The overall goals of the proposed research are to quantify the contributions of different mechanisms by which nonlegumes yield more in intercrops with legumes than in monocultures and to quantify how the contribution of each mechanism varies with interplant distance. The ultimate goal is to use this information to design optimal intercrop systems. Biomass and nitrogen yield of a corn plant surrounded by legumes may be higher than that of a corn plant surrounded by an equal density of corn plants either because of reduced competition for soil nitrogen (greater availability of soil nitrogen to the corn) and/or the transfer of atmospheric nitrogen fixed by the neighboring legume plants, i.e facilitation. These two sources can be distinquished by adding isotopic nitrogen to the soil. Reduced competition would result in an increase in the absolute quantity of labeled soil nitrogen in the intercropped relative to monoculture corn. Facilitation would result in a decrease in the proportion of labeled soil nitrogen in the intercropped corn (increase in proportion of atmospheric-derived N).
