Influence of invasive earthworms (Amynthas spp.) on Wisconsin forest soil microbial communities and soil chemistry
Introduction
The invasion of earthworm species into North American hardwood forests is a major threat to the stability and function of these ecosystems (Sutherland et al., 2011). Invasive earthworms transform hardwood forest soils by consuming and eliminating the leaf litter layer (Hale et al., 2005). During consumption, carbon in the leaf litter layer is homogenized between the organic and mineral horizons, altering the carbon available in these profiles (Frelich et al., 2006). The homogenization and partial digestion of this carbon shifts the soil microbial community from an oligotrophic community into a copiotrophic community (McLean et al., 2006). Most prior research on the impact of earthworm invasions has generally focused on Lumbricus species and other species of European origin (Greiner et al., 2012), but another group of earthworms, frequently referred to as “pheretimoids”, poses a ‘second wave’ of disruption to forest soil processes (Szlavecz et al., 2018).
Pheretimoids are a multi-species complex of earthworms formerly of the genus Pheretima. An invasive complex of pheremtimoid worms including the species Amynthas agrestis, A. tokioensis, and Metaphire hilgendorfi, have been repeatedly introduced to North America since 1905 and are commonly found to co-occur (Chang et al., 2018). Human activities, such as the spreading of contaminated mulch and recreational fishing (Callaham et al., 2003; Belliturk et al., 2015) are the primary means of their dispersal. Two of the most prevalent invasive Amynthas species, A. tokioensis and A. agrestis, were recently discovered in Wisconsin hardwood forests (Qiu and Turner, 2017). A. tokioensis and A. agrestis are both epi-endogeic species that primarily feed on litter and woody detritus (Chang et al., 2016c; Qiu and Turner, 2017). A. agrestis can grow to lengths of 14 cm (Gorres et al., 2016), while A. tokioensis can reach lengths of 12.5 cm (Chang et al., 2016a). Both species can reach maturity within about 60–90 days (Gorres and Melnichuk, 2012; Qiu and Turner, 2017). As a habitat, hardwood forests are an ideal environment for invaders like Amynthas due to seasonal differences in activity patterns between Amynthas and Lumbricid worms (Görres and Bellitürk., 2014; Nouri-Aiin and Gorres, 2019), the relative lack of strong detritivore competitors (Snyder et al., 2011), the abundant availability of food resources (Eisenhauer et al., 2012), and the lack of suitable predators (Gorsuch and Owen, 2014). Although other invasive earthworm species may occupy some of these forests, Amynthas species have been shown to invade these occupied forests and then competitively exclude established Lumbricus rubellus and Lumbricus terrestris populations (Zhang et al., 2010; Laushman et al., 2018).
The replacement of Lumbricus communities by Amynthas can alter the chemical characteristics of soils. In hardwood forests, Amynthas radically increase the average size of visible soil aggregates at the soil surface, even relative to other invasive earthworm species such as Lumbricus rubellus (Greiner et al., 2012). These changes in soil aggregate size following Amynthas invasion influence soil chemistry in the A horizon, affecting the carbon and nitrogen economy of these forest soils (Suarez et al., 2006; Stromberger et al., 2012; Wu et al., 2017). These changes to soil carbon and nitrogen chemistry by earthworms can influence the composition and functioning of soil microbial communities (Bohlen et al., 2004). Indeed, prior work evaluating the co-invasion of Amynthas and other earthworm species found that the soil microbial community shifts from a fungal-dominated to a bacterial-dominated community (McLean et al., 2006; Dempsey et al., 2011; Chang et al., 2017). This type of community shift has been correlated with changes in soil microbial community function such as increases in soil basal metabolic rates, increased denitrification activity, and increased mineralization of fixed carbon (Burtelow et al., 1998). These studies demonstrate the need to observe both the microbial communities and chemical characteristics of soils when assessing the effects of Amynthas invasions.
