Objectives Intercropping wheat with common vetch or hairy vetch is a new crop production system in northwest China. To provide theoretical base for the nutrient management of this intercropping system, we studied the effect of N fixation and N transfer efficiency of two legumes (common vetch and hairy vetch) on wheat.

Methods Identical pot experiments were conducted in two sites: Xining and Wuwei in Qinghai and Gansu Provinces of China, respectively. The treatments in the experiments included wheat monoculture (WC); common vetch monoculture (MC); hairy vetch monoculture (MH); wheat and common vetch intercropping (IWC); and wheat and hairy vetch intercropping (IWH). We did not apply N fertilizer in all the treatments. ¹⁵N natural abundance method was used to analyze the amount of N from biological fixation and the amount transferred to wheat.

Results The aboveground dry biomass of wheat and the two legumes in intercropping treatments were significantly lower than their biomass in monoculture treatments. Similarly, the individual N accumulation of the two legumes and wheat in intercropping treatments were significantly lower than their N accumulation in monoculture treatments. The total N accumulation in legume monoculture (MH and MC) and wheat-legume intercropping treatments (IWC and IWH) were higher than that of WC. The amount of N fixed by common vetch in IWC treatment in the two sites (Xining: 0.24 g/pot and Wuwei: 0.48 g/pot) were lower than that fixed in MC treatment (Xining: 0.88 g/pot and Wuwei: 0.78 g/pot). The amount of N fixed by hairy vetch in IWH (0.38 g/pot) was lower than that fixed in MH treatment (0.81 g/pot) in Xining, while those of Wuwei were similar. Both the amount of N fixed by hairy vetch and N transferred to wheat in Xining were significantly higher than those in Wuwei, whereas both N fixed by common vetch and N transferred to wheat were higher in Wuwei than Xining. The amount of N transferred from N fixation to wheat under IWC (Xining: 0.13 g/pot and Wuwei: 0.19 g/pot) accounted for 31.6% and 24.7% of the total N uptake by wheat in IWC treatment in Xining and Wuwei, respectively. The N transferred to wheat under IWH (Xining: 0.09 g/pot and Wuwei: 0.06 g/pot) accounted for 23.8% and 11.4% of the total N uptake by wheat in IWH treatment in Xining and Wuwei, respectively. Path analysis showed that shoot dry biomass was the most important factor affecting N fixation and N transfer from legumes to wheat.

Conclusions Overall, intercropping decreases the aboveground biomass of legumes and amount of fixed N. Our results also suggest that cultivar and growing environment affect N fixation of the two legumes. About 11.4%–31.6% of the N uptake by wheat is from leguminous N fixation, and the N transfer ability of common vetch is generally higher than that of hairy vetch. Therefore, further study is needed to identify the best techniques for efficient intercropping of wheat and vetch legumes.