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Mature deciduous forests can serve as important carbon (C) sinks, but the C storage differs significantly in dependency on the tree species. To specify the significance of overstory-specific effects of litter fall on the soil microbial C turnover, we have investigated the C-13 isotopic signature of microbial biomarker phospholipid fatty acids (PLFAs). Samples were taken under pure Fagus sylvatica and mixed overstory (F. sylvatica and Fraxinus excelsior or F. excelsior, Acer spp. and F. sylvatica) in a mature temperate deciduous forest in Central Germany 4 weeks prior to and 3 weeks after litter fall. Accordingly, the CO2 emission from soil was measured before, during and after the litter fall to investigate the response of decomposition. At all sites and at both sampling dates the fungal biomarker PLFA 18:2 omega 6,9 had predominantly lower delta C-13 values (from -32 to -43 parts per thousand) than the bacterial biomarker PLFAs (delta C-13 values from -23 to -39 parts per thousand). This difference indicated that fungi generally used preferentially plant derived C, whereas the bacterial populations include groups which used SOM derived C, independent on the overstory, trees. Under pure F. sylvatica overstory the delta C-13 values of microbial biomarker PLFAs were slightly decreased (up to 2 parts per thousand for 17:0br) or unchanged after litter fall. By contrast, under both variants of mixed overstory the delta C-13 values of biomarker PLFAs of fungi (18:2w6,9) were increased after litter fall (+3.5 and +3.8 parts per thousand). This might be explained partly by a faster initial decomposition of foliar litter from mixed overstory already during litter fall as confirmed by higher CO2 emission under mixed F. excelsior, Acer spp. and F. sylvatica than under pure F sylvatica in this period. However, the involved microbial populations differed overstory-specific. Bacterial biomarker PLFAs with strongest overstory-specific differences in the response on litter fall were 17:0br (Gram-positive bacteria), 18:1 and 19:0cy (Gram-negative bacteria). The present results indicate that a tree species conversion even exclusively between deciduous tree species might alter the soil microbial C turnover during litter decomposition and suggest that it would in the long-term change the SOM stability and C storage. (C) 2009 Elsevier B.V. All rights reserved. [References: 54]
