Using Stable Nitrogen Isotope Tracing to Indicate the Effects of Increasing Groundwater Depth on the Soil–Plant System in a Semi-Arid Region of Eastern China

Nitrogen isotopes have garnered increasing attention in the investigation of nitrogen (N) dynamics. However, there remains a significant knowledge gap concerning the dynamics of plant–soil nitrogen interactions and their driving factors under conditions of increasing groundwater depth. In this study, we assessed the response of soil and plant tissue 15N signatures of two dominant species (the herb Pennisetum centrasiaticum and the shrub Artemisia halodendron) to three groundwater depth treatments (30 cm, 50 cm, and 100 cm) with the addition of N compounds (15NH415NO3) in the Horqin Sandy Land. Our results suggested that soil δ15N increased with soil depth at 30 cm groundwater depth, and plant tissue δ15N were positively related to soil δ15N at 30 and 50 cm groundwater depth. Negative effects of groundwater depth variability on plant tissue δ15N and TN values were observed; our results also showed that the variability in SW and pH caused by groundwater depth was most responsible for the distribution in plant tissue N. These findings enhance our understanding of the profound impacts of climate change on plant and soil properties and their interrelationships in semi-arid regions and also provide critical insights to underpin sustainable water resources management.