Formation and use of microbial residues after adding sugarcane sucrose to a heated soil devoid of soil organic matter

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 (C4-sucrose with a δ13C value of −10.5‰), inorganic N and an inoculum for recolonisation and subsequently at day 33 with C3-cellulose (δ13C value of −23.4‰). 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 δ13C value over the whole incubation period was −10.7‰ in soil total C in the treatments without C3-cellulose addition. In the CO2 evolved, the δ13C values decreased from −13.4‰ to −15.4‰ during incubation. In the microbial biomass, the δ13C values increased from −11.5‰ to −10.1‰ at days 33 and 38. At day 67, 36% of the C4-sucrose was left in the treatment without a second amendment. The addition of C3-cellulose resulted in a further 7% decrease, but 4% of the C3-cellulose was lost during the second incubation period. Total microbial biomass C declined from 200 μg g−1 soil at day 5 to 70 μg g−1 soil at day 67. Fungal ergosterol increased to 1.5 μg g−1 soil at day 12 and declined more or less linearly to 0.4 μg g−1 soil at day 67. Bacterial muramic acid declined from a maximum of 35 μg g−1 soil at day 5 to a constant level of around 16 μg 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 C3-cellulose led to a small increase in C3-derived microbial biomass C, but to a strong increase in C4-derived microbial biomass C. At days 45 and 67, the addition of N-free C3-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.