ObjectiveIn Horqin Sandy area of northern China, the poplar plantations have shown different degree of degradation, and water is the main limiting factor to plant survival in this region. This study aims to explore the mechanism of water source and utilization, reveal the relationship between moisture and poplar plantation degradation, in order to provide the reference basis for the ecosystem restoration and reconstruction of poplar plantation in Horqin Sandy Land.

MethodIn this study, the poplar plantation(Populus simonii|×Populus nigra) in Horqin Sandy Land was studied by isotope technique, and the stable δ¹⁸O isotope value of poplar stem and xylem was measured using the "delta plus XP" stable isotope ratio mass spectrometry, combined with the automatic weather station (HOBO U30-NRC) for continuous observation of rainfall and meteorological environment factor, analysis of water use strategy of poplar plantation in rainy season and dry season by isotope mass conservation.

Result(1) The soil moisture of rainy season was higher than that of dry season. The soil moisture of 0-100cm soil layer in rainy season increased significantly with soil depth (P < 0.05), while the soil water content of 100-170cm soil layer decreased significantly (P < 0.05). In the dry season, the soil moisture content in the forest was higher than outside the forest, but the difference was not significant (P> 0.05). (2) The δ¹⁸O values of soil water and tree plant water were both higher than the δ¹⁸O values of groundwater. The δ¹⁸O values of soil water in dry season were lower than δ¹⁸O values of tree branch xylem, while the δ¹⁸O values of soil water in rainy season were higher than δ¹⁸O values of tree branch xylem. (3) In the dry season, poplar plantations mainly absorbed soil water and groundwater below 130 cm soil, and also had a small amount of absorption in shallow soil water. In the rainy season, apart from absorbing soil water above 130cm, the poplar plantation also absorbed a small amount soil water and groundwater from deep soil.

Conclusion(1) The solar radiation and rainfall have great inference on oxygen isotopes of shallow soil water, but little interference on deep soil water. In dry season, the soil moisture content in the forest was higher than outside the forest, especially in 80-170cm soil layer was more significant. It explained the phenomenon from the side aspect that poplar will release small amount water to the surface soil when utilizing the deep soil water and groundwater. (2) Poplar plantation in Horqin Sandy Land mainly use shallow soil water during the rainy season, but mainly use deep soil water and groundwater during the dry season. The root system of poplar plantations in Horqin Sandy Land distributed in both shallow and deep soils, the distribution of this root system ensured that poplar plantation adopted different water use strategies in the dry and rainy seasons, thus to ensure its own moisture. This manifestation is the adaptability of plants to the arid sand environment.