Successional changes in microbial biomass, activity and nutrient status in faecal material of the slug Arion rufus (gastropoda) deposited after feeding on different plant materials

Abstract

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 litter ( Fagus sylvatica L.) and fresh leaves of Allium ursinum L. and Mercurialis perennis L. were investigated. The following parameters were determined: basal respiration (O 2-consumption), microbial biomass (SIR, substrate-induced respiration method), microbial growth ability after C (glucose), CN, CP and CNP amendment, specific respiration ( qO 2), lag time (time between CNP addition and start of exponential increase in respiration rates) and decomposition time (time required to achieve the maximum respiration rate following CNP addition). Basal respiration and microbial biomass initially increased but then decreased in ageing cast materials. The increase in basal respiration usually predated 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 μl O 2 g −1 dry wt h −1), day 1 (1420 μl O 2 g −1 h −1) and days 1, 2 and 5 (average 1270 μl O 2 g −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 respiration of cast materials deposited after feeding on beech leaf litter was exceptionally high at day 1 (43.4 μl O 2 mg −1 C mic h −1) and 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 days C was the primarily limiting element and N and P were available for microbial growth. Between days 5 and 20, C-amended microorganisms were limited by N and CN-amended microorganisms were limited by P. Following day 40 P was mobilized in ageing cast materials of beech leaf litter. In cast materials deposited after feeding on A. ursinum and M. perennis leaves, microbial growth usually was limited by C. Requirement for P by microorganisms in 4- and 12-h-old C-amended cast materials was high. This P requirement decreased in ageing cast materials indicating P mobilization. In later successional stages sufficient P supply but a strong N demand by microorganisms occurred, which presumably was caused by immobilization of N in ageing cast materials. Decomposition time in cast materials deposited after feeding on beech leaf litter was shorter than in intact leaf litter. Lag time and decomposition time increased with age of casts produced after feeding on each of the food materials but remained almost constant in ageing beech leaf litter. The increase was more pronounced in A. ursinum and M. perennis casts than in casts deposited after feeding on beech leaf litter. Considering Piankas r K -concept the prolongation of the lag and decomposition times in ageing casts and the decline in specific respiration in casts deposited after feeding on beech leaf litter and leaves of A. ursinum with age indicate successional replacement of r-strategists by K-strategists in ageing casts. According to Grime's theory of life strategies, the short lag and decomposition times in early successional stages in casts indicate dominance of ruderal populations.