N2-fixing trees and the transfer of fixed-N for sustainable agroforestry: a review

Many tropical areas lack soil nitrogen (N), an essential nutrient for plant growth and the production of food. Commercial N fertilisers are expensive, with only a fraction of this nutrient reaching the plant, which limits efficiency and potentially increases water contamination. Dinitrogen (N-2)-fixing trees are a promising alternative to sustainably fertilise crops. The use of N-2-fixing trees in tropical agriculture has garnered attention from researchers, development organisations, governments, and farmers in recent years as a revival of pro-environmental practices. Dinitrogen (N-2)-fixing trees can establish in N-deficient soils, replace N lost in harvest and provide an as-of-yet not fully realised benefit to ecosystem services. High N-2-fixation rates, upwards of 92 %, have been measured in some N-2-fixing trees, using the N-15 natural abundance method. The recovery of this fixed-N by associated perennial crops is of particular interest in tropical agroforestry systems. Here, we review N transfer pathways from trees to perennial crops in agroforestry. We focus on Theobroma cacao and Coffea arabica. We also draw on agroforestry systems with herbaceous alleys. We identify three pathways of N transfer from N-2-fixers to non-N-2-fixing crops: (1) decomposition and mineralisation of organic compounds, e.g., litter, prunings, roots, and nodules, (2) root-to-root direct transfer via exudation, and (3) common mycorrhizal networks. Both N-15 natural abundance and N-15 enrichment techniques have been used to study N transfer. However, various factors limit the accuracy of estimates within agroforestry systems. Under field conditions, the major limits are (1) improper reference selection and (2) unrepresentative sampling of the receiver plant and/or donor N source. We highlight key findings and provide recommendations to tackle these obstacles.