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Microbial substrate use efficiency is an important property in process-based soil organic matter models, but is often assumed to be constant in mechanistic models. However, previous studies question if a constant efficiency is appropriate, in particular when evaluating carbon (C) cycling across temperatures and various substrates. In the present study, we evaluated the relation between substrate use efficiency, microbial community composition and substrate complexity in contrasting long-term management regimes (47-49 years of either arable, ley farming, grassland, or forest systems). Microbial community composition was assessed by phospholipid fatty acid analysis and three indices of substrate use efficiencies were considered: (i) thermodynamic efficiency, (ii) calorespirometric ratio, and (iii) metabolic quotient. Three substrates, d-glucose, l-alanine, or glycogen, varying in complexity, were added separately to soils, and heat production as well as C mineralization was determined over a 32-h incubation period at 12.5 °C. Microbial communities from forest systems were most efficient in utilizing substrates, supporting our hypothesis that maturing ecosystems become more efficient. These changes in efficiency were linked to microbial community composition with fungi and Gram-negative bacteria being important biomarkers. Despite our initial hypothesis, complex substrate such as glycogen was utilized most efficiently. Our findings emphasize that differences in land use management systems as well as the composition of soil organic matter need to be considered when modelling C dynamics in soils. Further research is required to establish and evaluate appropriate proxies for substrate use efficiencies in various ecosystems. [ABSTRACT FROM AUTHOR]