Soil processes related to organic matter modifications following Douglas-fir mature reforestation

The aim of the study was to analyse the pedological, chemical and biochemical properties of soil in order to assess the effect of plant cover species (chestnut, Douglas-fir and mixed vegetation) on soil processes. The selected area under Douglas-fir reforestation was homogeneous for climate, morphology and parent material. The study involved three soils: under chestnut forest (CS), Douglas-fir reforestation (DF) and the transition zone (T). A soil profile differentiation occurred after 50 years of Douglas-fir plantation. The thickness of soil horizons at the surface varied, and the OH horizon in T site was fourfold thicker than that in DF site. The A1 mineral horizon in T site was also thicker than that in DF site. The Munsell value of mineral horizons was significantly lower in DF than CS site. Morphological differences (thickness and colour) probably reflect soil organic matter quality changes: in DF higher values of humification index were found in surface layers (OF/OH or OH and A1) and lower in deep organo-mineral and transition horizons (A2 and BA or AB) with respect to CS site. Also, a slight increase of weathering process was observed under Douglas-fir reforestation. The morphological, chemical and biological properties of soil were effective to assess the impact of various vegetation types on soil organic matter properties related to pedogenetic process. The various methodological approaches allow studying soil processes using a small–medium spatial scale sampling scheme.