Evaluation of soil water evaporation and condensation using stable water isotopes under plastic mulching

The understanding of soil evaporation is crucial for optimizing water management in agriculture to improve ecohydrological modeling and increase crop yields. Stable isotope ratios of oxygen and hydrogen in soil water reflect phase changes owing to equilibrium and kinetic effects, which are valuable for examining water-mixing processes in soil. This study aimed to examine the effects of evaporation and condensation of water from soil wrapped in black and white films to quantify the differences in isotope ratio changes between soil columns with and without plastic mulch. A soil-column experiment was conducted to examine the isotopic fluctuation (enrichment) processes of soil water and condensed water using water stable isotopes (δ¹⁸O and δ²H) under the three treatments: black plastic mulching (BPM), white plastic mulching (WPM), and no-mulching (NM). The results showed that plastic mulching significantly reduced net soil evaporation and enhanced soil water storage compared with the NM treatment. The net evaporation ratio (net evaporation to irrigation) decreased more for plastic mulching (BPM: 23.1 %; WPM: 23.7 %) than for bare soil (75.1 %). The isotope ratios of δ¹⁸O and δ²H in soil water were lower in the mulched soil profiles than in bare soil at 0–60 cm depths, suggesting that mulching reduces evaporation because of water retention effect. The vertical profile of isotopic ratios of soil water varied among the three treatments owing to the evaporation effect on the soil surface and the addition of condensation water. Condensate under the plastic mulch exhibited higher (more enriched) δ¹⁸O and δ²H than soil water and source (irrigation) water owing to kinetic isotope fractionation during evaporation. The regression slopes of δ¹⁸O and δ²H decreased more in condensed water than in soil water under the mulching treatments. Therefore, the stable isotope techniques are effective for estimating the ratio of evaporated water that escapes to the atmosphere versus the water that returns to the soil as condensate. In addition, isotopic analysis of condensate reveals the moisture exchange between the topsoil and mulch, particularly vapor exchange obtained using various mulching methods.