During invasion, Amynthas may alter the soil microbial community through both direct and indirect effects. Directly, microbes associated with the worm gut may be introduced to the soil via discharged castings (Drake and Horn, 2007; Wu et al., 2015). Resident microbes may also be differentially affected when passing through a worm if they are sensitive to gut-associated processes such as digestion (Pedersen and Hendriksen, 1993). Indirectly, the structural and chemical changes associated with earthworm invasion, such as those reviewed above, may also drive changes in soil microbial communities (Dempsey et al., 2013; Paudel et al., 2016). These examples of the direct and indirect affects that Amynthas have on microbial communities demonstrate that different Amynthas species may exert unique species-specific effects on microbial communities relative to other invasive earthworm species. Furthermore, because many of these species-specific effects derived from Amynthas are likely a result of gut-associated processes, it is important to evaluate gut-associated microbes as a first step for assessing if the sloughing of Amynthas-associated gut microbes is a mechanism that influences soil microbial communities following Amynthas invasion.
In this study, we investigate the impacts of Amynthas invasion on soil properties and microbial communities by sampling newly-invaded sites and sites with multi-year occupancy along an active invasion front in a Wisconsin forest. Our first objective is to determine if the multi-year invasion of Amynthas alters the carbon mineralization, carbon metabolic diversity, total carbon, total nitrogen, C:N ratio, and microbial community composition, of invaded soils relative to soils that have been invaded by Amynthas for less than a year. We aim to determine if the guild replacement of other invasive earthworm species by Amynthas results in compositional and functional changes in the soil microbial community as well as soil chemical and functional changes. We anticipate that soils invaded by Amynthas for longer periods of time will have less total nitrogen, less total carbon, higher carbon mineralization rates, lower C:N ratios, and that soil microbial community composition will reflect an enhanced carbon metabolic function (i.e. increased diversity in the metabolization of carbon substrates). Our second objective is to determine if Amynthas invaders exert species-specific effects on the microbial community composition of cast and gut associated microbes. We anticipate that the gut and cast microbial communities collected from the Amynthas species A. agrestis and A. tokiensis will be different from one another and that microbes prevalent in Amynthas guts will also be prevalent in older invaded soils.
Section snippets
Site and plot description
Soil and earthworm samples were collected from a maple-oak hardwood forest (within the Gallistel Woods management unit) at the University of Wisconsin-Madison Arboretum in Madison, Wisconsin (43.045477N, −89.42853W). Samples were collected during the summer of 2017 adjacent to study plots recently described in an earlier study on plant and worm communities (Laushman et al., 2018). Samples were collected from a total of 10 plots across an Amynthas invasion front so that we could evaluate
Effects of Amynthas invasion on soils
For soil chemistry, we found no statistically significant difference in soil pH, nitrate, and total carbon (Fig. 1A) between newly-invaded plots and older invaded plots (Wilcoxon Signed-Rank tests p-value = 0.3524, p-value = 0.4762, p-value = 0.2571, respectively). However, total nitrogen was higher in older invaded plots than newly invaded plots (Wilcoxon Signed-Rank p-value = 0.0183, Fig. 1B), and consequently the C:N ratio of newly invaded soils was higher than in older invaded areas
Discussion
By comparing newly invaded plots to older invasions, we have shown that the prolonged occupancy by Amynthas was associated with decreased soil C/N, enhanced carbon mineralization rates, enhanced metabolism of multiple carbon sources, and altered microbial community composition. We have shown that two Amynthas species within the pheremtimoid invasion complex differ in gut and cast-associated microbial community composition. Furthermore, we have also demonstrated that Amynthas exert direct
Conclusions
In summation, our overall conclusions are that the prolonged invasion of Amynthas species alters the soil total nitrogen, C/N, carbon respiration, carbon metabolic profiles, and bacterial and fungal communities of soils previously invaded by Lumbricus species. We also found that Amynthas exhibited species-specific effects on gut and cast associated microbial communities, overall suggesting that Amynthas species drive changes in forest soils that are unique from other earthworm species and that
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
I would like to thank my wife Suzanne for sacrificing her weekends to help sample earthworms with me and Alex Kent and Lora Cheng for helping to process samples. I would also like to thank Tricia Bethke, Christopher Evans, and Martha Hellander for their advice, feedback, and expertise. This work was supported through the USDA National Institute of Food and Agriculture by the Hatch Act of 1887 project accession no. 1004684, AFRI Foundational and Applied Science Program grant no. 2018-67013-27537
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