Soil C and N isotope composition after a centennial Scots pine afforestation in podzols of native European beech forests in NE-Spain

The replacement of native European beech forests (Fagus sylvatica) with Scots pine (Pinus sylvestris) afforestation may exert changes in soil properties, particularly with respect to soil organic matter (SOM). Stable isotope composition of light elements (δ13C, δ15N) in soils are known proxies for the characterisation of SOM genesis and dynamics. In this research, C and N isotope composition of organic layers, classified as OL (fresh litter), OF (fragmented litter) and OH (humified litter), and the first mineral horizon (Ah) from what was, originally, a beech domain and from a domain of afforestation with pine were analysed by using EA-IRMS. Additionally, C and N isotope signatures were studied in complete soil profiles that were representative of each forest. Pine OL was found to be 13C enriched (δ13C = −28.08 ± 0.49‰) compared with beech (−29.87 ± 0.27‰). Along the soil profile, C isotope composition mirrors that of the standing vegetation down to the first mineral Ah horizon, with significantly higher δ13C in pine than in beech. Deeper in the soil, from the eluvial E horizon, no significant δ13C differences were found between soils, indicating a limited pine influence in depth, years after afforestation. Pine litter tended to be 15N enriched (δ15N = 4.43 ± 2.65‰) compared to beech (1.43 ± 2.80‰). Along the soil profile, a consistent 15N enrichment was observed with depth in the organic layers (O-layers) down to OH. No significant δ15N differences were found in the mineral horizons between soils, except for the E horizon that showed a lower δ15N in the beech than in the pine profile. This N trend could be explained by 1) a progressive biomass alteration and a concomitant 15N-enrichment being, in general, more pronounced in O-layers under alien pine than under beech, and 2) migration of more humified SOM forms from eluvial to deeper Bhs horizons, causing a relative accumulation of 15N-depleted SOM in the beechwood E horizon. The accumulation of fungal and root biomass in pinewood OF horizons could be reflected in its 15N-depleted signature.