Direction and magnitude of plant-soil feedbacks for native and invasive prairie grasses subjected to herbivory

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Heath McDonald, Eric Duell and Gail Wilson, Natural Resource Ecology and Management, Oklahoma State University, Stillwater, OK

Background/Question/Methods
Plant-soil feedback (PSF) is described as a two-step process; first, the population of plants within a system alter the soil microbial communities and secondly, this alteration then affects the growth and composition of future communities. Alterations to PSFs by non-native grasses can facilitate and strengthen biological invasions, however, non-natives may have little effect on PSFs. We conducted a greenhouse study in which 2 warm-season grasses and and 2 cool-season grasses were selected. Native Schizachyrium scoparium and Pascopyrum smithii were paired with non-native Bothriochloa ischaemum and Bromus inermis, respectively. This experiment was conducted in two phases: 1) soil conditioning phase and 2) experimental phase. During the soil conditioning phase, half of the plants were assigned a clipping treatment and half were left as a non-clipped control. Soil from the conditioning phase was then used as inoculum in the experimental phase. My objectives were to assess PSFs of common grass invaders of North American grasslands and determine the direction and strength of PSF dynamics, and examine the role of grazing (clipping), a common biotic stressor, in influencing PSF direction and magnitude.
Results/Conclusions
In the absence of simulated herbivory, native Schizachyrium scoparium and Pascopyrum smithii produced significantly greater biomass in soil conditioned by heterospecifics, leading to negative PSFs (self-limiting; promoting coexistence and community-level plant diversity). Conversely, non-native Bothriochloa ischaemum and Bromus inermis produced significantly greater biomass in soil conditioned by non-clipped conspecifics, thereby generating positive PSFs (self-promoting; promoting dominance and community-level monocultures). Notably, native grasses displayed neutral PSF in the presence of simulated herbivory, with both species producing similar biomass in soil trained by “grazed” conspecifics, as well as soil trained by “grazed” heterospecifics. Similarly, when conditioning species were subjected to simulated herbivory, PSFs created by both non-native invasive species were only slightly positive, suggesting that the pressure of “grazing” reduces the PSF effects of both native and non-native species by driving the direction closer towards a state of neutrality. Our results further our understanding of the intricate belowground mechanisms that help facilitate conspecific stands of non-natives and may influence future management decisions in mitigating non-native grass invasions in grassland systems.