A 67-day incubation experiment was carried out with a soil initially devoid of any organic matter due to heating, which was amended with sugarcane sucrose (C-4-sucrose with a delta C-13 value of -10.5 parts per thousand), inorganic N and an inoculum for recolonisation and subsequently at day 33 with C-3-cellulose (delta C-13 value of -23.4 parts per thousand). In this soil, all organic matter is in the microbial biomass or in freshly formed residues, which makes it possible to analyse more clearly the role of microbial residues for decomposition of N-poor substrates. The average delta C-13 value over the whole incubation period was -10.7 parts per thousand in soil total C in the treatments without C-3-cellulose addition. In the CO2 evolved, the delta C-13 values decreased from -13.4 parts per thousand to -15.4 parts per thousand during incubation. In the microbial biomass, the delta C-13 values increased from -11. 5 parts per thousand to -10. 1 parts per thousand at days 33 and 38. At day 67, 36\% of the C-4-sucrose was left in the treatment without a second amendment. The addition of C-3-cellulose resulted in a further 7\% decrease, but 4\% of the C-3-cellulose was lost during the second incubation period. Total microbial biomass C declined from 200 mu g g(-1) soil at day 5 to 70 mu g g(-1) soil at day 67. Fungal ergosterol increased to 1.5 mu g g(-1) soil at day 12 and declined more or less linearly to 0.4 mu g g(-1) soil at day 67. Bacterial muramic acid declined from a maximum of 35 mu g g(-1) soil at day 5 to a constant level of around 16 mu g g(-1) soil. Glucosamine showed a peak value at day 12. Galactosamine remained constant throughout the incubation. The fungal C/bacterial C ratio increased more or less linearly from 0.38 at day 5 to 1.1 at day 67 indicating a shift in the microbial community from bacteria to fungi during the incubation. The addition Of C-3-cellulose led to a small increase in C-3-derived microbial biomass C, but to a strong increase in C-4-derived microbial biomass C. At days 45 and 67, the addition of N-free C-3-cellulose significantly decreased the C/N ratio of the microbial residues, suggesting that this fraction did not serve as an N-source, but as an energy source. (c) 2007 Elsevier Ltd. All rights reserved.