Soil Biology & Biochemistry 38 (2006) 1933–1942 A comparison of methods to determine the biodegradable dissolved organic carbon from different terrestrial sources W.H. McDowell a,Ã , A. Zsolnay b , J.A. Aitkenhead-Peterson a , E.G. Gregorich c , D.L. Jones d , D. Jo¨demann e , K. Kalbitz f , B. Marschner e , D. Schwesig f a Department of Natural Resources, University of New Hampshire, Durham, NH 03824, USA b GSF Forschungszentrum fu ¨ r Umwelt und Gesundheit, Institut fu ¨ r Bodeno ¨kologie, D-85764 Neuherberg, Germany c Agriculture & Agri Food Canada, Ottawa Research Centre, Central Experimental Farm, Ottawa, Ont., Canada, K1A 0C6 d School of Agricultural and Forest Sciences, University of Wales, Bangor, Gwynedd LL57 2UW, UK e Ruhr-University Bochum, Geographical Institute, D-44780 Bochum, Germany f Department of Soil Ecology, University of Bayreuth, Bayreuth Institute for Terrestrial Ecosystem Research, D-95440 Bayreuth, Germany Received 23 December 2004; received in revised form 28 December 2005; accepted 31 December 2005 Available online 3 March 2006 Abstract The importance of dissolved organic matter (DOM) in many soil processes is determined in large part by its availability to microbial uptake and decomposition, as this biodegradation can yield both energy and limiting nutrients. Despite its importance in soil ecology, there are no standard approaches to measuring the biodegradable fraction of DOC (BDOC) in soils. Here, we evaluate the comparability and reproducibility of methods employed in six laboratories including batch, kinetic, and bioreactor methods. Solutions from a variety of sources (throughfall, soil solution and soil extracts) were analysed using methods typically employed in each of the six participating laboratories. Our results show that the precision of various BDOC methods was similar (5–15%) across a broad range of BDOC (from 12% to 56% of total DOC). Differences in mean BDOC for the various test solutions were statistically significant when results were pooled across all the methods, and only a 90-day incubation resulted in consistently higher values for BDOC than the other methods. For 4 of 6 test solutions, measured BDOC increased by 6–13% with added nutrients. Current methods produce largely comparable results, providing the justification for comparisons among existing data sets collected with different methodologies. We recommend two standard methods for future studies: (1) a rapid determination of relatively labile DOC (measurement of DOC removal after 7 days of incubation with added nutrients) and (2) a 42-day incubation with repeated analysis of CO 2 production when determination of decomposition rate constants and a labile and relatively refractory component of DOC is desired. r 2006 Elsevier Ltd. All rights reserved. Keywords: Biodegradation; Bioavailability; Dissolved organic carbon; Method; Mineralization 1. Introduction Dissolved organic matter (DOM) plays a key role in a wide variety of chemical, physical, and biological processes occurring in soils (Kalbitz et al., 2000; Zsolnay, 1996, 2003). It also facilitates the transport of metals and hydrophobic organic contaminants through soils (Kalbitz et al., 1997; Temminghoff et al., 1997), and thus DOM degradation can alter the dynamics of pollutant fate and transport in soils. DOM represents a potential supply of energy and organic nutrients (especially N and P) to soil microflora. Microbial degradation of DOM can regulate the production of greenhouse gases such as CH 4 and N 2 O by both reducing the O 2 content of soils and by providing the electrons required for methanogenesis and denitrifica- tion (Yavitt, 1997; Zsolnay, 1996; Lu et al., 2000). Finally, degradation of DOM in soils can alter its delivery to aquatic ecosystems, where DOM plays an important role in food webs, contaminant transport, and absorption of harmful UV-B radiation (Engelhaupt et al., 2003; Findlay et al., 2003; Houser et al., 2003). Recognition of the ARTICLE IN PRESS www.elsevier.com/locate/soilbio 0038-0717/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2005.12.018 Ã Corresponding author. Tel.: +1 603 862 2249; fax: +1 603 862 4976. E-mail address: bill.mcdowell@unh.edu (W.H. McDowell).