Temporal dynamics and variation with forest type of phospholipid fatty acids in litter and soil of temperate forests across regions

Microorganisms form the basis of soil food webs and represent key control points of carbon cycling and sequestration. Virtually all central European forests are managed and land-use regimes likely impact microbial abundance and community composition. Consequently, knowledge on how land-use intensity and abiotic variables, such as pH, C-to-N ratios, moisture regimes and concomitantly different stress levels, affect microbial communities is needed. We investigated phospholipid fatty acid (PLFA) profiles of leaf litter and soil from four forest types differing in foliage, age and management intensity, replicated in three regions across Germany. To account for temporal variation, samples were taken twice in the same season, but with an interval of three years. Total microbial biomass and microbial community composition differed between years, presumably due to between year variations in weather conditions. The litter layer was more prone to effects of drying, with a reduction of almost 30% of total PLFAs in the drier year. In soil effects of weather conditions depended on soil type and therefore differed between regions, with microorganisms in the sandy soils of the Schorfheide being more susceptible to water-stress, as evidenced by a ten-fold increase of the stress indicator cy/pre ratio in the drier year. Despite temporal variations in microbial biomass and community composition, the balance between the fungal and bacterial energy channel, as measured by fungal-to-bacterial ratios, remained rather constant in particular in soil. While total microbial biomass did not differ between forest types, microbial community composition differed significantly between beech and coniferous forests. Despite more acidic conditions, the fungal energy channel was less pronounced in leaf litter of coniferous forests than in broad-leaved forests, whereas the proportion of bacterial fatty acids was the highest in coniferous forests. Increasing management intensity presumably fosters the bacterial energy channel in the exposed litter layer. Supporting this assumption coniferous forests featured significantly higher values of the stress indicators cy/pre and SAT/MONO ratio. Bacterial community structure and biomass closely correlated with pH, with particular PLFAs dominating at high and low pH, respectively, indicating pH-specific microbial communities. In contrast, fungal abundance in leaf litter was correlated with C-to-N ratio. The results suggest that leaf litter and soil need to be considered separately when investigating changes in microbial community composition, since susceptibility of microorganisms to environmental stressors differs markedly between these layers. This, and repeated sampling events, may be particularly important when investigating subtle effects such as those related to climate change.