Deficit Irrigation Trades Shallow for Deep Water Uptake in Saline Cotton, up to a Salinity Limit

ABSTRACT

Plant‐available water, bounded by the matric contents between field capacity and the wilting point, assumes that all water in the rooting zone is equally usable. Under mulched drip irrigation on saline soil this fails: the emitter creates a desalinated wetting bulb in which roots concentrate, while water outside the bulb remains available yet poorly accessible because extracting it carries an osmotic cost. Whether irrigation can be managed to enlarge the accessed fraction is unknown. Over two seasons (2024–2025) in a saline oasis field at Wensu, southern Xinjiang, we grew film‐mulched drip‐irrigated cotton under four sustained regulated deficit irrigation levels (100%, 85%, 70% and 55% of the full drip‐irrigation quota), combining two‐dimensional water–salinity–potential monitoring, δ ² H and δ ¹⁸ O source partitioning with a depth‐controlled ² H‐labelling test, minirhizotron root tracking, and coupled root‐zone water and salt balances. Moderate deficit (70%) shifted uptake from the shallow bulb (0–40 cm: 72% → 45% of uptake) towards deeper layers, increased rooting depth (D ₉₅ 62 → 91 cm), and raised deep‐source accessed water—the seasonal transpiration sourced from below the ≈40 cm base of the shallow desalinated bulb—from 115 mm (95% credible interval 106–124) to 193 mm (178–208) while attaining the highest water‐use efficiency (13.1 kg ha ⁻¹  mm ⁻¹ ); minirhizotron imaging showed these deep roots grew actively, not passively stranded. Severe deficit (55%) collapsed the salt export ratio (0.31 → 0.11), re‐concentrating retained salt above the cotton tolerance threshold; deep‐root growth reversed, yield fell 27%, and deep‐source accessed water did not rise further (186 mm, 171–201), its interval overlapping that of the 70% treatment. Saturated conductivity fell only 25%, indicating a predominantly osmotic rather than structural limitation, although a residual structural contribution cannot be excluded. A moderate (≈70%) deficit, bounded by salt re‐concentration, optimised water productivity—field evidence for an accessibility‐based reading of regulated deficit irrigation in saline, mulched‐drip cotton.