Enzyme dynamics on decomposing leaf litter of Cistus incanus and Myrtus communis in a Mediterranean ecosystem

The decomposition of Cistus incanus leaf litter, a summer deciduous species, was compared to that of Myrtus communis, an evergreen species, during 15 months of incubation in a Mediterranean low shrubland. The litterbags were placed under randomly selected shrubs of Myrtus and Cistus, respectively. Owing to the different microclimatic conditions under deciduous and evergreen shrubs Cistus litter was also incubated under Myrtus shrubs. Microbial activity was evaluated by measuring litter respiration and enzyme activities (cellulase, xylanase, alpha -amylase, beta -amylase, laccase and peroxidase). During the first 8 months of incubation the decomposition rate of both litters was independent of litter quality and incubation conditions. The average decay constant (k) ranged between 0.29 +/- 0.03 and 0.33 +/- 0.03 yr(-1) Subsequently, it increased only for litters incubated under Myrtus shrubs (k = 0.57 +/- 0.15 and 0.48 +/- 0.16 yr(-1) for Cistus and Myrtus litters, respectively). The dry summer affected the decay rate of litters incubated under Myrtus but not under Cistus. Microbial respiration showed seasonal changes (from 25 to 150 mu mol CO2 g(-1) dry wt. 24 h(-1)), with low levels in summer, mainly because of the low litter water content. After samples were placed in the field, alpha -amylase activity decreased rapidly, dropping to zero in Cistus litter, whereas it remained detectable in Myrtus litter (>0.02 mu mol glucose g(-1) dry wt. h(-1)). The beta -amylase activity was low over the entire period. The activities of cellulase and xylanase ranged from 1 to 30 mu mol glucose equivalents (reducing sugar)g(-1) dry wt. h(-1). Both litters showed the lowest enzyme activities in summer, when litter respiration was also at the lowest level. Peroxidase activity was detected in the litter of Myrtus(from 0 to 50 mu mol o-tolidine oxidised g(-1) dry wt. h(-1)) and had a seasonal pattern similar to cellulase and xylanase. It was undetectable in Cistus. In both litters laccase increased significantly going from 10 to 140 mu mol o-tolidine oxidised g(-1) dry wt. h(-1) between eight and nine months when a large increase of fungal biomass occurred (from 0.5 to 2.5 mg g(-1) dry wt.). The analyses of these enzymes have shown qualitative and quantitative differences depending on the litter type and the microclimatic conditions, suggesting changes in the microbial succession. (C) 2000 Elsevier Science Ltd. All rights reserved.