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Accurate assessments of soil organic carbon (SOC) and total nitrogen (TN) stocks and controlling factors are essential for understanding carbon and nitrogen sequestration in soil. In this study, we analyzed the spatial distribution and influencing factors of SOC and TN stocks to the depth of 3 m in a desert oasis ecotone of northwestern China. The study area has experienced large-scale transition of dunelands to irrigated croplands in the past 40–50 years. The results showed that the vertical distributions of SOC and TN stocks were well demonstrated by quadratic function forms. The SOC and TN stocks at 0–3 m depths in nabkha dunes were generally significantly higher than that in bare sand dunes. Additionally, after converting nabkha dunes to croplands, both SOC and TN stocks at the top 0.4 m depths increased with the extension of cultivation years, the mean SOC and TN stocks significantly increased from 0.396 to 0.992 kg·m−2 and from 0.053 to 0.096 kg·m−2, respectively. The strongest heterogeneity of SOC and TN generally appeared at top 0–0.4 m depths and decreased with the depth. The SOC and TN stocks in the nabkha dunes near the water areas were relatively higher, which may be suitable for croplands reclamation. It is noteworthy that land use brought a major influence of SOC and TN at top 0–0.4 m depths, whereas soil texture (clay and silt contents) was the main factor that controlled SOC and TN at deep soil depths (1–3 m). The results indicate that the nabkha dunes also increased carbon and nitrogen stocks throughout the entire 3 m soil profiles. While the demand for food is almost certain to increase in arid and semiarid China and other regions worldwide, the conversion from non-traditional croplands such as dunelands to croplands is likely going to continue.

Tamarisk (Tamarix ramosissima) plants are widely distributed in semi-arid regions with saline soils. In these environments, the survival and sustainability of the Tamarisk plants are critically related to the availability and distribution of soil water and salt. In this study, we analyzed the soil water content (SWC) and soil electrical conductivity (EC) to the depth of 60 cm in natural Tamarisk communities, located in a semi-arid saline region of northwestern China, in July 2016 (a normal period) and July 2017 (a dry period). The results showed that SWC varied from 1.91 to 7.82% and soil EC ranged from 9.65 to 25.38 mS/cm in the 0–60 cm soil layer. Despite the deficiency of precipitation in the study period of 2017, the SWC in all plots was higher at the 20–60 cm depth than that of the normal study period in 2016. Compared to 2016, the soil was also less saline in 2017, representatively by lower soil EC values at the 0–40 cm depth. Additionally, the SWC decreased with Tamarisk density, indicating that Tamarisk plants increase water consumption. The SWC and soil EC both revealed “island” distribution, with highest level found inside or underneath the Tamarisk plant canopies. Both the SWC and soil EC presented a strong or moderate spatial dependency, and the spatial dependency in soil EC were significantly affected by community composition. Correlations (Pearson’s r) showed that there were positive correlations between SWC and soil EC. The results above indicate that Tamarisk communities can ameliorate the semi-arid high saline soil environment, and that the growth and distribution of Tamarisk plants influence the spatiotemporal variability of SWC and soil EC. In addition, the results also demonstrate that the possible appropriate density in the process of constructing Tamarisk plants should be considered in order to achieve the optimal water condition. The study may provide a scientific basis for the ecological restoration and halophytes establishment in the semi-arid saline region of northwestern China and other areas with similar ecologies.