Bacterial and enchytraeid abundance accelerate soil carbon turnover along a lowland vegetation gradient in interior Alaska M.P. Waldrop a, * , J.W. Harden a , M.R. Turetsky b , D.G. Petersen c , A.D. McGuire d, e , M.J.I. Briones f , A.C. Churchill e , D.H. Doctor g , L.E. Pruett a a USGS, Menlo Park, CA, United States b University of Guelph, Canada c Center for Geomicrobiology, Department of Bioscience, Aarhus University, Denmark d USGS, Alaska Cooperative Fish and Wildlife Research Unit, United States e University of Alaska, Fairbanks, AK, United States f Departamento de Ecología y Biología Animal, Universidad de Vigo, 36310 Vigo, Spain g USGS, Reston, VA, United States article info Article history: Received 12 September 2011 Received in revised form 22 February 2012 Accepted 27 February 2012 Available online 28 March 2012 Keywords: Bonanza Creek LTER Boreal wetland C and N mineralization Fungi Bacteria Enchytraeids Soil enzymes Black spruce Tussock Fen abstract Boreal wetlands are characterized by a mosaic of plant communities, including forests, shrublands, grasslands, and fens, which are structured largely by changes in topography and water table position. The soil associated with these plant communities contain quantitatively and qualitatively different forms of soil organic matter (SOM) and nutrient availability that drive changes in biogeochemical cycling rates. Therefore different boreal plant communities likely contain different soil biotic communities which in turn affect rates of organic matter decomposition. We examined relationships between plant communities, microbial communities, enchytraeids, and soil C turnover in near-surface soils along a shallow topographic soil moisture and vegetation gradient in interior Alaska. We tested the hypothesis that as soil moisture increases along the gradient, surface soils would become increasingly dominated by bacteria and meso- fauna and have more rapid rates of C turnover. We utilized bomb radiocarbon techniques to infer rates of C turnover and the 13 C isotopic composition of SOM and respired CO 2 to infer the degree of soil humification. Soil phenol oxidase and peroxidase enzyme activities were generally higher in the rich fen compared with the forest and bog birch sites. Results indicated greater C fluxes and more rapid C turnover in the surface soils of the fen sites compared to the wetland forest and shrub sites. Quantitative PCR analyses of soil bacteria and archaea, combined with enchytraeid counts, indicated that surface soils from the lowland fen ecosystems had higher abundances of these microbial and mesofaunal groups. Fungal abundance was highly variable and not significantly different among sites. Microbial data was utilized in a food web model that confirmed that rapidly cycling systems are dominated by bacterial activity and enchytraeid grazing. However, our results also suggest that oxidative enzymes play an important role in the C miner- alization process in saturated systems, which has been often ignored. Published by Elsevier Ltd. 1. Introduction Boreal ecosystems in Alaska include a diverse array of deciduous and conifer forests, tussock grasses, fens, and bogs which are struc- tured by local climate, disturbance, water table position, and nutrient availability (Jasieniuk and Johnson, 1982; Wahren et al., 2005; Hollingsworth et al., 2008). These systems contain large quantities of soil organic carbon that are susceptible to changes in climate and climate-induced disturbances (Juday et al., 2003; Benoy et al., 2007; Goetz et al., 2007). Although vegetation composition is a primary determinant of ecosystem productivity (Bonan et al., 1992; Benoy et al., 2007; Goetz et al., 2007; Myers-Smith et al., 2008), rates of organic matter turnover and C storage are mediated in part by the composition of the soil microbial community, food web interactions, and their physical environment (Waldrop et al., 2004; Chapin et al., 2009; van Dijk et al., 2009). Microbial communities are often strongly linked to the composition of the aboveground plant community (Zak and Kling, 2006; Taylor et al., 2010), making understanding the drivers of rela- tionships between vegetation composition and soil biota an important * Corresponding author. Tel.: þ1 650 329 5005; fax: þ1 650 329 4920. E-mail address: mwaldrop@usgs.gov (M.P. Waldrop). Contents lists available at SciVerse ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Published by Elsevier Ltd. doi:10.1016/j.soilbio.2012.02.032 Soil Biology & Biochemistry 50 (2012) 188e198