Soil degradation associated with water-level fluctuations in the Manwan Reservoir, Lancang River Basin

• Water-level fluctuations (WLFs) showed significant influence on topsoil quality.
• The scrubland exhibited the highest degradation level under the impacts of the WLFs.
• Soil degraded significantly with decreasing distance upstream from dam.

This study was conducted to understand the impacts of dam-induced water-level fluctuations (WLFs) on soil properties. The redundancy analysis (RDA) and two soil degradation indices were used to analyse soil degradation under different land uses and at different soil depths. In addition, the relationships among soil degradation and topographical and geographical parameters were determined. The sampling sites were located in two zones along the Manwan Reservoir, which is a section of the Lancang River in Southwestern China. These zones included a water-level-fluctuation zone (WLFZ) and an infralittoral reference zone (IRZ), which included forestlands, scrublands, and farmlands. The results indicated that WLFs increased soil pH, total phosphorus (TP) concentrations, and total potassium (TK) concentrations, and decreased soil total carbon (TC) and total nitrogen (TN) concentrations. Meanwhile, the WLFs increased the soil C/N ratio, which indicated that the WLFs had a stronger effect on TN than on TC. The degree of soil degradation based on the soil degradation index and the changes in the soil quality index indicated that WLFs significantly affected topsoil quality (0–5 cm), especially in the scrubland, which had the highest degradation level. The results from the RDA indicated that soil degradation significantly increased with decreasing distance along the river upstream of the dam (P < 0.05; F = 3.05). Our results identified soil degradation in the WLFZ along the Manwan Reservoir. However, further research is needed to determine the mechanisms of degradation in this zone.