The source of microbial C is thought to impact its stability in soil due to variations in cellular biochemistry. It has been hypothesised that a fungal-dominated community stabilises more C than a bacterial-dominated community, in part due to chemical recalcitrance of their non-living biomass, particularly cell wall components and pigments. We compared the turnover of 13C-labelled (99.9 atom %) temperate and tropical microbial isolates [i.e. fungi, Gram-positive bacteria (including actinobacteria) and Gram-negative bacteria] in temperate (California) and tropical (Puerto Rico) forest soils. While significant differences in 13C recovery and mean residence times occurred among some microbial additions, similar turnover rates were observed, and in general, results do not support the view that microbial biochemistry affects soil C maintenance. Different effects by microbial necromass additions in California and Puerto Rico suggest that ecosystem-specific effects may be as important to microbial C stabilisation as its macromolecular composition and recalcitrance.