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 decomposition. 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. (C) 1998 Elsevier Science B.V.