Soil Feedback of Exotic Savanna Grass Relates to Pathogen Absence and Mycorrhizal Selectivity

Abstract

Abstract. Enemy release of exotic plants from soil pathogens has been tested by examining plant-soil feedback effects in repetitive growth cycles. However, positive soil feedback may also be due to enhanced benefit from the local arbuscular mycorrhizal fungi (AMF). Few studies actually have tested pathogen effects, and none of them did so in arid savannas. In the Kalahari savanna in Botswana, we compared the soil feedback of the exotic grass Cenchrus biflorus with that of two dominant native grasses, Eragrostis lehmanniana and Aristida meridionalis. The exotic grass had neutral to positive soil feedback, whereas both native grasses showed neutral to negative feedback effects.Isolation and testing of root-inhabiting fungi of E. lehmanniana yielded two host-specific pathogens that did not influence the exotic C. biflorus or the other native grass, A. meridionalis. None of the grasses was affected by the fungi that were isolated from the roots of the exotic C. biflorus. We isolated and compared the AMF community of the native and exotic grasses by polymerase chain reaction-denaturing gradient gel elecrophoresis (PCR-DGGE), targeting AMF 18S rRNA. We used roots from monospecific field stands and from plants grown in pots with mixtures of soils from the monospecific field stands. Three-quarters of the root samples of the exotic grass had two nearly identical sequences, showing 99% similarity with Glomus versiforme. The two native grasses were also associated with distinct bands, but each of these bands occurred in only a fraction of the root samples. The native grasses contained a higher diversity of AMF bands than the exotic grass. Canonical correspondence analyses of the AMF band patterns revealed almost as much difference between the native and exotic grasses as between the native grasses.In conclusion, our results support the hypothesis that release from soil-borne enemies may facilitate local abundance of exotic plants, and we provide the first evidence that these processes may occur in arid savanna ecosystems. Pathogenicity tests implicated the involvement of soil pathogens in the soil feedback responses, and further studies should reveal the functional consequences of the observed high infection with a low diversity of AMF in the roots of exotic plants.

Facilitation and inhibition: changes in plant nitrogen and secondary metabolites mediate interactions between above-ground and below-ground herbivores

“…We transplanted seven-week-old seedlings individually into pots (16 cm height, 21 cm diameter) containing growing medium (50% topsoil and 50% sphagnum peat moss) that were arranged in an unheated greenhouse. To reduce potential adverse effects of host-specific soil biota, we collected topsoil from fields where no Triadica grew [25][26][27]. Subsequently, we enclosed each seedling in a nylon mesh cage (100 cm height, 27 cm diameter) that was fitted tightly to the rim of each pot to exclude insect herbivores.…”

Role and influence of soil microbial communities on plant invasion

“…Numerous soil microbes have been isolated from invasive plant species (Table 1). Soil biota can influence the alien plant invasion through direct and indirect mechanisms (Reinhart & Callaway, 2006;van der Putten et al, 2007). Assessment of shifts in the microbial communities and their subsequent influence on the competition between native and invasive plant species could provide a better understanding of post-invasion changes (Hawkes et al, 2005;Zhang et al, 2010).…”

“…The influence of nematodes feeding on an invasive grass A arenaria showed that plant roots were colonized by feeding generalist and not by feeding specialist in the non-native regions (van der Putten et al, 2005). The invasive soil pathogens that affect the natural vegetation have been mostly reported from the forest and ornamental plants.…”

“…Positive feedback promotes intraspecific competition and predominance of plants due to the presence of symbiotic mutualists that enhance plant growth thus, also increasing interspecific competition (Bever, 2003). However, the negative feedback regulates and maintains not only the plant abundance but also the coexistence of other plant species (van der Putten et al, 1993;Mangan et al, 2010). The soil biota associating with Centaurea maculosa Lam., have greater negative effects on the plant growth when grown on native European soils in the absence of competitor.…”

Investigation of fungal root colonizers of the invasive plant Vincetoxicum rossicum and co-occurring local native plants in a field and woodland area in Southern Ontario

“…Plants associating with arbuscular mycorrhizal fungi are typically more competitive and better able to tolerate environmental stresses than are non-mycorrhizal or poorly colonized plants (Biermann and Linderman 1983, Daniell et al 2001, Bianciotto and Bonfante 2002, Vandenkoornhuyse et al 2002, Bonfante 2003, Brundrett 2004, DeBellis and Widden 2006, Greipsson and DiTommaso 2006, Parniske 2008, Shah et al 2010. AMF are also known to provide protection from a variety of microbial pathogens to their host plants (Newsham et al 1994, Borowicz 2001, Klironomos 2002, Pozo and Azćon-Aguilar 2007, Van der Putten et al 2007, Appoloni et al 2008, Mogg et al 2008, Krüger et al 2009, Wehner et al 2009). …”