The effects of intercropping on dry weight (DW) of herbage and nitrogen (N) nutrition of plants of two winter cereals, barley and wheat, and two legumes, white lupin and common vetch, were investigated, and above- and below-ground competition were separated in a fully factorial additive design. Intercropping increased DW compared with the sole species and the increase was higher for the cereals and lupin than for cereals and vetch intercropping systems. Above-ground competition for light reduced DW of cereals and lupin while it did not influence the DW of vetch. Processes involved in below-ground competition increased shoot growth of cereals and reduced shoot growth of legumes. N nutrition of cereals was enhanced by below-ground competition with legumes and N nutrition of vetch was enhanced by above-ground competition with cereals. Cereals had a higher competitive ability than legumes as a result of their below-ground competitive ability. The interaction between above- and below-competition is not predictable: negative, positive and no interaction (additivity) between different types of competition were found. In low-input intercropping systems, when audN-fixing species is present, the mixture of the roots of components is important for the utilization of the soil resources and, when a climbing species is also present, the mixture of shoots can result in an increased utilization of light.
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