Belowground pathways for nitrogen transfer from a tropical legume tree to an associated fodder grass

Legume trees which form symbiosis with N2-fixing bacteria can help replenish and maintain soil fertility on tropical agricultural lands by supplying nitrogen to the system. However, the mechanisms of N transfer from the trees to associated crops are not well understood. The role of root exudation and common mycelial networks of mycorrhizal fungi (CMN) in interplant N transfer was analysed in this study. A cut-and-carry agroforestry system comprising a legume tree (Gliricidia sepium) and a fodder grass (Dichanthium aristatum) was used as a model system. Nitrogen transfer was measured by labelling the tree with 15 N. Variation in the isotopic composition of the N sources from the tree and its effects on N transfer estimates was analysed using experimental methods and mathematical modeling. Both plant species were observed to form symbiosis with the same subgroup of Rhizophagus intraradices in the field, indicating favourable conditions for the formation of CMN. In pot culture D. aristatum obtained up to 14% of its N from G. sepium via belowground pathways over 10 weeks, which was mainly accounted for N exudation. Nitrogen transfer via CMN contributed up to 2.5% of N in D. aristatum and corresponded to 18% of total N transferred. Nitrogen transfer via CMN increased with arbuscular colonisation of the N donor and with decreasing N concentration of the N recipient. Transfer estimates varied manifold depending on the assumed isotopic composition of transferred N, which highlights the need for careful estimation of the isotopic ratios of the actual N sources. The results suggest a significant role for root exudates and CMN in transferring N from legume trees to the associated crops, as opposed to the common perception of tree prunings and mulching as the primary N sources to the crops. Design and management options of agroforestry systems could be reviewed to foster belowground N transfer and improve the sustainability of farming systems.