Faba bean (Vicia faba L.) intercropped with oil crops – a strategy to enhance rooting density and to optimize nitrogen use and grain production?

Intercropping of faba bean (FB) and oil crops (OC) has two main features: competition in the areal parts and competition as well as facilitation in the underground parts. Root distribution, nitrogen use, and grain yields of faba bean (Vicia faba L.) intercropped with safflower (Carthamus tinctorius L.) (SAF) or white mustard (Sinapis alba L.) (MUS), were investigated in field experiments at two sites in the Rhineland region of Germany. The crop species were grown both as sole crops and as intercrops with reduced sowing densities. In intercropped FB, row spacing was 56 cm with two OC rows in between. This study aimed at optimizing the rooting pattern and therefore enhancing soil space use, nitrogen acquisition, and grain production. Intercropping resulted in a more regular horizontal root distribution compared with sole cropped FB. In comparison to sole cropped FB, intercropping with SAF and MUS enhanced the root-length density in the subsoil, whereas the amount of plant-available soil nitrogen was reduced at all sampling dates. Compared with sole-cropped FB, nitrogen accumulation in FB intercrop shoots was reduced because of reduced sowing density of FB. Total nitrogen accumulation in shoots was ranked as follows: sole-cropped FB > FB intercrops > sole-cropped OC. Regarding the individual FB plant, in intercrops nitrogen acquisition was enhanced at floodplain site ‘Wiesengut’ where the competiveness of OC was rather low. Compared with sole cropping land equivalent ratios for grain yield dry matter of intercrops were increased at the site ‘Klein-Altendorf’ but not at ‘Wiesengut’. However, land equivalent ratios for total nitrogen accumulation (LER-N) in FB intercropped treatments were generally enhanced by 12% (SAF) to 18% (MUS) when compared with sole cropping. Thus, intercropping greatly influenced yield performance, depending on site specific competition: at the floodplain site ‘Wiesengut’ OC grain yields were significantly reduced by shoot competition, whereas on the more fertile loessial soil at the site ‘Klein-Altendorf’ that released higher amounts of soil nitrogen, FB grain yields were significantly reduced by high oil crop competiveness. Consequently, the advantages (reduced plant-available soil nitrogen) and disadvantages (reduced FB grain yield) of growing FB intercropped with OC have to be weighed according to site conditions and the decision regarding which is the most valuable cash crop.

► We optimized the rooting pattern in order to enhance the use of soil-borne nitrogen.
► Intercropping (IC) gave higher root-length density in the subsoil.
► IC enhanced LER for total nitrogen accumulation.
► Advantages of IC have to be weighed according to site conditions and value of crops.