Jürgen Bauhus. Microbial biomass carbon (Cmic) contributes, on average, 1.64 % to total organic C in forest soils. The Cmic/Corg ratio in mineral soils is significantly higher in tropical forest soils (2.53%) than in temperate (1.39%) and boreal (1.42%) climates, indicating that organic matter is less recalcitrant in tropical regions. On average, microbial nitrogen (Nmic) contributes 4.2% of total soil N, and 14.3% of total P is found in microbial biomass. The latter shows that immobilisation of nutrients in microbial tissue is particularly important in P cycling. Estimates of microbial C, N and P in forest ecosystems range between 121-4220 kg ha-1, 18-605 kg ha-1, and 32-86 kg ha-1 respectively. Assuming that microbial biomass turnover times lie between 1-3 yrs, the quantities of microbial N and P released annually could satisfy a large proportion of the vegetation uptake demand. In many studies concentrations of microbial C, N, and P declined from the upper to the lower soil horizons, as can be expected for a relationship between microbial biomass and the decomposition stage of organic substrate. However, in several studies, the Cmic/Corg and the Nmic/Nt ratios decreased with profile depth in the forest floor only, and increased subsequently in the mineral soil. This indicates that relative concentrations of microbial biomass are not only dependent on the quality of SOM but also on the extent to which microbes are physically protected in substrates, such as in mineral soil. This needs to be taken into account when comparing different substrates. Many studies have shown that microbial biomass decreases following forest harvesting, and that these changes occurred before measurable changes in soil organic matter quantity were found. The decline of microbial C and N following tree removal ranged between 27% and 64%. When bacterial and fungal biomass were determined separately, it was found that fungal biomass declined more sharply than bacteria. The often rapid decrease in fungal biomass may be explained by a reduction in ectomycorrhizal fungi, which decline sharply once the root system of cut stems can no longer support them. It is therefore recommended that mycorrhizal associations be determined separately if the effect of harvesting on microbial biomass is to be investigated. Several authors have shown that seasonal fluctuations of microbial biomass and its activity are closely related to substrate water content and temperature. Fluctuations of microbial biomass C and N were between 22 and 90% of the maximum. Thus to obtain useful results for time series and comparison between treatments, the timing of sampling is extremely important . Microbial variables appear to be sensitive to changes in N and P supply. However, the short-term and long-term effects that fertilisation has on SOM quality and therefore microbial biomass and its activity are not fully understood. In summary, the substrate and nutrient limitation of microbial biomass, and its central role in the soil nutrient cycling, facilitate the use of microbial biomass as an indicator for changes in the qualilty of soil organic matter. However, microbial biomass is also highly variable in space and time and is influenced by a range of factors unrelated to soil quality. Therefore sampling regimes that can separate impacts of forest management from ecosystem internal and climatic influences need to be developed. |
