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

Abstract 

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‰)<pastures (+1.4‰)<croplands (+1.6‰)<meadows (+2.5‰)<gardens (+3.8‰). This increase in soil δ¹⁵N with the intensity of former land use was related to the former input of ¹⁵N-enriched manure, and to an activation of soil nitrification leading to ¹⁵N-depleted nitrate export on previously manured parcels. Fern δ¹⁵N increased in the same order as soil δ¹⁵N in relation to past land use. The mean δ¹⁵N of fern in ancient forests (–4.4‰) and former pastures (–3.4‰) was 5‰ lower than soil δ¹⁵N and the two variables were strongly correlated. The δ¹⁵N of fern in formerly manured parcels varied little (cropland: –2.7‰, meadows: –2.6‰ and gardens: –2.2‰) and independently of soil δ¹⁵N, suggesting that the soil sources of fern N differed between unmanured and manured parcels. Understorey plant δ¹⁵N and soil δ¹⁵N 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.