Soil chemical fertility change over four decades in the Morvan Mountains and influence of tree species (Burgundy, France)

Background:Intensive silvicultural practices and the planting of monospecific forests of coniferous,that are more productive compared to hardwoods may threaten over the mid to long-term the sustainability of soil chemical fertility of forest ecosystems,and are a major concern for forest managers and policy.Methods:We investigated the tree species effect(Quercus sessiliflora Smith,Fagus sylvatica L.,Picea abies Karst.,Pseudotsuga menziesii Mirb.Franco.,Abies nordmanniana Spach.and Pinus nigra Arn.ssp.laricio Poiret var.corsi-cana)on the change over time of soil chemical properties and nutrient pool sizes in the mineral and organic layers of the soil during the 45 years after the plantation of the Breuil-Chenue common garden experiment(Burgundy,France).The organic and mineral soil layers down to 70-cm depth were sampled in the different monospecific plots in 1974,2001 and 2019.Results:Exchangeable Ca and Mg pools and soil pH increased on average over time in the 0-70 cm soil profile in most stands.However,in the topsoil layers(0-15 cm),the decrease of pH,the increase of exchangeable acidity over time under the coniferous stands and the decrease of exchangeable K pools in most stands highlighted that soil acidification is still on-going at Breuil-Chenue site but the intensity of this process depends on the tree species.Indeed,three groups of species could be distinguished:i)Nordmann fir(Abies nordmanniana Spach.)/Norway spruce(Picea abies Karst.)where acidolysis and chelation occurred,resulting in the most pronounced pH decrease in the topsoil,ii)Douglas fir(Pseudotsuga menziesii Mirb.Franco.)/Laricio pine(Pinus nigra Arn.ssp.laricio Poiret var.corsicana)where acidification caused by elevated nitrification rates is probably currently compensated by larger weathering and/or atmospheric depositions fluxes,and iii)oak(Quercus sessiliflora Smith)/beech(Fagus sylvatica L.)where soil acidification was less intense.Counterintuitively,soil acidification at Breuil-Chenue site resulted in an increase in soil CEC which limited the loss of nutrient cations.This change in soil CEC was most likely explained by the precipitation/dissolution dynamics of aluminium(Al)(hydr)oxides in the interfoliar space of phyllosilicates and/or the increase in soil carbon(C)content in the topsoil layers.Conclusion:After 45 years,tree species continue to exert influence on the chemical fertility of the soil and the pedogenetic processes which in turn may impact forest ecosystem functions and services.