Above- and below-ground competition between intercropped winter wheat Triticum aestivum and white clover Trifolium repens.
Intercropping of white clover and cereals has recently been promoted for low-input farming systems because it offers several benefits for sustainability, but the practical use of this system has been limited by the relatively low wheat yields. Very little is known about competition between the two species in this intercropping system and its implications for production. To investigate the interaction between white clover and wheat, we separated above- and below-ground competition in the field in a fully factorial additive design. The treatments were with and without clover, with and without below-ground partitions between rows of the two species, and with and without above-ground partitions. Above-ground biomass of both species was harvested three times over the growing season, and the wheat biomass was analysed for nitrogen content. When wheat was grown without clover, its biomass was much reduced by below-ground partitions and slightly reduced by above-ground partitions. Presence of both above- and below-ground partitions did not result in lower wheat biomass than below-ground partitions alone. Total biomass was higher when both species were present, and this biomass was reduced by below-ground partitions and, to a lesser degree, by above-ground partitions. In mixture, below-ground partitions reduced wheat biomass and increased clover biomass at the last harvest, whereas above-ground partitions reduced clover biomass and increased wheat biomass. The interaction between the two species was dominated by competition for soil nutrients, but competition for light influenced the partitioning of biomass production between the two species. Mingling of the roots of both species is important for maximizing soil resource utilization, whereas shoots performed almost as well without mingling. Synthesis and applications. The results suggest that competition between wheat and clover for nitrogen is reduced by nitrogen fixation because clover obtains some of its nitrogen from fixation. Increased availability of nitrogen to the intercropped wheat late in the growing season could increase grain protein content. Our results suggest possibilities for improved management of competition between the two species to optimize resource utilization, biomass production and wheat yields. Goals for yield should be based on a specific component, such as total biomass, grain yield and grain quality, because these yield components can behave differently when another species is present.