Decomposition and Nutrient Release in Needle Litter from Nitrogen-fertilized Scots Pine (Pinus sylvestris) Stands

Decomposition of Scots pine needle litter originating from five stands treated with different amounts of nitrogen fertilizer was measured over a 4-year period in a mature Scots pine forest. The litter types, which differed in initial concentrations of nitrogen, phosphorus, potassium, and sulfur, but not in gross organic composition, were studied with respect to mass loss, ingrowth of total fungal mycelium, and net release of nutrients. During the first year of decomposition, rates of mass loss and ingrowth of fungal mycelium were highest in the nutrient-rich litter. Phosphorus concentration was found to be the main factor affecting mass-loss rate, and the rate of fungal ingrowth was positively correlated with initial nitrogen concentration. After this initial period, decomposition rates decreased, and after 4 years, accumulated mass loss and amounts of fungal mycelium were similar in all five litter types. These findings may be due, in part, to a lower rate of lignin decomposition in nitrogen-enriched litter. Of the elements, potassium and magnesium were most rapidly lost from the litter, and their release was most pronounced during the first year. Calcium release was proportional to the loss in organic matter. Initially, the release of nitrogen and phosphorus was positively related to their concentrations in litter, however, during later stages of decomposition the differences among litter types levelled out. There was a tendency for concentrations of all elements, except nitrogen, in the different litters to approach similar levels as decomposition proceeded. Thus, after 4 years the nutrient composition of the various litter types was very similar, except for higher nitrogen concentrations in the originally most nutrient-rich litters. The importance of the results in terms of substrate quality in fertilized as well as in non-fertilized forests is discussed.