Element fluxes, forest floor characteristics and microbial activities under deciduous tree species after conversion of a Norway spruce (Picea abies (L.) Karst.) stand

Forest management is currently confronted with major questions, such as how to adapt plantation forests to a changing world. This questioning is not only essential with regard to forest health and productivity, but also within the frame of climate mitigation. As Norway spruce monocultures (Picea abies) have been planted in Europe beyond their assumed natural range, are subjected to forest decline and have negative impacts on ecological conditions, conversion into mixed stands has been suggested. Tree species can influence nutrient inputs, soil microbial activity, soil chemistry and nutrient cycling in forest ecosystems. These tree species effects on biogeochemical cycles may vary according to soil type, site characteristics and land use history. The objective of the present study was to quantify element fluxes in throughfall and seepage water, forest floor exchangeable element pools and nitrogen transformations, 12 years after conversion from Picea abies monocultures to a mixed forest stand. Measurements were performed under young and mature Picea abies, Alnus glutinosa (L.) Gaertn., Quercus robur L. and Sorbus aucuparia L. Thus trees have grown on the same site, sharing identical initial soil conditions and site history, so that potential effects on microbial processes and soil properties can be imputed to tree species. Results showed that conversion had a short term impact on nutrient budgets and nutrient cycling in the upper soil layer; in particular on input fluxes of acidifying cations, soil base saturation, net N mineralization and nitrification.