The effect of soil hydrology on the oxygen and hydrogen isotopic compositions of plants’ source water

Many studies have demonstrated that the isotopic composition of plants’ source water is the main factor affecting the isotopic composition of tree rings. Because of soil hydrological processes, soil water as the source water for plants may isotopically differ from precipitation that contains climatic information (such as surface temperature). This study addresses the effects of soil hydrological processes on the isotopic compositions of soil water and discusses how these effects affect interpretations of tree ring data in isotopic dendroclimatology. We collected precipitation, soil gas at two depths (20 and 50 cm), and twigs from a maple tree (Acer saccharum) on a biweekly basis during the growing season in 1997–1999 at Hanover, NH, USA. Water was extracted from the twig samples by vacuum distillation. All water samples were analyzed for both δD and δ18O. Soil CO2 was extracted from soil gas and measured for the δ18O values, and using the soil temperature and assuming isotopic equilibrium between CO2 and H2O, we calculated the δ18O values of soil water. Comparisons among the isotopic time series of each type of sample indicate the following. (1) The isotopic composition of soil water is much less variable than that of precipitation, suggesting isotopic mixing between waters of different precipitation events. (2) In early spring, soil water at all depths is isotopically similar to winter precipitation, but with time the surface soil water becomes progressively enriched in deuterium and 18O due to infiltration of summer rain and enrichment through soil water evaporation. (3) The influence of summer precipitation decreases with increasing depth, and soil at 50 cm can only receive water from large storms. (4) Replacement of old soil water with new infiltrating water is dependent upon frequency and intensity of growing season precipitation, and it is generally more efficient in a wet year than in a dry year. (5) The tree we studied uses water mainly from near-surface soil layers. (6) The δD–δ18O relationship in twig water indicates that soil water has experienced isotopic enrichment by evaporation. These results have important implications for selecting sites for paleoclimatic studies using isotopic data of tree rings.