Tree species effects on soil microbial activities in a young stand

Tree species effects on soil characteristics and biogeochemistry are mediated by several factors including microclimatic conditions, ground vegetation cover, quality and quantity of litter and roots exudates, interception of atmospheric particules and aerosols, as well as secondary metabolites from litter. Moreover, the effects depend on the activities and characteristics of the microbial populations. Due to complex interactions, tree species effects on biogeochemical cycles may vary according to soil type, site characterisitcs and history, and climate. However, these questions about tree species effects on biogeochemical cycles are of central interest to forested ecosystems functions, such as soil quality (restoration) and soil water protection.
This work is part of a long-term study on concentrations and fluxes in main compartments of 2 forested watersheds (Waroneu and Robinette, east Belgium) in relation with forest management. After 2 spruce generations, the Robinette catchment was partially clear-cut in 1996. Since 1998, this watershed is experiencing an ‘extensive’ afforestartion with a mixture of main and secondary tree species, adapted to specific site conditions. This site, provides the opportunity to study tree species effects on the same soil, with the same history. Morever, tree species have different ecological characteristics: N2 fixing species (Alnus glutinosa), secondary broadleaved species (Betula pendula, Sorbus aucuparia, Salix aurita), main broadleaved sepcies (Quercus robur and Fagus sylvatica) and spruce (Picea abies).
Here, we analysed the effects of tree species on soil pH and soil microbial activities in the organic layer in relation to carbon and nitrogen cycles: microbial biomass, basal respiration, labile carbon, nitrogen net mineralisation and potential nitrification.
Twelve years after plantation, our results showed differences below the different tree species: (1) a higher microbial biomass and a higher substrate use efficiency and organic matter accessbility for microbial populations below spruce as compared with other tree species; (2) higher pH and microbial biomass below secondary than below main broadleaved species; (3) an enhanced nitrification below alder; (4) a higher leaching of nitrate below broadleaved species than below spruce. These results show a short term impact of forest tree species on microbial activities in upper soil layers. Results are discussed in relation to ecological characteristics of tree species.