Growth characteristics and soil respiration rates with different coverages of Suaeda salsa at coastal beaches

Coastal beaches are one of the most important components of coastal wetlands. Studies on vegetation growth characteristics and soil respiration in coastal beaches are essential for evaluation of the ecological and environmental functions of coastal wetlands. In the present study, differences in vegetation growth, root distribution, and soil respiration rate of Suaeda salsa with four coverage types (bare flat and low-coverage, medium-coverage, and high-coverage) were determined to explore the impact of vegetation coverage on the growth characteristics of S. salsa and soil respiration rates at the coastal beach of the Yellow River Delta. Significant differences were observed in the soil physicochemical properties and vegetation growth of S. salsa on coastal beaches with different coverages. Soil salt content and bulk density were lower in various coverage areas than those in bare flats, whereas soil porosity and nutrients contents were greater than those in bare flats. The growth indices of S. salsa, such as biomass, plant height, and branch number, were positively correlated with vegetation coverage (P<0.05), indicating better growth in soils with higher vegetation coverage. The underground S. salsa biomass in saline land was mainly distributed in the 0–20 cm soil layer, showing a shallow distribution pattern. Roots with a 2–5 cm diameter were dominant components, accounting for 72.53%, 59.72%, and 39.30% of the underground biomass in the low-, medium-, and high-coverage areas, respectively. The root length, surface area, tip number, branch number, and cross number of fine roots increased with coverage, and the differences in these indices between the different coverage areas were significant (P<0.05). Soil respiration rates were low, at 0.26–1.01 μmol∙m⁻²∙s⁻¹, owing to the low soil organic carbon content and microbial activity in the study area. Soil respiration rates were significantly affected by vegetation coverage and showed an increasing order of value with coverage (high-coverage area > medium-coverage area > low-coverage area > bare area). Soil respiration rate was measured as an evident daily change as a low-high-low single peak curve, with the maximum value appearing at 12:00 in the low-coverage and bare areas and at 14:00 in the high- and medium-coverage areas. S. salsa growth indicators were significantly negatively correlated with soil salt content, demonstrating that soil salt was the main limiting factor for vegetation growth in coastal wetlands. However, the soil salt content was affected by vegetation coverage. Soil respiration rate was highly and positively correlated with plant growth indicators. We concluded that soil physicochemical properties, vegetation growth of S. salsa, and soil respiration rate were significantly affected by vegetation coverage on the coastal beach of the Yellow River Delta. High vegetation coverage improves soil properties and vegetation growth, further promoting ecological restoration in coastal wetland areas. The results of this study provide a theoretical basis for the vegetation and ecological restoration of coastal beaches in the Yellow River Delta. However, long-term field observations are recommended to determine the permanent effects of vegetation coverage on vegetation growth characteristics and soil respiration on coastal beaches.