Soil Water Stable Isotopes Reveal Surface Soil Evaporation Loss Dynamics in a Subtropical Forest Plantation

Line-conditioned excess (lc-excess), the deviation of the relationship between delta D and delta O-18 in soil water from that of precipitation, is often used to indicate soil evaporation loss, but the conditions of using lc-excess under the influences of precipitation infiltration or percolation had not been identified. The interaction effects of climate, soil and vegetation on soil evaporation in forests are not well known. We collected soil water at 0-5, 15-20 and 40-45 cm depths and event-based precipitation from 2011 to 2015 in a subtropical forest plantation and calculated the lc-excess. Precipitation on the sampling day and percolation of upper soil water with low lc-excess affected the capacity of the lc-excess to indicate the soil evaporation fractionation signals. Lc-excess of soil water at 0-5 cm depth indicated a reliable soil evaporation loss estimate over 30 days prior to the sampling day. Soil evaporation loss was dominated by the cumulative soil temperature (T-ss) during drought periods and was dominated by the relative soil water content (RSWC) during non-drought periods. High T-ss decreased soil evaporation loss by increasing transpiration and relative humidity. Our results emphasize the importance of sampling the upper-most soil layer when there is no rain and vegetation during drought periods in forests when studying soil evaporation loss dynamics.