PLFA pro®les of microbial communities in decomposing conifer litters subject to moisture stress S.C. Wilkinson a , J.M. Anderson a, * , S.P. Scardelis b , M. Tisiafouli b , A. Taylor c , V. Wolters c a Department of Biological Sciences, University of Exeter, Hatherly Laboratories, Prince of Wales Road, Exeter EX4 4PS, UK b Department of Ecology, Aristotle University, 54006 Thessalonika, Greece c Department of Animal Ecology, Justus-Liebig University, Stephanstrasse 24, 35390 Germany Received 6 October 2000; received in revised form 20 September 2001; accepted 20 September 2001 Abstract The in¯uence of moisture stress on microbial communities in decomposing coniferous litters was investigated using phospholipid fatty acid PLFA) pro®ling. 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 in¯uences 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 signi®cantly 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 ®nal 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. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Phospholipid fatty acid; Microbial communities; Conifer litter; Climate; Decomposition; Litterbags 1. Introduction Litter decomposition in forest soils is mainly carried out by microorganisms particularly fungi), but many studies have demonstrated the importance of soil fauna on the composition and activities of these microbial communities Anderson, 1988; Verhoef and Brussaard, 1990; Teuben, 1991; Scholle et al., 1992). The initial phases of litter decomposition generally involve a succession of animal and microbial species Dilly and Irmler, 1998; Frankland, 1998) determined by mode of colonisation by different organisms, resource composition and biotic interactions Kendrick and Burges, 1962; Jensen, 1974; Ponge, 1991; Berg et al., 1998). Fungal communities in litter can be highly variable, even between adjacent pine needles Kendrick and Burges, 1962), as a consequence of the graz- ing activities of invertebrates Visser, 1985; Maraun et al., 1998), litter comminution Anderson and Ineson, 1984) and higher-order effects of predators Wardle and Yeates, 1993); but do not necessarily respond to these factors in predictable ways MacLean et al., 1996). Similar mechanisms may affect the structure of bacterial communities Allen-Morley and Coleman, 1989; Bardgett et al., 1996; Yeates et al., 1997) but have not been documented for conifer litters under ®eld conditions where fungal dominance is often assumed. These biotic factors are moderated by micro- climatic conditions in the surface litter layers which can undergo diurnal freezing and thawing cycles, or frequent wetting and drying after rain events. Consequently fungi and microarthropods occurring in surface litter layers tend Soil Biology & Biochemistry 34 2002) 189±200 0038-0717/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0038-071701)00168-7 www.elsevier.com/locate/soilbio * Corresponding author. Tel.: 144-1392-263790; fax: 144-139- 2263700. E-mail address: j.m.anderson@exeter.ac.uk J.M. Anderson).