Carbon and nitrogen turnover in the early stage of Scots pine (Pinus sylvestris L.) needle litter decomposition: effects of internal and external nitrogen

The effects of internal and external nitrogen (N) on the turnover of carbon (C) and N in the early stage of decomposition of Scots pine (Pinus sylvestris L.) needle litter were studied in a 142-d laboratory incubation experiment. Different doses of external N (as NH4Cl) were added to litter with different internal N contents. Both internal and external N contributed in covering the N demand under the given circumstances. For needles with an internal N content of 12.9 mg N g−1 dry litter, the mass loss and respiration rate were more than 40 and 30% higher, respectively, than for needles with an internal N content of 5.4 mg N g−1 dry litter. The importance of the initial internal N content for the decomposition rate decreased with time. The decay rate increased by more than 60% when 1.1 mg NH4-N g−1 dry litter (14 d)−1 was applied compared to no addition of external N. No further increase in decomposition rate was observed when the external dose increased to 3.2 mg NH4-N g−1 dry litter (14 d)−1. The positive effect of the external N addition was significantly lower for needles with a high content of internal N. The releases of dissolved organic compounds (DOC and DON) were not significantly affected by internal or external N. In spite of the effective N retention, the leaching of inorganic N generally increased with higher levels of both internal and external N. More than 85% of the N loss from the litter with added external N was in the form of leached NH4+. The N2O emission increased with increased N status, whereas no clear relationship was found between N2O emission and NO3− leaching. The increased N status due to N fertilisation or atmospheric N deposition may result in considerable changes in a forest ecosystem. The overall effect on the organic matter storage and the cycling of important nutrients depends, however, on the influence of N in later decomposition stages and on litter producing related processes.