The effect of three collembolan species on decomposition and N mobilization was studied in a Pinus nigra plantation, using field enclosures and by trenching tree roots. The fauna were added to defaunated organic material. Estimates were made of weight and N losses from fresh litter, fragmented litter and humus after 7 months exposure. N mobilization was also measured as leachable mineral N using suction cup lysimeters, and was found to be dominated by NO3(-)-N. In the presence of the surface dwelling Orchesella cincta (L.) a net loss of 310 mmol m-2 of total N in humus was observed, as opposed to a small gain of 76 mmol N m-2 in the control. Tomocerus minor (Lubbock) induced a nearly two-fold increase in the mobilization of NO3(-)-N to the leachate compared to Isotoma notabilis Schaffer, possibly as a consequence of differences in grazing saprotrophic fungi. The influence of I. notabilis on the experimental systems differed from the effects of the other Collembola in effecting an increased mass loss of humus material. Tree roots suppressed fragmented litter decomposition significantly by 2.0%, and had considerable effect on N dynamics. In the presence of roots, leachable quantities of NO3(-)-N were reduced to half the amounts that were collected in their absence. The amount of total-N of fragmented litter was nearly unchanged in non-rooted plots, but was reduced at a rate of 274 mmol m-2 after 7 months. It was concluded that C and N mineralization in soils is influenced by the ecological characteristics of the dominant species and is not simply a function of trophic group or biomass.
