PLFA profiles of microbial communities in decomposing conifer litters subject to moisture stress

The influence of moisture stress on microbial communities in decomposing coniferous litters was investigated using phospholipid fatty acid (PLFA) profiling. Studies were carried out in German and Greek forest plots under contrasting climatic conditions from the late summer to the early winter periods. Litterbags containing spruce (Germany) or pine (Greece) needles were subjected to different irrigation treatments over 4 months. The influences of climate and litter type on microbial community structure were larger than those imposed by irrigation or moisture stress treatments. In the German spruce litter, the PLFA signatures indicated that there was initially a larger bacterial than fungal biomass and both components decreased with time. Concentrations of individual PLFA, proportions of PLFA subgroups and principal component (PC) scores showed that, apart from sample date, mesh size was more important than irrigation treatment in determining microbial community structure; though treatment effects were less apparent in the third (winter) sample. Pine litter in the Greek site, with a Mediterranean climate, had a larger fungal than bacterial biomass. Little effect of treatment on individual PLFA concentrations or PC scores was measured, though both fungal and bacterial communities increased significantly with regular irrigation in the third (winter) sample. Effects of mesh size in the German spruce litter were related to differences in the abundance of microarthropods. This effect was absent from the Greek pine litter where there was a relatively low abundance of fauna. The final spruce litter sample, taken in winter, exhibited very different PC scores from other samples, suggesting marked changes in the microbial community in response to snow melt. Certain long chain fatty acids associated with eukaryotes were only found on this occasion. This study has shown that structure of bacterial communities associated with decomposing conifer litters is highly sensitive to changes in environmental conditions. There was, however, little indication that these differences in biota were functionally important for the initial phases of plant litter decomposition.