Legume Performance and Nitrogen Acquisition Strategies in a Tree-Based Agroecosystem

Legume N-2 fixation is a critical plant-mediated nutrient pathway highly dependent on site and management conditions, yet little is known of annual crop N-2 fixation associated with established trees in tree-based intercropping systems. Our objective was to investigate the performance and N acquisition strategies of soybean in a tree-based intercropping system with four tree species (genera Juglans, Fraxinus, Acer, and Populus) at two distances from the tree row (1- and 4-m tree-crop interface) in comparison to monocropped soybean. Soil nitrification rates were inversely related to soybean nodule production at the 1-m tree-crop interface (r = -0.64; p = 0.0260). However, regardless of inhibited nodule production, soybeans at the 1-m tree-crop interface were fixing N at high rates as compared to monocropped soybeans, possibly stimulated by belowground interspecies competition; nitrogen derived from atmosphere (Ndfa) in soybeans was highest when intercropped with spruce (75% Ndfa) and poplar (65% Ndfa) at the 1- and 4-m tree-crop interface, respectively. We show a shift in crop N acquisition strategies, as modified by tree species and distance. We suggest that an often overlooked nutrient pathway, nitrate deposition via tree canopy wash, may be an unaccounted abiotic factor influencing nodule formation of associated legume crops.