Seasonal reorganization of vertical soil-water partitioning among contrasting crops in an arid oasis agroecosystem

Abstract. Water availability in arid agroecosystems is strongly structured along the soil profile, yet how contrasting crop functional types redistribute their water uptake across this vertical gradient through the growing season remains poorly constrained. Here, we combined stable hydrogen and oxygen isotope measurements of precipitation, irrigation water, soil water, and crop xylem water with soil-water content observations and Bayesian source apportionment to examine seasonal soil-water partitioning among alfalfa, maize, and Lycium chinensis in an oasis agroecosystem of northwestern China. Soil-water isotopes showed the strongest variability in the upper profile, consistent with greater evaporative modification of near-surface water. Alfalfa exhibited comparatively stable reliance on shallow soil water, with estimated 0–20 cm contributions of 54.2–64.5 % across the growing season. Lycium chinensis relied more strongly on middle and deep soil-water stores, whereas maize showed the greatest seasonal reorganization, shifting from dominant shallow-water use early in the season towards increasing contributions from deeper layers as crop development progressed. These contrasting uptake-depth patterns were accompanied by differences in soil-water availability and evaporative enrichment among crop fields. Although isotope-based source fractions remain sensitive to end-member definition and temporal sampling, the combined evidence indicates that crop functional type and phenological development regulate access to vertically structured soil-water storage in irrigated drylands.