Irrigation and phosphorus fertilization regulate trade-offs among yield, forage quality, and resource-use efficiency in alfalfa production under arid and semi-arid conditions.

INTRODUCTION

Optimizing water-phosphorus coupling is essential for resolving the trade-offs among high yield, improved forage quality, efficient resource use, and soil nutrient sustainability in alfalfa production in arid and semi-arid regions. Yet, how irrigation regimes and phosphorus fertilization jointly regulate these interconnected agronomic, physiological, quality-related, and soil nutrient responses remains unclear.

METHODS

A two-year field experiment was during the 2024 and 2025 growing seasons, using three irrigation regimes: W1, moderate water stress; W2, mild water stress; and W3, full irrigation, combined with three phosphorus levels: P0, P50, and P100. Alfalfa yield, irrigation water use efficiency (IWUE), phosphorus agronomic efficiency (AEP), plant morphology, forage quality, leaf photosynthetic traits, and soil available nutrients in the 0-60 cm soil profile were measured across three cuttings. Principal component analysis and correlation analysis were used to evaluate the integrated effects of water-phosphorus coupling.

RESULTS

Irrigation, phosphorus fertilization, and year significantly affected alfalfa yield, IWUE, plant morphology, and photosynthetic characteristics, while their interactions significantly influenced yield, IWUE, agronomic phosphorus efficiency (AEP), plant morphology, forage quality, and photosynthetic characteristics, as well as soil nutrient status, whereas interaction effects varied among indicators. Across treatments, W3P50 produced the highest yield, with no significant difference from W3P100. Under P50, W2P50 and W3P50 increased yield by 5.3% and 7.9%, respectively, compared with W1P50, while under P100, W2P100 and W3P100 increased yield by 4.0% and 5.0%, respectively, compared with W1P100. Under W2 and W3, P50 and P100 produced significantly higher IWUE than P0, but no significant difference was observed between P50 and P100, indicating that moderate phosphorus input was sufficient under improved irrigation conditions. W2P50 and W3P50 showed higher AEP than W1P50 and W3P100. W2P50 and W3P50 also significantly increased crude protein content compared with W1P50. Full irrigation enhanced Pn, Tr, and Gs, while W2P50 and W3P50 maintained higher intrinsic water-use efficiency (WUEi). The entropy-weighted TOPSIS evaluation showed that W2P50 had the highest comprehensive score, followed by W3P50 and W2P100.

DISCUSSION: Water-phosphorus coupling regulated alfalfa production by coordinating soil nutrient availability, photosynthetic capacity, plant growth, forage quality, and resource-use efficiency. W2P50 showed the best overall performance by maintaining stable yield while improving AEP, IWUE, and forage quality, and is therefore recommended as an optimized water-saving and phosphorus-efficient strategy.