Dynamics of Microbial Communities during Decomposition of Carbon-13 Labeled Ryegrass Fractions in Soil

decomposition (Smith et al., 1998), less is known about the interactions of substrate characteristics and microThe soluble fraction of ryegrass [Lolium perenne L. ssp. multibial decomposers during residue decomposition (Schiflorum (Lam.) Husnot.] straw comprises a major component of residue C and its presence or absence should influence the succession of mel, 1995). decomposer communities. Changes in phospholipid fatty acid (PLFA) Residue decomposition is often mathematically deprofiles are indicative of shifts in microbial community structure. We scribed by dividing the residue C into two, or more, comused 13C-labeled ryegrass to track substrate-derived C into microbial partments that decompose at faster and slower rates lipids during decomposition in a microcosm-based study. Treatments (e.g., van Veen et al., 1984; Saviozzi et al., 1997). Alwere unleached straw, leached straw, and leachate, plus an unamended though these compartments do not neatly correspond control. Destructive sampling took place after 0.6, 1.6, 15, 18, 50, and to chemically defined pools of C, many studies have 80 d of incubation. Phospholipid fatty acids were extracted from bulk shown that the fast or labile pool is comprised primarily soil and isolated straw (detritusphere), and analyzed by gas chromaof soluble C compounds (Reinertsen et al., 1984; Cogle tography combustion isotope ratio mass spectroscopy (GC-C-IRMS). et al., 1989; Collins et al., 1990; Marstorp, 1996; TrinsouDistinct temporal shifts occurred in PLFAs in bulk soil samples; cy19:0 was an indicator for late succession communities in unleached straw trot et al., 2000). The slow pools are made up of strucand leachate treatments, and 18:2 6,9 characterized late samples in tural, polymeric C compounds, such as hemicelluloses, the leached straw treatment. The temporal shift was affected by the cellulose, and lignin (Wessén and Berg, 1986; Saviozzi presence of the soluble fraction of straw. Microscale spatial effects et al., 1997). Thus, as decomposition proceeds, the chemwere clear. Bulk soil and detritusphere communities were different ical composition of the residue changes (Horwath and and more 13C was detected in the 16:0 and 18:2 6,9 PLFAs of the Elliott, 1996). It is tempting to postulate that the utilizadetritusphere than bulk soil. Carbon-13 was slowest to appear in tion of these C pools is associated with different groups PLFAs of leached straw samples, illustrating the importance of the of microorganisms, resulting in an orderly microbial sucsoluble fraction in promoting growth of the biomass in bulk soil and cession that is linked to the changes in residue chemistry in detritusphere. The 18:2 6,9 PLFA, a fungal biomarker, was the during decomposition. The real situation is more commost highly labeled in all treatments. Carbon-13 PLFA analysis provides greater insight into microbial community structure and functionplex, of course, and each succession is unique (Franking, facilitating more concrete conclusions regarding the role of funcland, 1998). tional groups of organisms. Plant litter is already colonized with microorganisms before its senescence (Tester, 1988), but bacterial and fungal biomass increase as decomposition proceeds (e.g., D of plant residues is a major ecosysWessén and Berg, 1986; Parmelee et al., 1989; Henriksen tem process that is important in recycling nutriand Breland, 1999; Malosso et al., 2004). Many studies ents, maintaining organic matter, and fueling food webs have shown an increase in the relative amounts of fungi in soils. Consequently, litter decomposition has been studvs. bacteria during decomposition (Neely et al., 1991; ied extensively (e.g., Swift et al., 1979; Cadisch and Giller, Beare et al., 1992; Lundquist et al., 1999; Henriksen and 1997; Berg and McClaugherty, 2003). Regulation of deBreland, 2002), but some have observed fairly constant composition is a function of the intrinsic quality of the or even declining fungal/bacterial ratio (Broder and Wagsubstrate (the chemical constituents of the residue), exner, 1988; Lundquist et al., 1999). The different patterns trinsic environmental factors (such as temperature, moisin fungal and bacterial biomass that have been observed ture, etc.), and the composition of the decomposing commay be due in part to differences in the methods used munity. Although the effects of microclimatic variables in these studies. The ratio of culturable copiotrophic to and substrate quality on decomposition are well underoligotrophic bacteria was found to increase during the stood and are the primary controllers used in models of first 3 wk of decomposition and then decline (Hu et al., 1999) and the types of fungi isolated changed during litS.K. McMahon, M.A. Williams, P.J. Bottomley, and D.D. Myrold, ter decomposition (Frankland, 1998). The newer methDep. of Crop and Soil Science, Oregon State Univ., Agric. Life Sci. ods of community level physiological profiles, PLFA Bldg. 3017, Corvallis, OR 97331-7306; P.J. Bottomley, Dep. of Microanalysis, denaturing gradient gel electrophoresis, and biology, 220 Nash Hall, Oregon State Univ., Corvallis, OR 97331-3804; RNA fingerprinting have also found microbial comS.K. McMahon, Biological Sciences, Ecology, Evolution & Marine Biology, Univ. of California, Santa Barbara, CA 93106; M.A. Williams, munity composition to shift as plant materials decomDep. of Crop and Soil Sciences, Univ. of Georgia, 3121 Miller Plant pose (Sharma et al., 1998; Thirup et al., 2001, 2003; Sciences Bldg., Athens, GA 30602. Received 28 Aug. 2004. *CorreNakamura et al., 2003; Aneja et al., 2004). How tightly sponding author ([email protected]). Abbreviations: FAME, fatty acid methyl ester; GC-C-IRMS, gas chroPublished in Soil Sci. Soc. Am. J. 69:1238–1247 (2005). Soil Biology & Biochemistry matography-combustion-isotope ratio mass spectrometry; IRMS, isotope ratio mass spectrometer; ISA, indicator species analysis; MRPP, doi:10.2136/sssaj2004.0289 © Soil Science Society of America multi-response permutation procedure; PCA, principal components analysis; PLFA, phospholipid fatty acid. 677 S. Segoe Rd., Madison, WI 53711 USA 1238 Published online June 28, 2005