Abstract
Although both living roots and mycorrhizal fungi are well known to interact with saprotrophic microbes to affect litter decomposition, their relative importance is largely unclear. Here, a two-year pot experiment was conducted with two ectomycorrhizal (Pinus elliottii and Pinus massoniana) and four arbuscular mycorrhizal (Cinnamomum camphora, Cunninghamia lanceolata, Michelia maudiae and Schima superba) subtropical tree species to evaluate the relative effects of living roots and mycorrhizal fungal hyphae on their own root litter decomposition and to test whether these effects differed between ectomycorrhizal and arbuscular mycorrhizal trees. To achieve these objectives, litterbags with 50-µm and 1-mm mesh sizes filled with root litter of a given tree species were simultaneously installed in pots planted with the same species and unplanted pots filled with composite soil for all species. Effects of living roots alone were calculated as differences in root litter decomposition between 50-µm and 1-mm mesh litterbags installed in planted pots. Mycorrhizal hyphal effects were calculated as differences in root litter decomposition between 50-µm litterbags installed in planted and unplanted pots. The presence of mycorrhizal fungal hyphae significantly reduced root litter mass loss and inhibited the activities of β-glucosidase and phenol oxidase, while effects of living roots alone were non-significant when all tree species were pooled and inconsistent at the tree species level. Mycorrhizal fungal hyphae induced decreases in root litter mass loss that were markedly related to their inhibitory effects on β-glucosidase and phenol oxidase activities. When tree species were grouped by their mycorrhizal types, non-significant differences were observed between ectomycorrhizal and arbuscular mycorrhizal trees in their living root or mycorrhizal fungal effects on root litter decomposition. These findings highlight the important roles of mycorrhizal fungi in mediating litter decomposition via interacting with saprotrophic microbes and suggest that changes in tree carbon allocation to mycorrhizal fungi owing to global change may affect soil carbon storage.
Highlights
Root litter decomposition is an essential process in terrestrial ecosystems, as it releases nutrients for plant growth and returns photosynthetically fixed carbon (C) to the atmosphere [1]
When tree species were grouped by their mycorrhizal types, no significant differences were observed between ECM and arbuscular mycorrhizal (AM) trees in their living root and mycorrhizal hypha effects on root litter decomposition (Figure 5).When analyzed at the tree species level, in contrast with the consistently negative mycorrhizal hypha effects, living root effects on root litter mass loss and enzyme activities are inconsistent (Figures 6 and 7), indicating that living root effects are species specific and far from generalizable
When analyzed at the tree species level, our results revealed that there was a lack of consensus among tree species on whether living roots markedly differed from mycorrhizal hyphae in their effects on root litter decomposition (Figures 6 and 7), general patterns may not be made across all tree species or even when tree species were grouped by mycorrhizal types
Summary
Root litter decomposition is an essential process in terrestrial ecosystems, as it releases nutrients for plant growth and returns photosynthetically fixed carbon (C) to the atmosphere [1]. Through producing a suite of hydrolytic and oxidative enzymes, free-living saprotrophic microbes are the most important organisms participating in decomposition processes in soils [2]. Soil conditions and litter chemical traits can affect the activities of saprotrophic microbes and, subsequently, influence root litter decomposition [1]. Living roots and their associated mycorrhizal fungi are suggested to affect decomposition by interacting with saprotrophic microbes [3,4]. Root-microbe interactions could play important roles in litter decomposition, given that living roots, mycorrhizal fungi and Forests 2019, 10, 502; doi:10.3390/f10060502 www.mdpi.com/journal/forests. Evidence for living root and mycorrhizal fungal effects on root litter decomposition is conflicting, as both positive and negative effects have been reported [3,7]. It is still unclear what controls the direction and magnitude of living root and mycorrhizal fungal effects on root litter decomposition
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