Dynamics and stratification of bacteria and fungi in the organic layers of a Scots pine forest soil

The abundance and micro-stratification of bacteria and fungi inhabiting the organic layers of a Scots pine forest (Pinus sylvestris L.) were investigated. An experiment using stratified litterbags, containing organic material of four degradation stages (fresh litter, litter, fragmented litter and humus) was performed over a period of 2.5 years. Dynamics and stratification of fluorescent stained bacteria and fungi, ratios between bacterial and fungal biomass, and relationships with moisture and temperature are described. Average bacterial counts in litter and fragmented litter were similar, i.e., approximately 5 x 10(9) bacteria g-1 (dry weight) organic matter, and significantly exceeded those in humus. The mean bacterial biomass ranged from 0.338 to 0.252 mg carbon (C) g-1 (dry weight) organic matter. Lengths of mycelia were significantly below the usually recorded amounts for comparable temperate coniferous forests. The highest average hyphal length, 53 m g-1 (dry weight) organic matter, was recorded in litter and decreased significantly with depth. The corresponding mean fungal biomass ranged from 0.050 to 0.009 mg C g-1 (dry weight). The abundance of bacteria and fungi was influenced by water content, that of fungi also by temperature. A litterbag series with freshly fallen litter of standard quality, renewed bimonthly, revealed a clear seasonal pattern with microbial biomass peaks in winter. The mean hyphal length was 104 m g-1 (dry weight) and mean number of bacteria, 2.40 x 10(9) bacteria g-1 (dry weight). Comparable bacterial and fungal biomass C were found in the freshly fallen litter [0.154 and 0.132 mg C g-1 (dry weight) organic material, respectively]. The ratio of bacterial-to-fungal biomass C increased from 1.2 in fresh litter to 28.0 in humus. The results indicate the existence of an environmental stress factor affecting the abundance of fungi in the second phase of decomposition. High atmospheric nitrogen deposition is discussed as a prime factor to explain low fungal biomass and the relatively short lengths of fungal hyphae in some of the forest soil layers under study.