Effects of Irrigation Methods on Root Distribution, Water Uptake Patterns, and Water Use Efficiency in Transplanted vs. Direct-Seeded Cotton

The transplanted cotton–wheat rotation enables dual cropping but alters root system architecture, typically suppressing taproot growth and promoting shallow lateral and fibrous roots, with unclear implications for irrigation response and water use efficiency (WUE). Field experiments were conducted in 2021 and 2022 to investigate root growth, spatial distribution, and water uptake sources, using a minirhizotron system and stable hydrogen and oxygen isotopes. The study examined the effects of two cultivation modes (direct seeding and transplanting) and three irrigation methods (border irrigation, micro-spray tape irrigation, and surface drip irrigation) on cotton root traits and WUE. Results showed that transplanted cotton roots were predominantly concentrated in the 0–30 cm soil layer (75.35–77.13% of total root length), significantly higher than those of direct-seeded cotton (63.10–74.71%). Under micro-spray tape and drip irrigation, the root length density (RLD) of transplanted cotton was 18.55% and 23.46% higher, respectively, than that of direct-seeded cotton, whereas under border irrigation it was 5.09% lower. Transplanted cotton mainly extracted water from the 0–40 cm soil layer (utilization rate: 65.49%), while direct-seeded cotton primarily relied on water from the 20–60 cm layer (53.20%). Although no statistically significant difference in yield was observed between the two cultivation modes, transplanted cotton exhibited a 15.37% higher WUE than direct-seeded cotton. Moreover, surface drip irrigation substantially enhanced WUE, exceeding that under border irrigation and micro-spray tape irrigation by 37.35% and 14.07%, respectively. This study enhances understanding of root traits in transplanted cotton and demonstrates that irrigation methods regulate WUE by modifying root distribution and water uptake patterns.