Soil water content and its stable isotope composition are two main indexes used to study soil hydrological processes. The two indexes are closely correlated with and also differed from each other. For example, soil evaporation changes both soil water content and its stable isotope composition, but plant transpiration only depletes soil water content. Additionally, soil water stable isotope composition shows its unique advantages in distinguishing evaporation and transpiration.

In karst regions in southwest China, which is characterized by high outcrop ratio, shallow and discontinuous soil and dual hydrological structures, soil water movement is complicated and lack of clear understand. Thus, study the spatial variability of soil water content and its stable isotope composition together could help to better understand the ecohydrological processes in this region.

Researchers in the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA) employ Geostatistics and Kriging interpolation methods to study the spatial patterns of shallow soil water content and its hydrogen isotope composition (δD value) and their influencing factors in karst regions in southwest China in early (April 15) and vigorous (August 18) growing seasons in 2011, respectively.

The team found the spatial variability of soil water content and its δD value were rather different. Firstly, soil water content decreased with soil depth but its δD values in all depth were similar and close to the δD value of recent rainwater. Variance of soil water δD value was smaller than that of soil water content. This kind of difference was more obvious in second sampling time when recent rainfalls had similar δD values. Besides, soil water content showed similar spatial pattern with its influencing factors, which had moderate to strong spatial dependence. However, differ from its influencing factors, soil water δD value exhibited weak spatial dependence and random spatial pattern.

Moreover, significant correlations between soil water δD value and its influencing factors disappeared when they used soil water content as a control factor to conduct partial correlation analysis. This suggested that environmental factors affect soil water δD value through influencing soil water content. The differences between the spatial variability of soil water content and its δD value indicated that the effects of other factors on soil water δD value were covered by rainfall. Spatial variability of soil water δD value was mainly dominated by factors in smaller scales.

The results highlight the importance and necessity of studying soil water content and its stable isotope composition separately, and was helpful for better understand the hydrological processes in karst regions by isotope tracer method.

This research was supported by the National Key Research Program of China (2015CB452703) and the National Natural Science Foundation of China (41171187 and 51379205).

The study entitled "Spatial variability of shallow soil moisture and its stable isotope values on a karst hillslope" has been published in Geoderma, details could be found at http://www.sciencedirect.com/science/article/pii/S0016706115300987.