Moisture eects on microbial activity and community structure in decomposing birch litter in the Alaskan taiga Joshua P. Schimel a, *, Jay M. Gulledge b, 1 , Joy S. Clein-Curley b , Jon E. Lindstrom b , Joan F. Braddock b a Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, CA 93106, USA b Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775, USA Accepted 22 October 1998 Abstract We carried out a ®eld experiment to evaluate the eect of moisture regime on microbial biomass and activity in birch litter in the Alaskan taiga. Litter bags were placed in one of three treatments: continuously moist (0.5 cm water d  1 ), cycling (0.5 cm water weekly), and `natural', which experienced two natural dry±wet cycles of 2 weeks dry followed by rain. The experiment lasted for 1 month. Each week we collected litter bags and analyzed microbial respiration and biomass C and N. In the last two cycles we analyzed bacterial substrate use on Biolog GN plates. There were strong overall correlations between biomass, respiration and litter moisture content. However, the dierent treatments had signi®cantly dierent rates of respiration, biomass and respiratory quotient (qCO 2 ) that could not be explained by moisture content directly. The natural treatment had lower respiration rates and biomass than the wet or cycling samples, indicating that the 2-week droughts in the natural treatment reduced microbial populations and activity to a greater degree than did shorter droughts. Episodic drying and rewetting considerably decreased the number of Biolog substrates used. This experiment showed that the size and functioning of the litter microbial community was strongly aected by its stress history. # 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction Physiological stress is common in the taiga forests of interior Alaska. Not only are winters extremely cold, but summers are usually dry with only occasional rainfall events. Drought, in fact, is a major factor con- trolling tree growth (Yarie et al., 1990) and soil respir- ation (Schlentner and Van Cleve, 1985; Gulledge and Schimel, 1998). Soil moisture is a major control on mi- crobial and microfaunal community structure and ac- tivity in the forest ¯oor (Wagener and Schimel, 1998). Dry conditions make the litter layer an unsuitable habitat for a number of soil faunal groups, including dipteran ¯y larvae that are important litter shredders deeper in the forest ¯oor (Wagener, 1995). Total mi- crobial biomass in the forest ¯oor correlates well with soil moisture (Wagener and Schimel, 1998) and soil drying limits microbial activity even in winter when soil is frozen (Clein and Schimel, 1995). Occasional rainfall events in the Alaskan taiga may place additional stress on the soil microbial commu- nity. The water potential `upshock' associated with rewetting a dry soil can kill a large fraction of the mi- crobial community (Bottner, 1985; Kieft et al., 1987). Clein and Schimel (1994) found that even a single short drying±rewetting event reduced microbial activity in Alaskan birch litter by greater than 25% over the following 2 months laboratory incubation. They hy- pothesized that this was due to the loss of important litter degrading organisms. While the eects of drought and rewetting on total microbial biomass and activity have been studied, the eects on microbial community structure are poorly Soil Biology and Biochemistry 31 (1999) 831±838 0038-0717/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-0717(98)00182-5 PERGAMON * Corresponding author. Tel.: +1-805-893-7688; fax: +1-805-893- 4724; e-mail: schimel@lifesci.ucsb.edu 1 Current address: Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.