Soil organic matter degradability as a function of forest species, a study based on Py-GC–MS analysis

• High SOM molecular diversity is observed under silver fir, spruce, beech, Douglas fir. • SOM degradability is higher under silver fir than under the other species. • Molecular markers of deciduous vegetation are preserved in coniferous forest soils. A significant proportion of the global organic carbon reservoir is stored in forest soils, which cover 30 % of continental surfaces. This carbon stock could be affected by ongoing climate change, but also by changes in land use such as forest management. In particular, changes in forest species will alter the quantity and molecular quality of organic matter added to the soil. However, how this will affect the degradability of soil organic matter and, consequently, the potential mineralisation of carbon in forest soils, is poorly understood. In this study, the overall degradability potential of soil organic matter (top 20 cm) was estimated for four forest types: mixed beech, i.e., the potential natural vegetation, and three coniferous species planted at least 50 years ago (spruce, Douglas fir and silver fir). Molecular results obtained using Py-GC–MS show a higher overall degradability potential of organic matter in soils under silver fir compared with soils under Douglas fir, with higher polysaccharide content and lower aliphatic compound content. This is consistent with higher water-extractable soil organic carbon concentration in silver fir samples. Mixed beech stands exhibit intermediate degradability. Pyrolysis-GC–MS revealed a good correlation between degradability potential, based on relative proportions of molecular families, and the level of molecular diversity estimated by the Shannon index. These results suggest that promoting molecular diversity would lead to lower degradability and thus reduced carbon losses through mineralisation.