N transfer Rate is Dependent on N Fixation by Legume in Alfalfa (Medicago sativa L.)/Maize (Zea mays L.) Intercropping System

Abstract: Cereals can stimulate commonly the symbiotic nitrogen fixation of legume presumably through the competition for N. In addition, there was some evidence showing that root exudates of cereals can also help legume nodulation. However, it still unclear to what extent NO₃^- and NH₄⁺ competition and root exudates contribute N fixation in the rhizosphere of alfalfa and maize. Therefore, field and greenhouse experiments were conducted to explore the relative effects of N competition and root interaction on N fixation in a maize/alfalfa intercropping system with two N levels. Our results showed that under no N addition, alfalfa significantly increased both nodulation and symbiotic N₂ fixation when intercropped with maize. On average, nitrogen derived from the atmosphere (%Ndfa), nodules number and leghemoglobin content in intercropped alfalfa were increased by 43.66%, 85.53%, 141.05%, compared with monoculture alfalfa. The high %Nfda of intercropped alfalfa was not only due to the depletion of ammonia and nitrate caused by cereals, but also significantly correlated with the decrease of NO₃^-/NH₄⁺ ratio. Furthermore, the content of citric acid in alfalfa root exudates was dramatically increased by 130% in intercropping, and it was positively correlated with nitrogen fixation and nodulation, indicating that the enhancement of root exudates by root interaction can facilitate mobilizing resources for alfalfa N fixation. Overall, 15.4%-21.5% of N fixed by alfalfa was transferred to associated maize, and there was a strong positive correlation between %Ndfa, nodule numbers of alfalfa and the N transfer rate, suggesting N transfer is reliant on N fixation by legume. Our findings indicate the importance and relevance of NO₃^-/NH₄⁺ ratio and citric acid in the rhizosphere for N fixation and an emphasis on the strong positive relationship between N transfer rate and N fixation.

Keywords: intercropping; N fixation; citric acid; NO₃^-/NH₄⁺ ratio