Interseeded alfalfa N 2 fixation and transfer to maize are reduced by N fertilizer

Establishing alfalfa ( Medicago sativa L.) under a maize ( Zea mays L.) silage companion crop is a promising approach for increasing forage production and profitability, but the role of biological N 2 fixation in this system has not been explored. This study utilized 15 N natural abundance techniques to assess biological N 2 fixation by interseeded alfalfa and transfer of fixed N to maize under three N fertilizer application rates. Across two locations in Wisconsin, USA, the results showed N 2 biological fixation supplied 59% of herbage N to interseeded alfalfa with no N fertilizer, though this was reduced to 15% when fertilized with 224 kg N ha −1 . Transfer of newly fixed N to maize was observed at both locations, with maize at harvest obtaining on average 28% of N uptake from fixed N without N fertilizer application, and 7% when fertilized with 112 or 224 kg N ha −1 . Additionally, both solo-seeded maize and tall fescue ( Schenodorus arundinaceus (Schreb.) Dumort.) interseeded into maize system could produce appropriate reference crops for N 2 fixation and transfer calculations, provided the N-fertilizer strategies were similar to the interseeded alfalfa-maize system. Furthermore, results suggest the natural abundance method may not be effective if high rates of N fertilizer are applied at sites having inherently high levels of available soil N. The evidence for transfer of biologically fixed N 2 from interseeded alfalfa to maize offers interesting possibilities to leverage this process to ensure efficient N cycling, but it must be balanced against the need to ensure an adequate N supply for high maize silage yields.