Lupin-barley intercropping: Root to root interactions drive nitrogen transfer from legume to cereal

Intercropping cereals and legumes is a means to reduce fertilizer input in agriculture. Transfer of biologically fixed N often occurs in cereal-legume intercropping and this study aims to understand the mechanism behind. Lupins are legumes of agronomical interest due to their high protein content and effective soil P extraction. However, as lupins are commonly described as non-mycorrhizal the transfer route of N from lupin to barley remains to be elucidated. We investigated the growth and nutrient content of barley intercropped with lupins, to test whether transfer of symbiotically fixed N from lupins to barley occurs, with focus on any role of arbuscular mycorrhizal (AM) fungi in this transfer.

Lupin species and barley were grown in pots (as sole crop or intercropped) ± mesh enclosures restricting mycelial and/or root growth between compartments. Plant growth and AM fungal root colonization were recorded, and plant ¹⁵N natural abundance was measured to determine potential transfer routes of fixed N from lupin to barley.

Intercropped treatments showed increased barley growth and N contents, most pronounced if root-root intermingling of the two species was allowed. Also, ¹⁵N natural abundance in plants corroborated N transfer from lupins to barley. As lupin roots remained non-mycorrhizal, even in presence of a mycorrhizal donor plant, hyphal translocation of N was unlikely.

We conclude that N transfer from non-mycorrhizal lupins to mycorrhizal barley primarily occurred through bulk flow, stimulated by interspecific root-to-root contact. This may contribute to the success of lupin and barley intercropping.