SUCCESSIONAL CHANGES IN MICROBIAL BIOMASS, ACTIVITY AND NUTRIENT STATUS IN FECAL MATERIAL OF THE SLUG ARION RUFUS (GASTROPODA) DEPOSITED AFTER FEEDING ON DIFFERENT PLANT MATERIALS

Microbial biomass and activity in casts of the slug Arion rufus L. of different ages (0 to 160 days) deposited after feeding on beech leaf l itter (Fagus sylvatica L.) and fresh leaves of Allium ursinum L. and M ercurialis perennis L, were investigated. The following parameters wer e determined: basal respiration (O-2-consumption), microbial biomass ( SIR, substrate-induced respiration method), microbial growth ability a fter C (glucose), CN, CP and CNP amendment, specific respiration (qO(2 )), lag time (time between CNP addition and start of exponential incre ase in respiration rates) and decomposition time (time required to ach ieve the maximum respiration rate following CNP addition). Basal respi ration and microbial biomass initially increased but then decreased in ageing cast materials. The increase in basal respiration usually pred ated that in microbial biomass. Basal respiration in cast materials of beech leaf litter, A. ursinum leaves and M. perennis leaves was at a maximum at day 5 (521 mu l O-2 g(-1) dry Wt h(-1)), day 1 (1420 mu l O -2 g(-1) h(-1)) and days 1, 2 and 5 (average 1270 mu l O-2 mg(-1) dry wt h(-1)), respectively. Microbial biomass (C-mic) for the respective materials was at a maximum at day 5 (29.9 mg C-mic g(-1) dry wt), day 2 (64.4 mg C-mic g(-1)) and day 1 (83.1 mg C-mic g(-1)). Specific resp iration of cast materials deposited after feeding on beech leaf litter was exceptionally high at day 1 (43.4 mu l O-2 mg(-1) C-mic h(-1)) an d decreased with age. Specific respiration of cast materials deposited after feeding on A. ursinum leaves also significantly decreased with age but no significant trend occurred in cast materials of M. perennis leaves. In cast materials of beech leaf litter of an age of 1 and 2 d ays C was the primarily limiting element and N and P were available fo r microbial growth. Between days 5 and 20, C-amended microorganisms we re limited by N and CN-amended microorganisms were limited by P. Follo wing day 40 P was mobilized in ageing cast materials of beech leaf lit ter. In cast materials deposited after feeding on A. ursinum and M. pe rennis leaves, microbial growth usually was limited by C. Requirement for P by microorganisms in 4- and 12-h-old C-amended cast materials wa s high. This P requirement decreased in ageing cast materials indicati ng P mobilization. In later successional stages sufficient P supply bu t a strong N demand by microorganisms occurred, which presumably was c aused by immobilization of N in ageing cast materials. Decomposition t ime in cast materials deposited after feeding on beech leaf litter was shorter than in intact leaf litter. Lag time and decomposition rime i ncreased with age of casts produced after feeding on each of the food materials but remained almost constant in ageing beech leaf litter. Th e increase was more pronounced in A. ursinum and M. perennis casts tha n in casts deposited after feeding on beech leaf litter. Considering P iankas r/K-concept the prolongation of the lag and decomposition times in ageing casts and the decline in specific respiration in casts depo sited after feeding on beech leaf litter and leaves of A. ursinum with age indicate successional replacement of r-strategists by K-strategis ts in ageing casts. According to Grime's theory of life strategies, th e short lag and decomposition times in early successional stages in ca sts indicate dominance of ruderal populations. Copyright (C) 1996 Else vier Science Ltd