Short-term N Transfer from Alfalfa to Maize is More Highly Dependent on Arbuscular Mycorrhizal Fungi (AMF) rather than Root Exudates

Abstract: In ecosystems, common mycorrhizal networks and root exudates have been considered as direct pathways for helping plant–plant short-term N transfer. However, the underlying mechanism of the relative contribution of arbuscular mycorrhizal fungi (AMF) and root exudates on short-term N transfer are largely unknown in agroecosystems. A field experiment was conducted to explore these aspects for two consecutive years in a split-plot design with main plots treated with two nitrogen levels and subplots treated with maize and alfalfa grown alone or intercropped. The ¹⁵N foliar labeling technique was used for measuring N transfer between crops. Overall, short-term N transfer rate ranged from 4.4 to 12.85%, and it was significantly decreased by 20.6% in N fertilizer treatments, compared to no N addition, indicating that N transfer from alfalfa to maize was improved in N deficient soils. Irrespective of N level, maize had greater hyphal length density (HLD) and mycorrhizal colonization intensity (M%) than alfalfa, indicating that more N were transported to maize by AMF than alfalfa. Regardless of N level, short-term N transfer rate was highly correlated with HLD and M% of maize and alfalfa, however, strong correlations of soluble sugar and citric acid of root exudates in maize and alfalfa with N transfer rate were only observed under N addition, suggesting that short-term N transfer from alfalfa to maize is more dependent on AMF rather than root exudates, especially in low N soils.