Stable Isotope Fractionation in an Agricultural Field during Wet-Dry Cycles

In an agricultural field, crops in the Rabi and Zaid seasons are exposed to repeated wet-dry cycles between precipitation and/or irrigation events. There is a general consensus in the literature that no isotopic fractionation occurs during water uptake by the plants. The isotopic shifts in soil water at different soil tensions, if occurs during repeated wet-dry cycles, are expected to influence the isotopic composition of the plant’s xylem water. The objective of this study was to investigate the isotope enrichment and/or depletion during repeated wet-dry cycles in an agricultural field within the plant-available water range, specifically from field capacity to wilting point. The pressure-saturation curves and isotope retention patterns were compared to observe the changes in the isotopic compositions of plant-available water.

The laboratory experiments were conducted with soil (silty sand) from an agricultural plot that undergoes regular cultivation of 2-crops a year (Rice-Wheat) with no tillage. A modified pressure plate apparatus was fabricated to simultaneously measure the pressure vs. saturation and isotope compositions at each pressure. The pressure plate apparatus was designed to ensure mass balance across the imbibed, exuded, and retained water at each suction pressure, throughout all the wet-dry cycles. The experiments were conducted over five wet-dry cycles with the same reference water of known isotopic composition. The exuded water was analyzed directly, and the retained water content at each suction pressure of five wet-dry cycles was extracted using cryogenic vacuum distillation. The isotopic composition of all the samples was analyzed using an LGR OA-ICOS liquid water isotope analyzer.  

The pressure-saturation curves across all five cycles exhibited no significant changes for drainage. Drained water, even at a small suction pressure of 0.1 bar, was enriched in both δ²H and δ¹⁸O compared to the isotopic composition of the imbibed water. Within a cycle, both δ²H and δ¹⁸O in the exuded water showed depletion as the suction pressure increased. The δ²H composition of the exuded water became enriched with the progression of the wet-dry cycles. The δ¹⁸O composition of the exuded water, on the other hand, showed depletion with the progression of the wet-dry cycles. These results indicate that plant xylem water may show a mismatch with irrigation water due to fractionation during the wet-dry cycles.