Abstract: Stoichiometry is an essential indicator of ecosystem nutrient cycling. However, the effects of floodinduced sediment deposition on soil physicochemical properties and soil stoichiometric relationships remain unclear. In this study, three sites (100 m, 1000 m, and 1800 m to the main channel) of floodplain with three replications were selected in the middle and lower reaches of the Yellow River. Soil samples at depths of 0-10 cm, 10-20 cm, and 20-30 cm were collected to analyze the concentrations of soil organic carbon (SOC), total carbon (TC), total nitrogen (TN), total phosphorous (TP), and other soil properties. The results showed that SOC, TC, TN and TP contents were 3.49±0.29, 15.13±0.76, 0.46±0.03 and 0.59±0.01 g·kg^-1, respectively, and C/N, C/P, and N/P were 37.14±2.40, 25.38±1.17 and 0.78±0.06, respectively, indicating that the soil in the study area was under seriously nitrogen limitation. In addition, the contents of soil C, N and the ratios of C/N, C/P, N/P significantly decreased with soil depth (P<0.05), but the variation of soil TP content was minor. In contrast, the contents of soil C, N and the ratios of C/N, C/P, N/P significantly increased with the distance from the river (P<0.05). The results of the principal component analysis showed that the higher contents of C, N, and P in the soil occurred in the farther site from the river, which were mainly related to soil particle size composition. The stoichiometric ratios were related to the contents of C, N, and P. These results suggest that the floodplain floods in the middle and lower reaches of the Yellow River have different siltation degrees in the three sites. The degree of siltation caused by the distance from the river channel was the main factor affecting soil particle size composition. The different distances to the river affected soil particle size composition resulting in the variation of soil nutrient condition and thus affecting plant growth in floodplain. The sediment interception of the front vegetation in the middle and lower reaches of the Yellow River is helpful to soil nutrient retention and vegetation development in the site far from the river course.

Key words: soil organic carbon, Yellow River, stoichiometry, floodplain.