Effects of walnut trees on biological nitrogen fixation and yield of intercropped alfalfa in a Mediterranean agroforestry system

•Alfalfa yield was reduced in the shade of walnut trees, but BNF was significantly increased.•The light use efficiency of alfalfa increased by 35% in the shade.•BNF increase may result from N competition or mineralization inhibition under the trees. While intercropping annual non nitrogen-fixing crops with deciduous hardwood species is now well documented, there is a need to investigate if nitrogen-fixing intercrops may succeed in agroforestry systems. Intercropping with trees usually leads to a decline in crop yield, and could in addition possibly reduce the biological N fixation (BNF) over time due to the competition for resources. In a Mediterranean experimental site, 17 year-old hybrid walnut trees (Juglans nigra x Juglans regia L.) planted in East-West oriented lines were intercropped with alfalfa (Medicago sativa L.) to assess the impact of competition for light and water on alfalfa yield and BNF. Alfalfa yield and shoot δ15N values (a proxy for the proportion of N derived from the air, %Ndfa) were measured during one year at different distances from the tree row in two directions (north and south). Alfalfa yield was reduced close to the tree row (−28% and −22% on the northern and southern sides respectively), but less than the reduction of irradiation (−59% and −33% respectively). Shading improved by 35% the apparent light use efficiency (LUE: aboveground biomass produced per unit of global radiation) of alfalfa, indicating that alfalfa was shade tolerant at this Mediterranean site. Alfalfa shoot δ15N values were lower close to the tree rows than at mid inter-row: BNF was stimulated close to the trees. Compensative and facilitative mechanisms between trees and alfalfa plants led to a rise in LUE and%Ndfa in shaded areas. These results contradict the frequent assumption that N fixation is reduced in the shade of trees. Appropriate tree canopy management may help maintain light competition between trees and alfalfa to a level that still enhance complementary, which would further improve the sustainability of the use of alfalfa as an intercrop in Mediterranean regions.