Effects of precipitation on water use characteristics of Caragana intermedia plantations with different stand ages in alpine sandy land

Abstract

To clarify the changes of water sources for Caragana intermedia plantations at different ages (4, 9, 17 and 31 years) in response to rainfall in the Gonghe Basin of Qinghai Province, China, we used the stable isotope technique to identify δ2H and δ18O compositions of soil water, xylem water, groundwater, and rain water before and after rainfalls. The proportions of different water sources were calculated by the Iso-Source model. The results showed that the δ2H and δ18O compositions of the shallow soil layer (0-40 cm) of all plantations responded significantly to the precipitation. The isotopic values of plant xylem water, soil water, and groundwater of each plantation were spotted on the lower right of the local meteoric water line (LMWL) either before or after rainfall, with lower intercepts and slopes than LMWL and the global meteoric water line (GMWL). The isotopic compositions of xylem water and soil water of C. intermedia plantations were closer to LMWL after rainfall. The 4- and 9-year-old C. intermedia plantations mainly used shallow soil water, the 17-year-old plantation mainly used middle layer soil water (40-90 cm), and the 31-year-old plantation primarily use deep soil water before rainfall. After rainfall, the shallow soil layer became sources of water absorption for all plantations. The utilization proportions of groundwater for all plantations were only 1.8%-11.9%. In general, water sources of different aged C. intermedia plantations showed similar responses to rainfall, by primarily absorbing the shallow soil water supplied by rainfall and reducing the use of groundwater.