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

Abstract

tWhile intercropping annual non nitrogen-fixing crops with deciduous hardwood species is now welldocumented, there is a need to investigate if nitrogen-fixing intercrops may succeed in agroforestrysystems. Intercropping with trees usually leads to a decline in crop yield, and could in addition possiblyreduce the biological N fixation (BNF) over time due to the competition for resources. In a Mediterraneanexperimental site, 17 year-old hybrid walnut trees (Juglans nigra x Juglans regia L.) planted in East-Westoriented lines were intercropped with alfalfa (Medicago sativa L.) to assess the impact of competition forlight and water on alfalfa yield and BNF. Alfalfa yield and shoot 15N values (a proxy for the proportionof N derived from the air, %Ndfa) were measured during one year at different distances from the treerow 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: abovegroundbiomass produced per unit of global radiation) of alfalfa, indicating that alfalfa was shade tolerant at thisMediterranean site. Alfalfa shoot 15N values were lower close to the tree rows than at mid inter-row: BNFwas stimulated close to the trees. Compensative and facilitative mechanisms between trees and alfalfaplants led to a rise in LUE and%Ndfa in shaded areas. These results contradict the frequent assumptionthat N fixation is reduced in the shade of trees. Appropriate tree canopy management may help maintainlight competition between trees and alfalfa to a level that still enhance complementary, which wouldfurther improve the sustainability of the use of alfalfa as an intercrop in Mediterranean regions.