Root and mycorrhizal fungal foraging responses to fruit removal in apple trees

Background and aims: Root and mycorrhizal fungal foraging in nutrient-rich patches is an energy-intensive process, and shifts in carbon (C) availability may affect foraging strategies. We hypothesize that when trees are C limited, they will prioritize root and mycorrhizal hyphal growth in nutrient-rich soil patches. Methods: Apple (Malus domestica Borkh.) trees with fruit were compared to trees with fruit removed to investigate the effect of reproductive effort and associated shifts in belowground C availability on root and arbuscular mycorrhizal (AM) fungal growth in unfertilized soil and localized nitrogen (N)-rich patches (containing inorganic or organic nitrogen). Results: Across nutrient treatments, fruit removal enhanced root production compared to fruiting trees. In fruiting trees, about four times more roots proliferated in the inorganic-N patch than in unfertilized soil or the organic-N patch. However, in trees with fruit removal, root proliferation was similar among nutrient treatments. Arbuscular mycorrhizal extramatrical-hyphal biomass was not affected by fruit removal but was greater in the organic-N patch than the inorganic-N patch or unfertilized soil. Fruit removal and N addition had modest effects on AM fungal colonization of apple roots and no effect on non-mycorrhizal fungal colonization. Conclusions: Root and AM foraging for nutrients should be considered in the context of C availability. Apple trees may manipulate root foraging more than AM fungal foraging when C belowground is constrained.