Does short-term heating of forest humus change its properties as a substrate for microbes?

Prescribed burning is known to reduce the size of the microbial biomass in soil, which is not explained by preceding clear-cutting or the effects of ash deposition. Instead, burning induces an instant heat shock in the soil, which may either directly kill soil microbes or indirectly alter the soil organic matter. We heated dry forest humus at temperatures from 45 to 230°C, inoculated them to ensure equal opportunities for microbial proliferation and incubated the heated humus samples at 14°C. After 1, 2, 4 and 6 months we studied the microbial community structure of the samples by determining the phospholipid fatty acid pattern (PLFA), microbial substrate utilization pattern using Biolog Ecoplates and total microbial biomass (Cmic) by substrate-induced respiration (SIR). The chemical structure of humus was scanned by Fourier-transform infrared (FTIR) and 13C NMR spectroscopy. Heating at 230°C caused changes in the chemical structure of the humus as indicated by FTIR spectroscopy, increased the pH of the humus by 1.1 units, reduced Cmic by 70% compared with the control and caused changes in substrate utilization patterns and proportions of PLFAs. More interestingly, the heat treatments from 45 to 160°C, which did not increase humus pH, resulted in differences in both microbial community structure and substrate utilization patterns. The severely heated samples (120–160°C) were relatively richer in 16:1ω7t, cy19:0 and 18:1ω7, while the mildly heated samples (45–100°C) showed higher proportions of 16:1ω5, 16:1ω9, 10me16:0 and a15:0. The t/c ratio calculated from trans and cis configurations of 16:1ω7 increased from 1 to 6 months in the severely heated humus, possibly indicating nutrient deprivation. The control showed a decreasing t/c ratio and a stable amount of Cmic indicating sufficient amount of decomposable organic matter. After incubation for 1 month, similar amounts of Cmic had reestablished in 160°C-treated and control samples. However, the Cmic in 160°C-treated samples decreased over 5 months. This might have been caused by a heat-induced flush of easily decomposable carbon, which was later exhausted. We conclude that changes in chemical properties of humus during dry heating at 230°C were capable of causing changes in microbial community structure of the humus.