Mixed intercropping of spring barley (Hordeum vulgare L.) with field pea (Pisum sativum L.), faba bean (Vicia faba var. minor L.) or narrow-leafed lupin (Lupinus angustifolius L.) was compared with sole cropping in two field experiments at different locations, on a sandy loam soil and a sandy soil, in Denmark in 2001.

Grain legumes were dominant in intercrops on the sandy loam soil, except for lupin, whereas barley was dominant in intercrops on the sandy soil site. Combined intercrop grain yields were comparable to grain yields of the respective sole cropped grain legume or sole cropped, fertilized barley on each soil site. On the sandy loam soil, pea–barley and faba bean–barley intercrops increased the proportion of plant N derived from N2 fixation in grain legumes and increased the barley grain N concentration (from 1·7 to 2·2 mg/g) compared with sole cropping. However, the later maturity of faba bean compared with barley caused problems at harvest. The grain N concentration of intercropped barley was increased where grain legumes were the dominant intercrops and not on the sandy soil site. Lupin-barley intercrops did not show intercropping advantages to the same degree as faba bean and pea, but lupin constituted a more stable yield proportion of the combined intercrop yield over locations.

Furthermore, the study indicated that the natural 15N abundance at certain locations might not always be sufficient to ensure a reliable estimate of N2 fixation using the 15N natural abundance method.