δN 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 N 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 δN (fraction <50 µm and total soil), C/N, P and N content and fern (Dryopteris carthusiana) δN. The mean δN of soil increased in the order ancient forests (+0.0‰)<pastures (+1.4‰)<croplands (+1.6‰)<meadows (+2.5‰)<gardens (+3.8‰). This increase in soil δN with the intensity of former land use was related to the former input of N-enriched manure, and to an activation of soil nitrification leading to N-depleted nitrate export on previously manured parcels. Fern δN increased in the same order as soil δN in relation to past land use. The mean δN of fern in ancient forests (-4.4‰) and former pastures (-3.4‰) was 5‰ lower than soil δN and the two variables were strongly correlated. The δN of fern in formerly manured parcels varied little (cropland: -2.7‰, meadows: -2.6‰ and gardens: -2.2‰) and independently of soil δN, suggesting that the soil sources of fern N differed between unmanured and manured parcels. Understorey plant δN and soil δN appear to be excellent tracers of previous land use in forests, and could be used in historical studies. The persistence of high isotopic ratios in previously manured parcels, almost a century after afforestation, suggests a long-term influence of former land use on the N cycle in forest soils.