LITTER DEGRADATION STAGE AS A PRIME FACTOR FOR ISOPOD INTERACTION WITH MINERALIZATION PROCESSES

The involvement of the common macroarthropod, the isopod Porcellio scaber, in decomposition processes was studied, in relation to different degradation stages of leaf litter. To obtain litter at different stages of degradation, poplar leaves (Populus x canadensis) were sampled from the litter layer in October, January, March and May, and stored until the start of the experiment. Initial dissolved organic C and PO42- concentrations decreased with the sampling sequence; total mineral N concentrations increased .Nitrogen concentration was higher in litter sampled in January compared to litter sampled in October, March and May. During experimental incubations C:N ratios decreased in all litter degradation stages; NH4+-N concentrations decreased and NO3--N concentrations increased. Immobilizations of mineral N was found in litter sampled in March and May. After 8 weeks, isopods had a negative effect on microbial respiration except for litter sampled directly after litter fall; isopods had a positive effect on net N mineralization in aged litter. The general trend appears to be that the influence of litter-feeding soil animals is strongest in relatively low quality litter, i.e. in freshly fallen litter with physical and chemical defences against microbial attack, and in decayed litter with a low C:N ratio, in which carbon content and composition may be limiting for microbial growth. In the case of poplar litter this is litter that is probably not preferred by isopods as food. The experiment stresses the importance of litter degradation stage as a prime factor for detritivore-microbial interactions.