Impact of different tree species on the soil microbial community under temperate-oceanic climate

Forests all over the world are endangered by different factors such as fragmentation, landscape change, deforestation, pollution or inadequate management, with the detrimental effects that this cause on biodiversity or even climate change, since these ecosystems are crucial carbon sinks, both above and below-ground, even though the soil section is quite often excluded from the C pools estimations. The soil microbial community plays a key role in the stabilization of organic matter in the forest soil systems and harbours numerous ecosystems services, but can be affected, among other factors, by the different tree species present in the forest canopy. This study is focused on forests located in Galicia (on the north-west of Spain) with a temperate climate, specifically Cbf climate (Köppen classification), with no dry season and mild summers. This climate implies that the forests in this region are very productive, and this has a clear impact in the tree species that grow on then naturally and the species planted with commercial purposes. We analyse soil properties and soil microbial activity in 54 forest plots, both natural and plantations, with different plant cover: birch, chestnut, eucalyptus, walnut, pines, oak and shrublands. Soil samples were collected at 0-20 cm depth in spring-summer 2024 and the soil microbial activity was estimated by the respiration rate (CO₂ production) using a gas chromatograph (FID-ECD-TCD).

These forest soils have in general an acid pH (between 4.2 and 5.4) and a high content of organic matter (between 6.3 and 31 %), with C and N concentrations ranging 3.6-17.9% and 0.2-0.9% respectively, but a small amount of phosphorous (between 3.5 and 28.3 mg P kg^-1). These properties are mainly related with the climatic conditions of this region, namely the elevated precipitation. Soils under pines had the most acidic conditions and the lower amount of N, meanwhile the soils under walnuts presented the highest amount of C and organic matter and pH values. The soil under a eucalyptus plantation was the driest (7 % of H₂O) vs the moistest with a 36% of H₂O under a chestnut plantation. The results regarding the microbial activity showed that soil under broadleaf species (birch, chestnut, walnut and oaks) have bigger respiration rates than soil under pines and specially under eucalyptus. These preliminary results show that the forest management in terms of which tree is cultivated have an impact on the soil general properties and the soil microbial activity and should be considered when elaborating forestry exploitation plans, especially in the current scenario of climate change where the C that healthy forest soils will be able to fix becomes crucial.