The addition of labile carbon alters litter fungal communities and decreases litter decomposition rates
Introduction
It is generally accepted that carbon (energy) availability is the major factor limiting the activity of soil decomposer microorganisms (Daufresne and Loreau, 2001). Litter and upper mineral soil of boreal forests contain large amounts of organic carbon, but the major part of this carbon is bound in complex and recalcitrant compounds like lignin and humic acids, and is not easily available for decomposers (Swift et al., 1979). Dissolved organic carbon represents a small but functionally very important fraction of the soil organic matter (SOM) (van Hees et al., 2005). Many field and laboratory experiments have demonstrated that microbial activity increases strongly in response to the addition of labile carbon (Mikola and Setälä, 1998, Ekblad and Nordgren, 2002). Moreover, the addition of easily available carbon (as carbohydrates or other easily degradable compounds) often increases rates of decomposition of cellulose, plant litter or humified SOM (‘priming effect’: Kuzyakov et al., 2000, de Nobili et al., 2001, Hamer and Marschner, 2005, Kuzyakov et al., 2007). However, others studies reveal negative or zero effect of the addition of energy-rich compounds on the decomposition of plant residues or SOM (Dalenberg and Jager, 1989, Wu et al., 1993, Magill and Aber, 2000, Boberg et al., 2008). It was suggested that the influence of labile carbon on the rates of SOM decomposition results largely from the competition for energy and nutrients between the fast-growing microorganisms specialized in the utilization of easily available resources (r-strategists) and those feeding on polymerized SOM (K-strategists) (Fontaine et al., 2003).
Microbial communities of boreal forest soils are often dominated by biotrophic ectomycorrhizal (EM) fungi (Smith and Read, 1997). EM fungi possess a wide set of enzymes that enable them to forage complex organic materials in the litter layer, which is also densely populated by diverse communities of saprotrophic fungi. Ectomycorrhizal and saprotrophic fungi compete for nutrients, and antagonistic interactions between them are presumably common in forest soils (Leake et al., 2002, Lindahl et al., 2002, Hättenschwiler et al., 2005, Leake, 2006). These interactions can influence litter decomposition rates, but experimental evidence is scarce and controversial. Gadgil and Gadgil, 1971, Gadgil and Gadgil, 1975 suggested that ectomycorrhizal fungi might retard decomposition by competing with saprotrophic microorganisms. However, some further studies did not confirm ‘Gadgil effect’ (Cheng and Coleman, 1990, Entry et al., 1991, Zhu and Ehrenfeld, 1996). The effect of EM fungi on the rates of litter decomposition may depend on a range of factors. In particular, Koide and Wu (2003) have demonstrated that the negative influence of ectomycorrhizal fungi on litter decomposition was mediated by decreased water availability in mycorrhizal plots.
On the other hand, EM mycelial network provides efficient route of carbon flow from plant roots into the soil system. Field studies showed that EM mycelium is a very important or even a dominant pathway through which labile carbon enters the SOM pool (Simard et al., 2002, Godbold et al., 2006). In boreal forests this root-derived carbon may turn over very rapidly and contribute strongly to the soil respiration (Högberg et al., 2002). It is plausible to suggest that this carbon flux contributes to the regulation of microbial decomposition of various soil C pools, as it was often observed in rhizosphere soil (Cheng and Kuzyakov, 2005).
We performed a long-term field experiment aimed to assess the influence of the increased carbon availability and the presence of ectomycorrhizal roots on the rates of leaf litter decomposition and on the taxonomic and functional diversity of saprotrophic fungi in decomposing litter. We hypothesized that (1) the addition of labile carbon will result in changes in the functional structure of saprotrophic fungal communities and in the litter decomposition rates; and (2) the effects of EM roots on decomposition and fungal communities (if any) will depend on the level of carbon availability.
Section snippets
Study site, experimental design
The experiment was carried out at the Malinky biological station (Moscow region, 55°45′N, 37°23′E). The mean annual air temperature is 2.7–3.8 °C, rainfall 580–620 mm. The experimental site was situated in a ca. 50-year-old spruce (Picea abies (L.) Karst.) stand on albeluvisol. At the study site, 28 experimental plots (ca. 40 cm × 60 cm) were prepared.
Litterbags were used to determine decomposition rates of three litter species (Populus tremula L., Quercus robur L. and P. abies (L.) Karst.) over a
Decomposition rates
The aspen litter decomposed significantly faster than oak and spruce litter. The addition of the sucrose solution caused a significant decrease in decomposition of all three litter types, but the most pronounced effect was observed in the aspen litter (PLANT SPECIES × SUCROSE interaction: F = 18.3; P < 0.001, Table 1; Fig. 1).
After the first year, the presence of roots slightly decreased decomposition (overall mean weight loss 40.1 ± 1.1[SE] and 42.6 ± 1.5% with and without roots, respectively). During
Discussion
The serial dilution technique was used for assessing the structure of saprotrophic microfungal communities. This method has well-known limitations, such as an overestimation of the density of species that produce large numbers of spores or conidia and an underestimation of species in mycelial state or those that have slow growth in culture (Kirk et al., 2004). However, there are strong indications that cultural methods still reflect principal patterns of microbial communities composition and
Acknowledgements
This study was supported by the Russian Foundation for Basic Research. We thank O. Kozlitina (MPSU) for counting collembolans.
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