Impact of woody debris of different tree species on the microbial activity and community of an underlying organic horizon
Introduction
Woody debris (WD) is a component of the carbon (C) and nutrient cycle in forest ecosystems. Its relevance for element cycling strongly varies among forest ecosystems and relies on production, stock and decay of WD (Harmon et al., 1986, Brassard and Chen, 2008). Chemical and physical wood traits associated with tree species as well as the decomposer community have great impact on the initial decay of WD (Kahl et al., 2017, Weedon et al., 2009). Gymnosperms and ring-porous angiosperms like Fraxinus excelsior and Quercus robur/petraea decay slower than diffuse-porous angiosperms with the fastest decay rates of Carpinus betulus and Fagus sylvatica among 13 temperate tree species (Kahl et al., 2017). Another important factor of the decay process is the location of WD in the forest. Contact of downed WD to soil accelerates the decay by many years relatively to standing snags (Vanderwel et al., 2006).
While many studies focus on the decay of downed WD, the impact of WD on forest soils is less well investigated and the few findings are divergent so far. Soils under WD can have higher concentrations of nutrients and organic C (Bade et al., 2015, Goldin and Hutchinson, 2013, Kappes et al., 2007, Stutz et al., 2017) or experience negative to no changes in element concentrations (Spears et al., 2003, Spears and Lajtha, 2005, Kahl et al., 2012, Zalamea et al., 2016). Even large inputs of WD do not inevitably increase the organic C stock of temperate forest soils (Krüger et al., 2017). Pathways of movement of WD components into the soil are incorporation of WD fragments, leaching of dissolved organic and inorganic matter (Bantle et al., 2014a, Bantle et al., 2014b) or transfer by fungal hyphae (Hughes and Boddy, 1994).
Fungi could play an important role in the change of forest soils by overlying WD. Fungal hyphae have the ability to immobilize and translocate nutrients between different compartments including deadwood (Chigineva et al., 2011, Lindahl et al., 2001, Watkinson et al., 2006). The transfer between different compartments may be bidirectional (Lindahl et al., 2001), but is often directed towards the mycelial front (Guhr et al., 2015, Lindahl et al., 2002). Several studies reported N uptake from soil by fungal hyphae to overcome the N deficiency in deadwood or in other N deficient litter (Boberg et al., 2010, Rinne et al., 2017). Nutrient transfer is presumably closely linked to fungal diversity in WD, the underlying soil and particularly to those fungi connecting the two habitats with their hyphae.
Communities of fungal decomposers usually differ among substrates. WD is mostly colonized by fungi specialized on this substrate (Dix and Webster, 1995), though the wood-inhabiting communities often differ among tree species (Hattori, 2005, Yamashita et al., 2010, Stokland et al., 2012, Hoppe et al., 2015). A horizonwise specialization by different fungal communities also occurs along vertical forest soil profiles (Lindahl et al., 2007, Peršoh et al., 2013). WD alters the structure of ectomycorrhizal (EcM) and yeast fungal communities in soil (Walker et al., 2012, Yurkov et al., 2012). The entire community of soil fungi was also shown to be altered by the presence of decaying spruce logs in a spruce forest (Mäkipää et al., 2017). However, it remains unclear if WD of other tree species has different effects on the fungal communities in underlying forest soil.
Here, we addressed the hypotheses that (1) presence and (2) origin of WD alters microbial CO2 production, microbial biomass and fungal community structure in the underlying organic soil horizon. We tested the hypotheses in a laboratory experiment and assumed that the effect of fast decomposing tree species (Acer pseudoplatanus, Betula pendula, Fagus sylvatica) is greater on soil microbial parameters than of slow decomposing tree species (Fraxinus excelsior, Quercus robur, Larix decidua, Picea abies, Pinus sylvestris). Oe horizon and WD were derived from a Norway spruce forest where WD of the eight tree species was subjected to initial decay on the forest floor for 1.5 years.
Section snippets
Experimental design, wood and soil characteristics
Woody debris (WD) and Oe horizon were taken from a field experiment in a 63-year-old Norway spruce forest at the Waldstein, Fichtelgebirge, Germany, in April 2016. The field experiment was initiated in November 2014 where WD of eight different tree species was exposed to the forest floor in three WD plots to study the decay process under the climate conditions at the Fichtelgebirge (MAT 5.3 °C, MAP 1160 mm). WD comprised five angiosperms: Acer pseudoplatanus, Betula pendula, Fagus sylvatica,
CO2 production
In the Oe treatment, WD of fast decomposing tree species (Acer, Betula, Fagus) significantly (p = 0.018) increased cumulative CO2 production of the Oe horizon compared to the control by 44–74% during the 78-days incubation period (Fig. 1a). Higher or even lower cumulative CO2 production occurred in the Oe horizon under Fraxinus, Quercus, Larix, Picea and Pinus, but the differences to the Oe from the control subplots were not statistically significant. In the WD treatment, CO2 production
CO2 production and microbial biomass of Oe horizons
Two treatments were conducted to test the hypothesis that WD of fast decomposing tree species increases the CO2 production and microbial biomass C of the underlying Oe horizon in a Norway spruce forest. In the Oe treatment, WD of Acer, Betula and Fagus had a positive effect on the Oe horizon, which persisted for the incubation period of 78 days. WD of all other tree species had no effect, although Quercus decomposed as fast as Betula. It seems that the effect of WD on the microbial activity in
Acknowledgements
We like to thank Uwe Hell and Karin Söllner for technical support and chemical analyses. We grateful acknowledge Christina Groβ and Merve Ay for conducting the laboratory experiment and preparation of samples for DNA analyses of fungal communities. Andreas Brachmann (Munich) supported library preparation and conducted the Illumina sequencing.
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