Limnetica 39

The aquatic microbial decomposition of leaf litter has been the subject of many field studies throughout the world. However, field experiments cannot always separate the effects of the multiple biotic and abiotic factors involved in the process. In this laboratory experiment, we controlled the abiotic factors and the fungal decomposer community during decomposition of alder, oak and eucalypt leaf litter in order to determine if variation in leaf carbon, nitrogen and phosphorus (CNP) ratios during decomposition was similar among the three species. Initial CNP values differed among the three species with alder being the richest (C:N=16, C:P=903, N:P=57) and oak being the poorest (C:N=55, C:P=1779, N:P=32) species. In all leaf species, nitrogen was immobilized during decomposition (final < initial C:N ratios), while phosphorus was released (final > initial C:P ratios). Final CNP values were lowest in alder (C:N=11, C:P=2495, N:P=224) but there was a change in the ranking of oak and eucalypt regarding nutrient contents. Leaf species were similar regarding the variation in C:N (final/initial=0.7 to 0.8) but C:P and N:P increased more in eucalypt and oak than in alder (final/initial C:P=5.9, 3.9 and 2.8, final/initial N:P=7.5, 4.7, 3.9, respectively for eucalypt, oak and alder). The lowest decrease in P of alder leaves may explain the highest mass loss observed in this species, most probably due to a higher fungal colonization despite the controlled fungal decomposer community. In conclusion, CNP ratio in leaves seems to determine the fungal-mediated mass loss of leaf litter.