Effect of green manure returning pattern on water utilization of spring wheat under reduced irrigation in arid irrigation areas

To address the challenges of high water consumption and low water use efficiency (WUE) in spring wheat cultivation in arid irrigated regions, this study investigated the regulatory effects of green manure on water utilization characteristics of spring wheat under reduced irrigation quotas, aiming to provide a theoretical basis for efficient water-saving cropping systems. A field experiment was conducted from 2021 to 2023 in the Hexi Oasis irrigation zone using a split-plot design. The main plot included three irrigation quotas: 420 mm (local conventional, I3), 370 mm (reduced by 50 mm, I2), and 320 mm (reduced by 100 mm, I1). The subplot comprised three green manure treatments: full biomass return after wheat harvest (WG), root and stubble return only (WR), and fallow (W). Results showed that reduced irrigation significantly decreased grain yield; However, green manure effectively offset yield losses. Full green manure return (WG) compensated for the yield loss under a 100 mm irrigation reduction, while WR compensated for a 50 mm reduction. Compared with I3, grain yield under I1 and I2 decreased by 11.6% and 3.2%, respectively. WG and WR treatments increased yield by 11.2% and 5.0% compared to W. Notably, WGI2 increased yield by 7.7% relative to WI3, and both WGI1 and WRI2 showed no significant difference from WI3. Both reduced irrigation and green manure return decreased seasonal water consumption, with WG more effective than WR. Compared to I3, water consumption under I1 and I2 decreased by 24.7% and 10.5%, respectively; WG and WR decreased consumption by 7.8% and 5.3% relative to W. This was mainly due to: (1) lower inter-row evaporation under I1 and I2 (36.8% and 21.0% less than I3) and under WG and WR (24.5% and 17.8% less than W); (2) reduced E/ET under I1 and I2 (by 16.5% and 12.0%) and under WG and WR (by 18.5% and 13.6%). Although the wheat-green manure intercropping system increased total water consumption by 25.1% compared to monoculture wheat, reduced irrigation effectively constrained this increase. Both strategies improved WUE and irrigation water use efficiency (WUE_i), with the greatest gains observed under the lowest irrigation quota combined with full green manure return. Compared with I3, WUE and WUE_i increased by 17.4% and 34.9% under I1, and by 7.9% and 16.9% under I2. Compared with W, WUE and WUE_i increased by 21.0% and 11.5% under WG, and by 11.4% and 5.0% under WR. The WGI1 treatment yielded a 37.4% increase in WUE and 51.2% in WUE_i compared to WI3, indicating that combining full green manure return with reduced irrigation is particularly effective for improving irrigation efficiency. Therefore, in arid oasis irrigation areas, returning full green manure to the field after wheat, under a 320 mm irrigation quota, can maintain stable wheat yields while significantly enhancing water use and irrigation water use efficiencies. This approach offers a viable strategy for achieving both yield stability and improved water productivity in spring wheat systems.