The patterns and controlling factors of soil carbon and nitrogen in the water level fluctuation zone of the Three Gorges Reservoir

This study aims to investigate the spatial variations in soil carbon and nitrogen in the water-level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) and explore the influence of soil factors on these properties.

For this research, six sampling points were selected along the WLFZ of the TGR, with the slope of the riparian zone categorized into four elevation ranges: 145–155 m, 155–165 m, 165–175 m, and above 175 m. A total of seventy-two soil samples were randomly collected to analyze the variations in soil physicochemical and biological properties along and perpendicular to the direction of river flow.

Compared with the upstream areas, the downstream areas in the flow direction presented significantly greater soil organic carbon (SOC), microbial biomass carbon, total nitrogen (TN) contents, soil sucrase activities and soil respiration intensities. In the vertical direction, the SOC, TN and microbial biomass carbon increased with increasing elevation in the riparian zone. Notably, as the elevation of the riparian zone increases and along the flow direction, both the geometric mean diameter (GMD) and the mean weight diameter (MWD) of the soil water-stable and nonwater-stable aggregates gradually increase, whereas the fractal dimension (D) decreases, indicating an increase in stability. There was a strong positive correlation between the soil water-stable aggregates and the soil carbon and nitrogen levels.

The soil carbon and nitrogen contents gradually increase along the flow direction, with a distinct boundary in the midstream region, and the differences in soil physical and chemical properties are the main factors influencing these changes. The soil carbon and nitrogen contents also increased with elevation, with 165 m being a clear boundary. Above 165 m, water-stable soil aggregates have a significant effect on the soil carbon content, and soil biological properties are the primary factors driving this variation.