Long-term decomposition of successive organic strata in a nitrogen saturated Scots pine forest soil.

The objective of this research was to investigate possible destabilising effects of high deposition of inorganic nitrogen on long-term decomposition of organic matter in forest soil. Degradation of successive organic layers, representing different stages in the process of decomposition, was studied in a nitrogen-saturated. Pinus sylvestris forest soil with a long history of high atmospheric-N deposition. Stratified litterbag sets, filled with litter, fragmented litter, or humus, were used to obtain information on decay rates, in addition to changes in C and N concentration of the residues, during a two-and-a-half-year period. Time series with contrasting incubation recovery sequences of litterbag sets, where equal decay stages in each sequence experienced different environmental conditions, were used to untangle the effects of time initiation from length of incubation. A series of two monthly renewed litterbags, filled with freshly fallen needles, was used as a measure of the potential influence of season on decomposition rate. Decay rate of litter was tightly linked to changes in C and N concentration and the C/N ratio of the residues. These relationships were independent of differences in introduction and recovery sequences of the litterbags and stress the importance of C and N concentration as the major control of the rate of litter decompression. Mass loss, residual N concentration, and C/N ratio approach rapidly to asymptotic values, almost within the first year of incubation; this may be seen in the light of C or nutrient requirements for the growth of the microbial community during the decomposition of recalcitrant organic compounds. All observations support the hypothesis that enhanced atmospheric-N deposition aggravates C limitation for microbial degradation which may explain the strong reductions in long-term decomposition.