2.4. Calculation of N transfer
The fraction of N in the grass derived from G. sepium(%Ntransf)was calculated
(Sierraet al., 2007):%Ntransf¼d15Nso d15Nogd15Nso d15Nex100 (3)where d15N is the per mill15N enrichment relative to atmosphericN (i.e. 0.3663%);d15Nsois the reference value corresponding to thegrass growing without contact with the leguminous tree, andrepresents N coming from soil mineral N; d15Nexis the referencevalue corresponding to the tree N source, and represents N comingfrom tree exudates; d
15Nogcorresponds to the grass in the opengrassland which takes N from both N sources (i.e. soil mineral Nand exudates). The value of d15Nsowas set equal to that estimatedbySierra and Nygren (2006) in a study carried out in the same plot
used in the present work ( d15Nso= 3.47). The value of d15Nexwas setequal to that measured by Sierra et al. (2007) in a greenhouseexperiment carried out with G. sepium trees originating from thesame agroforestry plot used in this study ( d15Nex= 0.42).The amount of grass N derived from the tree (in mg N m2) wascalculated by multiplying the total amount of grass N by %Ntransf
.The amount of grass N derived from the soil was calculated as the
difference between the total amount of grass N and the amount of
grass N derived from the tree.