Incorporating species capable of biological nitrogen fixation into agroforestry systems can improve nutrient cycling and soil health while reducing the need for synthetic nitrogenous fertilizers. Research has confirmed multiple pathways through which biologically fixed nitrogen is transferred to non-nitrogen fixing plants, including indirect transfer through decaying biomass and direct transfer through belowground root exudates and shared fungal networks. I analyzed the benefits of cover crops to the growth and nutrition of important agroforestry tree species through experiments in the field and greenhouse. In the field study, I examined the contributions of Trifolium pratense to the health and growth of hybrid poplar (Populus nigra L.×P. maximowiczii A. Henry 'NM6') in an alley cropping system at varying seed rates and tree row spacing. Soil and poplar leaf nutrient analysis revealed higher nitrogen levels in treatments with T. pratense at regardless of seed rate at the wider spacing, while narrow spacing treatments showed signs of competition. In the greenhouse study, the transfer of N through root exudates from white clover (Trifolium repens) to a hybrid poplar (Populus nigra L.×P. maximowiczii A. Henry 'NM6') and a hybrid chestnut (Castanea sativa x C. crenata 'Colossal') was quantified via 15N isotope tracing methods. Results revealed that over the course of 9 weeks, white clover supplied 33.3% (±7.4) of chestnut N and 12.1% (±1.1) of poplar N through root eudates. White clover (T. repens) was found to exude 0.048 g N m-2 day-1 and roughly 3.4% of its' total N over the course of the 63 day experiment. Together these studies reinforce the potential of nitrogen-fixing species to benefit mixed systems through both direct and indirect means.
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