Spatial variability of shallow soil moisture and its stable isotope values on a karst hillslope

• Spatial variability of soil moisture and its δD value were studied together.
• Soil moisture showed structural variability while δDθ varied stochastically.
• Environmental factors affected δDθ indirectly by influencing soil moisture.

Soil moisture (θ) and its stable isotope values are two of the most commonly used parameters for studying hydrological processes in soils. Despite their unique ability to aid in distinguishing between soil water evaporation and plant transpiration, the spatial variability of soil water isotope values is not fully understood. In the current study, 10 m × 10 m grids were established within a 90 m × 120 m plot on a highly heterogeneous karst hillslope. Two sampling campaigns were conducted during the early growing season, on April 15, and during the mid-growing season, on August 18, 2011. Stratified soil samples were collected from the shallow soil layer (0–30 cm) to measure θ and its stable isotope values, which were represented by soil water δD values (δDθ). Related soil properties, land cover and topography were also measured and treated as influencing factors. On both sampling dates, θ decreased with depth, while δDθ stayed constant for all soil layers and were similar to the most recent rainfall values. Additionally, the variance of δDθ was smaller than that of θ, especially on August 18 when the most recent rainfalls had similar δD values. The high contrast between θ and δDθ caused the impacts of other factors on δDθ to be masked by the impact of the recent rainfall. Soil moisture presented a moderate to strong spatial dependence, which was consistent with the spatial variability of the influencing factors that were significantly correlated with θ. Soil water δD value exhibited weak spatial dependence and random spatial patterns that were different from the other influencing factors. Moreover, significant correlations between these same influencing factors and δDθ disappeared after a partial analysis with θ as a controlled variable, which means δDθ was indirectly affected by these influencing factors through θ. This suggests that the spatial variability of δDθ was being controlled at fine scales. Our results highlight the importance of analyzing the spatial variability of δDθ, its influencing factors and θ in shallow soil layers separately.