Ectomycorrhizal fungi contribute to soil organic matter cycling in sub-boreal forests

The ISME Journal (2014) 8, 699–713 & 2014 International Society for Microbial Ecology All rights reserved 1751-7362/14 www.nature.com/ismej ORIGINAL ARTICLE Ectomycorrhizal fungi contribute to soil organic matter cycling in sub-boreal forests Lori A Phillips1,2, Valerie Ward1 and Melanie D Jones1 1 Department of Biology, University of British Columbia, Kelowna, British Columbia, Canada and 2Department of Environment and Primary Industries, Biosciences Research Division, Bundoora, Victoria, Australia Soils of northern temperate and boreal forests represent a large terrestrial carbon (C) sink. The fate of this C under elevated atmospheric CO2 and climate change is still uncertain. A fundamental knowledge gap is the extent to which ectomycorrhizal fungi (EMF) and saprotrophic fungi contribute to C cycling in the systems by soil organic matter (SOM) decomposition. In this study, we used a novel approach to generate and compare enzymatically active EMF hyphae-dominated and saprotrophic hyphae-enriched communities under field conditions. Fermentation-humus (FH)-filled mesh bags, surrounded by a sand barrier, effectively trapped EMF hyphae with a community structure comparable to that found in the surrounding FH layer, at both trophic and taxonomic levels. In contrast, over half the sequences from mesh bags with no sand barrier were identified as belonging to saprotrophic fungi. The EMF hyphae-dominated systems exhibited levels of hydrolytic and oxidative enzyme activities that were comparable to or higher than saprotroph-enriched systems. The enzymes assayed included those associated with both labile and recalcitrant SOM degradation. Our study shows that EMF hyphae are likely important contributors to current SOM turnover in sub-boreal systems. Our results also suggest that any increased EMF biomass that might result from higher below-ground C allocation by trees would not suppress C fluxes from sub-boreal soils. The ISME Journal (2014) 8, 699–713; doi:10.1038/ismej.2013.195; published online 31 October 2013 Subject Category: Microbial ecology and functional diversity of natural habitats Keywords: carbon cycling; ectomycorrhizal fungi; enzyme activity; hyphal trap bags; mycelia; soil organic matter Introduction Northern temperate and boreal forests represent a large carbon (C) sink, with an estimated half a gigatonne of C sequestered in above ground biomass every year (Myneni et al., 2001). Much of this C is eventually transferred below ground, either after the death or annual senescence of vegetation or directly via photosynthetic transfer to root systems and associated ectomycorrhizal fungi (EMF). These soils store up to three times the amount of C as is stored in the above ground vegetation, primarily in the form of soil organic matter (SOM) (Schmidt et al., 2011). These C sinks have the potential to become C sources under the projected global climate change (IPCC, 2007). Although some climate models assume increased C storage in temperate forest soils because of the increased plant allocation of photosynthetic C to roots and symbiotic fungi (Drigo et al., 2010; Orwin et al., 2011; Clemmensen et al., 2013), there is a high degree of uncertainly in this Correspondence: LA Phillips, Department of Environment and Primary Industries-AgriBioSciences Centre, Biosciences Research Division, 5 Ring Road, Bundoora 3083, Victoria, Australia. E-mail: Received 8 July 2013; revised 25 September 2013; accepted 26 September 2013; published online 31 October 2013 assumption (Higgins and Harte, 2012). Increased C allocation with elevated CO2 may not only be shortlived (Norby et al., 2010), but concomitant increases in mycorrhizal and mycorrhizosphere activity may stimulate decomposition of previously recalcitrant SOM (Cheng et al., 2012; Phillips et al., 2012b). Fundamental gaps in our understanding of terrestrial C cycling, including the relative contributions of EMF and other fungi to the release of SOM-C (Bargett, 2011), limit the predictive capability of current models (Von Lützow and Kögel-Knabner, 2009; Hayes et al., 2012). Fungi, especially saprotrophic fungi, are thought to dominate organic matter turnover in temperate forest soils (Baldrian, 2008). Certain groups of saprotrophic fungi, specifically the white rot fungi, have unique abilities to degrade wood because they possess numerous copies of class II peroxidases (Floudas et al., 2012). Both saprotrophic and EMF species, however, produce a range of hydrolytic and oxidative enzymes that have the potential to break down C-containing compounds and mobilize nutrients from SOM (Courty et al., 2010; Floudas et al., 2012). Although photosynthates are likely the primary source of C used by EMF under ideal conditions (Treseder et al., 2006; Wolfe et al., 2012), current research suggests that these fungi may also EMF and soil organic matter cycling LA Phillips et al 700 directly (Vaario et al., 2012) or indirectly (Rineau et al., 2012) access SOM-C pools. Recalcitrant SOM, including lignin and humic complexes, is predominantly found in soil horizons dominated by EMF mycelial networks (Lindahl et al., 2007). These mycelia are present at up to 600 kg of hyphae per hectare (Wallander et al., 2001; Hendricks et al., 2006) and account for up to one-third of the total microbial biomass in coniferous forests (Högberg and Högberg, 2002; Cairney, 2012). Although EMF mats are known to increase total soil CO2 respiration (Phillips et al., 2012a), the full contribution of these fungi, and their extensive mycelia, to decomposition processes remains unknown (van der Wal et al., 2012). Given the current interest in the C sequestration potential of boreal and temperate forests (De Luca and Boisvenue, 2012), as well as the influence of any increased C allocation below ground by plants, the role of EMF mycelia in SOM-C turnover warrants increased investigation (Talbot et al., 2008; Ekblad et al., 2013). Current research to estimate the extent and activity of EMF mycelia rely on methods to exclude roots and most saprotrophic fungi from a given zone in the soil profile. Hyphal in-growth cores or bags are commonly used (Wallander et al., 2001). These hyphae traps are usually filled with an inert substrate, such as sand, that minimizes colonization by saprotrophic fungi (Korkama et al., 2007; Lindahl et al., 2007; Wallander et al., 2010). In some cases, additional nutrient sources are added to ensure sufficient hyphal colonization or to evaluate the impact of limiting nutrients on EMF biomass production (Hagerberg and Wallander, 2002; Hagerberg et al., 2003; Hedh et al., 2008; Potila et al., 2009; Berner et al., 2012). However, the captured extramatrical hyphae may not accurately represent the composition or abundance of EMF communities in the surrounding soil (Hendricks et al., 2006; Kjøller, 2006). Further, enzyme expression and activity is downregulated when no nutrients are present (Wright et al., 2005). In order to estimate the contributi (...truncated)