S15 - Session O2 - Tree root interactions revealed by metabarcoding in agroforestry systems

Authors: Shahla Bai *, Rachel Wilson, Godfrey Hannet, Alexander Keller, Alison Shapcott, Birte Komolong, Michaël B Farrar, Helen M Wallace

Both cacao and coffee are important cash crops that are usually interplanted under shade trees in agroforestry practices. Root interactions between cash crops and neighbouring trees remain a major gap in knowledge. Ideal shade trees would develop their root systems below the cacao and coffee root zones, minimizing plant competition for nutrient uptake in the soil layers. Assessment of root development in soil remains highly challenging. Destructive excavation provides root biomass and root morphology data. However, the abundance of fine roots of an individual plant species in a mixed-species plantation remains unknown because it is difficult to identify the species of fine roots that are present. We selected to adjacent cocoa and coffee plantations where Canarium indicum was used as a shade tree. We used DNA meta-barcoding to determine the presence and abundance of fine roots of each species in the soil profile to a depth of 80 cm. Cacao and coffee had higher fine-root abundance at 0-30 cm depth, whereas canarium had higher fine-root abundance at 30-80 cm depth. Foliar total nitrogen and potassium in cocoa and coffee were over ideal levels whereas foliar phosphorus was not. The system was originally phosphorus limited. Our study indicated that canarium could be a suitable shade tree for cacao and coffee. DNA metabarcoding allowed us to develop a deeper understanding of root development through the tropical soil profile.