Spatial Variability in Soil Hydraulic Properties Under Different Vegetation Conditions in a Coastal Wetland

Understanding the spatial variability in soil hydraulic properties (SHPs) and their influencing variables is critical for ecohydrological and biogeochemical studies in coastal wetlands, where complex landscapes make it challenging to accurately delineate the spatial patterns of SHPs. In this study, soil samples were collected from two transects covered by Suaeda salsa (S. salsa) and Phragmites australis (P. australis) from the Beidagang Wetland Nature Reserve in northern China, and a comprehensive dataset on soil physical properties and SHPs was obtained by laboratory experiments. The results showed that soil physical properties (e.g., soil particle size, bulk density (BD), and soil organic matter (SOM)) displayed significant spatial variability, which was related to the physiological characteristics of S. salsa and P. australis and to soil depth. As a result, SHPs, including saturated hydraulic conductivity (Ks) and parameters of the van Genuchten model (θs-saturated soil water content, including α, the reciprocal of the air-entry value, and n, the pore size distribution index) varied considerably along the two transects. Specifically, Ks, θs, and α were negatively correlated with BD and pH, while positively correlated with SOM, which promoted soil aggregation to enlarge soil pores. Soil depth was shown to significantly affect SHPs, whereas the differences in SHPs between the two transects were not statistically significant, suggesting vegetation type did not directly impact SHPs. Soil water retention capacities were noticeably higher in surface soils, especially when soil suctions were less than 1000 cm, whereas their differences between depths largely diminished with further increasing soil suctions. This study highlights the complex interplay of SHPs with surrounding environments, providing critical insight for characterizing the spatial patterns of SHPs in coastal wetlands.