-fold higher (p<0.05) in the natural forest catchment compared to fertilized agricultural catchments. However, because of higher sediment loads from the smallholder agriculture catchment, the total sediment-associated loads of TC, TN and TP were higher compared to the natural forest and tea-tree plantation catchment. The higher C:N 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 a mixture of mineral and organic 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.

中文翻译：

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热带非洲山地流域的大量常量营养素出口表明农业土壤贫瘠

随着自然生态系统的转换，热带地区的农业流域经常产生高浓度的悬浮沉积物。被侵蚀的细颗粒通常富含源自农田表层土壤的碳（TC）和养分（TN和TP）。与沉积物有关的TC，TN和TP是对陆地生态系统的重要损失，并且与河流颗粒TC和养分输出的增加紧密相关。土壤养分的消耗会限制作物的生长和单产，而溪流中的养分过多会导致淡水系统富营养化。东非的小河，土地从森林到农业的广泛转化，预计将吸收大量与沉积物有关的TC，TN和TP。在这项研究中，我们评估了土地利用对颗粒状TC，TN和TP浓度的影响。 肯尼亚西部在2018年的湿采样期和2019年的更干燥采样期的km ²）流域。与施肥农业相比，天然森林流域的颗粒TC，TN和TP浓度最高高3倍（p <0.05）流域。但是，由于小农农业流域的沉积物负荷较高，因此与天然林和茶树人工林流域相比，TC，TN和TP的总沉积物相关负荷较高。较高的C：N天然林中的比率表明，由于养分循环更严格，颗粒状养分是有机来源，而两个农业流域的比率较低则表明矿物质和有机沉积物源混合在一起。这项研究的结果表明，随着天然林的流失，在当前低施肥率和土壤管理策略下，固有的土壤肥力将逐渐丧失。