The influence of omnivorous elaterid larvae on the microbial carbon cycle in different forest soils

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Abstract
Data are presented on the influence of Athous subfuscus larvae (Coleoptera, Elateridae) on the microbial carbon cycle in the biotically most active horizons of three contrasting beech forest soils: the Ah horizon of a mull soil on limestone (Göttinger Wald, FRG), the F/H horizon of a moder soil on new red sandstone (Solling area, FRG) and in the F/H horizon of a lime ameliorated area close to the second site. Gut content analyses demonstrated that the larvae of A. subfuscus are humiphagous and that this unspecific feeding behaviour is widely independent of soil conditions. Differences in 14C incorporation demonstrated that only the larvae in the F/H horizon of the limed moder soil directly affected primary decomposer organisms. However, the burrowing activity of the larvae in the topsoil indirectly modified the time course of beech leaf-litter decomposition in the litter layer of all three soils. The microflora of the mull soil contained 2.6%, that of the moder soil 0.7% and that of the limed moder soil 2.2% of total C. The metabolic quotient (qCO2, 10°C) of the soil microflora was 0.0010 (mgCO2-C·mg-1 biomass-C·h-1) in the mull soil, 0.0034 in the moder soil and 0.0012 in the limed moder soil. The A. subfuscus larvae generally reduced the size of the microbial C pool (<-30%) and increased the metabolic quotient of the microflora (>+50%). Considering these soil-independent effects of A. subfuscus on the C turnover of the soil microflora, the burrowing activity of humiphagous soil arthropods may generally increase nutrient availability to primary producers. The results of this study reveal that some of the micro- and mesoscale effects of humiphagous arthropods on the microbial carbon turnover in beech forest soils are surprisingly similar, even under very different soil conditions. The long-term modification of the time course of leaf litter decomposition, in contrast, indicates that the influence of humiphagous arthropods on the formation of the humus layer is soil-specific. There are profound differences in the role of humiphagous arthropods in limed moder soils and in naturally base-rich soils. It is concluded that liming increases competition within the microfloral population due to accelerated humification. The negative effect of A. subfuscus on 14C mineralization in the limed substrate could thus be explained by its effects on a microflora that was strongly limited by the availability of carbon.