Succession of litter-decomposing microbial organisms in deciduous birch and oak forests, northern Japan

Biological litter decomposition and the litter-associated microbial organisms were monitored for three years to characterize litter decomposition in early and late successional stages. Two forests were used for the investigation: pioneer a forest dominated by birch (Betula platyphylla var. japonica) and a climax forest by oak (Quercus mongolica var. grosseserrata) in the cool-temperate region of northern Japan. Three types of litter were used: birch, oak and mixed litter. The litter decomposition was effective during the first year but 50% of the original litter remained even after three years. Carbon-to-nitrogen ratios in the litter decreased largely in the first year and became stable thereafter. The litter decomposition rates were not different among the litter types and between the forests. The temporal changes in phospholipid fatty acids (PLFAs) showed that fungal biomass reached its peak in the first year and the bacterial biomass increased steadily until the end of the experiment. The concentrations of fungal PLFAs in the litter did not differ between the litter types but were lower in the oak forest. The litter decomposition was performed mostly by fungi, in particular in the early stages, while bacterial decomposition depended on the litter types and/or the forest types. Gram-negative bacteria reached their peak of PLFAs in the second year while gram-positive bacteria PLFAs increased gradually during the three years. Therefore, the succession of microorganisms in the litter occurred from fungi to bacteria and from gram-negative bacteria to gram-positive bacteria in the two forests. Unlike in the case of coniferous or monotonic forests, the effects of forests and litter types on litter decomposition for the first year were weak. The forest types on litter decomposition appeared only for the long-term litter decomposition. The successional changes of microorganisms occurred from fungi to bacteria for long-term litter decomposition processes with increasing N concentration in the litter