Is site productivity permanently changed by repeated fertilisation

Nitrogen (N) fertilisation increases tree production in boreal forests, but it is poorly known how long the fertiliser effect will remain after the end of the fertilisation. We studied a Scots pine stand during 17 years of annual fertilisation with N and other nutrients followed by a period of 19 years of no fertilisation. Fertilisation increased needle N concentrations, but once fertilisation was stopped the concentrations dropped to the level of the unfertilised trees within five years. Leaf area index was 10% higher in fertilised than in unfertilised plots 19 years after the end of fertilisation. Basal area and stem volume in fertilised plots were twice those in unfertilised plots at the end of the fertilisation period. The growth in the fertilised plots remained higher, even when the effects of tree size were accounted for. Fourteen years after the end of fertilisation, fertilised plots had higher soil carbon (C) pools, but similar annual C mineralisation as unfertilised plots. Fertilised plots had higher soil N pools and higher field net N mineralisation. The fertilisation resulted in a long-term increase in the forest production and probably caused a shift in N uptake from organic towards inorganic forms.