δ15N of forest soil and understorey vegetation reflect the former agricultural land use.

Since the middle of the 19th century, the area covered by forests in France has doubled. These new forests grow on previous agricultural lands. We have studied the influence of this agricultural history on the 15N abundance of present-day forests planted on farmlands in the Vosges mountains (north-eastern France) between 1898 and 1930. Different types of land use were identified from old cadastres (1814-1836) of 16 farms. Ancient forests adjacent to farmlands were used as controls. Former pastures, meadows, croplands, gardens and ancient forests were compared for soil δ15N (fraction <50 µm and total soil), C/N, P and N content and fern (Dryopteris carthusiana) δ15N. The mean δ15N of soil increased in the order ancient forests (+0.0‰)