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

Authors: Osterholz, William - Will; RUARK, MATT - University Of Wisconsin; RENZ, MARK - University Of Wisconsin; Grabber, John

Submitted to: Nutrient Cycling in Agroecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2023
Publication Date: 3/31/2023
Citation: Osterholz, W.R., Ruark, M.D., Renz, M.J., Grabber, J.H. 2023. Interseeded alfalfa N fixation and transfer to maize are reduced by N fertilizer. Nutrient Cycling in Agroecosystems. 126:67-79. https://doi.org/10.1007/s10705-023-10276-y.
DOI: https://doi.org/10.1007/s10705-023-10276-y

Interpretive Summary: Simultaneous planting of alfalfa and corn for silage (interseeding) is a forage production system that holds promise for increasing overall yields and profitability, but nitrogen (N) availability for corn production has been identified as a factor limiting corn production. As a legume, alfalfa can fix atmospheric nitrogen, thereby increasing the amount of N in the crop-soil system. However, the role of this N fixation in the N dynamics of the interseeding system remains unclear. We measured N fixation by alfalfa and uptake of newly fixed N by corn, as well as crop yields, in the interseeding system. Fixation provided 59% of alfalfa N when no N fertilizer was applied, but this was reduced to 15% at a high rate of N fertilization. Newly fixed N was also transferred to the corn crop: when no N fertilizer was applied 28% of corn N was newly fixed, but this was reduced to 7% when N fertilizer was applied. Our results also provided insight into the method used to calculate N fixation, revealing that both solo-seeded corn or corn interseeded with fescue grass were effective as "reference crops". The evidence for fixed N as an important source of N for interseeded corn 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.

Technical Abstract: Establishing alfalfa under a maize silage companion crop is a promising approach for increasing forage production and profitability, but the role of biological N fixation in this system has not been explored. This study utilized 15N natural abundance techniques to assess biological N 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 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. Evidence of N transfer to maize was observed at both locations, with maize at harvest obtaining on average ~28% of N uptake from biologically fixed N when no N fertilizer was applied, and ~7% with 112 or 224 kg N ha-1. Furthermore, we explored alternative reference crops for use in the N fixation and transfer calculations and found that solo-seeded maize or interseeded maize and tall fescue could serve as effective reference crops as long as N fertilizer strategies are similar to the interseeded alfalfa-maize system, although the natural abundance method may not be effective under excessive N fertilization rates as estimates of N fixation were observed. The evidence for transfer of biologically fixed N 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.