Objectives Continuous cropping of maize in Hexi Oasis irrigation areas is of great concern in terms of low efficiency and environmental risks as a result of excessive nitrogen (N) application. We studied the availability of reducing 25% of chemical nitrogen input through green manure intercropping with maize.

Methods A field experiment was conducted at Wuwei, Gansu Province, China, in 2019–2020, the tested green manure crops were common vetch (Vicia sativa L.) and rape (Brassica campestris L.). Split plot design was used for the experiment. The main plot was three cropping patterns, i.e., maize-common vetch intercropping (M/V), maize-rape intercropping (M/R), and sole maize (M). The sub-plot was N application rate, i.e., 25% N input reduction (N1), conventional N input 360 kg/hm² (N2). The aboveground part of green manure crops were cut and covered in maize strips at full flowering stage (about 50 days after maize emergence). Dry matter accumulation (DMA) of maize was determined every 15 days from seedling stage while grain yield and yield components were assessed at harvest stage.

Results Compared with N2, N1 decreased maize grain yield and biomass by 15.7% and 12.9%, respectively in sole maize, however, N1 elicited similar grain yield and biomass with N2 in the two maize-green manure intercropping systems. From the emergence to pre-tasseling stage of maize (0–60 days), N1 treatment recorded 15.8% lower DMA than N2, whereas M/R and M/V had 8.8% and 31.5% lower DMA than M treatment, respectively. From pre-tasseling to early grain-filling stage (60–105 days), N1 recorded 7.2% lower DMA than N2, while M/R and M/V recorded 10.2% and 8.8% higher DMA than M, respectively. From the early grain-filling stage to maturity stage (105–150 days), the DMA of N1 treatment was 4.2% lower than that of N2, but those of M/R and M/V were 5.8% and 8.2% higher than M, respectively. Intercropping increased the maximum growth rate (V_max) of maize under N reduction, M/R and M/V were 16.1% and 27.3% higher than that of M, respectively, while the V_max of M/V under N1 was 16.6% higher than that of M under N2. Intercropping increased grain number per spike and 1000-grain weight of maize under N reduction. Compared with M, the grain number per spike and 1000-grain weight of maize were increased by 11.9% and 19.6% with M/R, and 13.9% and 22.3% with M/V, respectively. The grey correlation analysis showed that 1000-grain weight, grain number per spike, and spike number per unit area were the top three factors related to yield.

Conclusions Intercropping green manure compensates for dry matter accumulation of maize from pre-tasseling to maturity stage and increases the photosynthate input of maize kernel under N reduction, which offsets yield loss of maize caused by N reduction and eventually ensures yield stability. Therefore, intercropping green manure combined with reducing 25% N input is an efficient agronomic technology for achieving N reduction and enhancing grain yield of maize in oasis irrigation region.