Nitrification, denitrification and microbial biomass N in soil from two N-fertilized and limed Norway spruce forests

Laboratory measurements of ammonification, nitrification, denitrification and microbial biomass N (Nmic) were made on samples of forest soil humus layer from two Norway spruce (Picea abies L.) experimental sites which had been repeatedly fertilized over the past 30 years with liming (Ca, total 6000 kg ha−1) or with N (N, total approx. 900 kg ha−1) or both together (CaN). N plots showed the greatest NH4+N production. Net formation of mineral N (accumulation of NH4+N + NO2−N + NO3−N), was greatest in N and CaN plots. Nitrification and denitrification were shown in both CaN plots and in one Ca plot. Overall, N fertilization alone decreased the immobilization of N in microbes and increased the net N mineralization, whereas combined liming and N fertilization stimulated both nitrification and denitrification, possibly leading to loss of N.