The relationship of δD and δ18O in surface soil water and its implications for soil evaporation along grass transects of Tibet, Loess, and Inner Mongolia Plateau

Abstract

Soil evaporation plays an important role in non-productive water loss and local climate regulation. The deviation of the relationship between δD and δ18O in surface soil water from that in precipitation is used to indicate the integrative results of the soil evaporation process, but the distinction between the slope of the soil water evaporation line (SEL) and line-conditioned excess (lc-excess) as individual indicators has not been identified. Furthermore, spatial variability and interaction effects of climate, soil, and vegetation on soil evaporation in arid and semi-arid grasslands remain poorly understood. We established 3 west-to-east sampling transects along with increasing precipitation and decreasing aridity index gradients in grasslands of the Tibetan (TP, 1500 km), Loess (LP, 600 km), and Inner Mongolia (MP, 1200 km) Plateaus. Slope of SEL and weighted mean lc-excess were calculated according to the δD and δ18O in soil water, which was extracted from soils collected at 10 cm intervals to a depth of 1 m along with soil profiles during the vigorous growing season. We found that lc-excess appeared relatively more robust than slope in representing soil evaporation process because lc-excess was determined by evaporation fractionation factors and soil residual water storage, while slope was regulated only by evaporation fractionation factors. Soil evaporation, as indicated by lc-excess, exhibited decreasing trends from west to east, and was controlled by relative humidity in TP, and by vegetation coverage in LP and MP. Our results highlighted that the soil evaporation process may be more sensitive to climate change in TP, and vegetation restoration in LP and MP.