Soil moisture ecological characteristics of typical shrub and grass vegetation on Shell Island in the Yellow River Delta, China

The construction of a shrub-grass community is the main action of vegetation restoration and ecological reconstruction on Shell Island, which is located in the Yellow River Delta, China. Three vegetation types—a mixed shrub-grass pattern, pure shrub, and pure grass—were studied, with bare land as the control, to elucidate the water retention characteristics of shell sand and the factors influencing the typical shrub and grass vegetation on Shell Island in the Yellow River Delta. Soil moisture physical parameters, such as the bulk density, particle size composition, particle fractal dimension and water storage capacity, were measured and analysed under different vegetation types, and the potential and main factors influencing the soil water retention were comprehensively evaluated. The results showed that (1) the bulk densities of the 0–40 cm layer of soil under the shrub-grassland, shrub land, and grassland were significantly lower and the total porosity was significantly higher than that under bare land. The coarse sand particle content in the shell sand was the highest (55.32%), followed by the fine sand particles and gravel contents. The silt-clay content was low (2.37%). The planting of mixed shrub-grass could reduce the gravel and coarse sand contents and increased the fine sand and silt-clay contents. (2) The ranking of the soil particle fractal dimension, capillary water retention, soil water retention, effective retention and rainfall retention of the different vegetation types was as follows: mixed shrub-grassland > shrub land > grassland > bare land. In terms of the vertical structure, the soil water retention in the 0–20 cm soil layer was higher than that in the 20–40 cm layer under all vegetation types. (3) The improvements to the soil physical properties and water storage capacity under the three vegetation types on Shell Island, the mixed shrub-grassland was the best, followed by shrub land and then, grassland. Moreover, the improvement in the 0–20 cm layer of the shell sand was better than that in the 20–40 cm layer. The soil bulk density, coarse sand, fine sand, capillary porosity, void ratio and fractal dimension were closely related to the soil water retention capacity and can be used as indicators to evaluate the water storage capacity of shell sand.