Abstract:We studied Pinus elliottii, Casuarina equisetifolia, Eucalyptus urophylla × E. grandis, Acacia cunninghamia and A.aulacocarpa plantations in the coastal sandy land of Fuzhou City, and determined stable carbon and nitrogen isotope abundance values (δ13C, δ15N) in unevenaged leaves (new and oldyear leaves) and topsoils (0-10 cm). we studied the relationship between stable carbon and nitrogen isotope abundance, water use efficiency and soil nitrogen saturation in order to reveal the mechanism of water use efficiency, nitrogen saturation and the difference of carbon and nitrogen cycle rate of different species. The results showed that: (1) the δ13C values in leaves of different tree species ranged from -31.682‰ to -29.323‰ in the sequence as P.elliottii>A.cunninghamia>C.equisetifolia>A.aulacocarpa> E.urophylla × E. grandis. The δ13C values in newyear leaves were higher compared with those in oldyear leaves for most species, except for E.urophylla × E. grandis. The variation range of δ15N in leaves of each tree species was from -5.548‰ to -2.167‰ with the highest value in A.cunninghamia, followed by, A.aulacocarpa, C.equisetifolia, P.elliottii, and E.urophylla × E. grandis. The δ15N values in newyear leaves were higher compared with those in oldyear leaves. (2) The variation range of δ15N in the topsoils of different tree species was from -4.675‰ to -2.975‰ with the highest value in A.aulacocarpa, followed by, A.cunninghamia, C.equisetifolia, E.urophylla × E. grandis, and P.elliottii. However, there was no significant difference in the C content in the topsoils of different tree species. (3) The water use efficiency of P.elliottii, C.equisetifolia, A.cunninghamia and A.aulacocarpa decreased with the aging of leaves. The water use efficiency of newyear leaves of different tree species varied from 39.09 μmol·mol-1 to 76.57 μmol·mol-1, with P.elliottii> A.cunninghamia> C.equisetifolia>A.aulacocarpa>E.urophylla × E. grandis. The water use efficiency of oldyear leaves varied from 38.56 μmol·mol-1 to 62.59 μmol·mol-1, with P.elliottii>C.equisetifolia>A.cunninghamia> E.urophylla × E. grandis>A.aulacocarpa. (4) The water use efficiency of different tree species was significantly positively correlated with that of new leaves, indicating that the water use efficiency of newyear leaves plays a more significant role. The water use efficiency of stand was also affected by stand type.