LONG-TERM DECOMPOSITION OF SUCCESSIVE ORGANIC STRATA IN A NITROGEN SATURATED SCOTS PINE FOREST SOIL

The objective of this research was to investigate possible destabilisi ng effects of high deposition of inorganic nitrogen on long-term decom position of organic matter in forest soil. Degradation of successive o rganic layers, representing different stages in the process of decompo sition, was studied in a nitrogen-saturated, Pinus sylvestris forest s oil with a long history of high atmospheric-N deposition. Stratified l itterbag sets, filled with litter, fragmented litter, or humus, were u sed 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 peri od. Time series with contrasting incubation-recovery sequences of litt erbag sets, where equal decay stages in each sequence experienced diff erent environmental conditions, were used to untangle the effects of t ime initiation from length of incubation. A series of two monthly rene wed litterbags, filled with freshly fallen needles, was used as a meas ure 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 independen t of differences in introduction and recovery sequences of the litterb ags and stress the importance of C and N concentration as the major co ntrol of the rate of litter decomposition. Mass loss, residual N conce ntration, 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 d uring the decomposition of recalcitrant organic compounds. All observa tions support the hypothesis that enhanced atmospheric-N deposition ag gravates C-limitation for microbial degradation which may explain the strong reductions in long-term decomposition. (C) 1998 Elsevier Scienc e B.V.