How Does Fire Retardant Influence Arbuscular Mycorrhizal Fungi?
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Smoke from forest fires in Idaho has covered the ranch for the past month and made fieldwork difficult. Aerial firefighters drop retardants that contain high levels of nitrogen (N) and phosphorus (P). Both N and P are plant nutrients that can promote growth, but high concentrations can kill plants and cause eutrophication of waterways. Previous research has shown that invasive cheatgrass (Bromus tectorum) increases in abundance and outcompetes native plants in the presence of fire retardant (Besaw et al. 2011). However, the effects of fire retardant on soil microbes are virtually unknown.
MPG Ranch worked with UM undergraduate Abigail Marshall to assess how fire retardants impact arbuscular mycorrhizal fungi (AMF). AMF are soil-dwelling mutualists that colonize plant roots and provide pathogen protection and nutrients in exchange for plant carbon. The high P levels in fire retardants can reduce AMF abundance and the services they provide. Abigail grew slender wheatgrass (Elymus trachycaulus) and blanket flower (Gaillardia aristata) with and without AMF and fire retardant in the greenhouse (Picture 1). Fire retardant increased growth in all plants, but the high concentration caused some burned tips. Preliminary data indicate reduced AMF colonization with retardant, especially in slender wheatgrass.
Picture 1. We tested the effect of fire retardant (FR) and arbuscular mycorrhizal fungi (AMF) on slender wheatgrass and blanket flower in a greenhouse experiment.
To relate greenhouse results to field conditions, Abigail and I chose an area that burned last year. We located sites with and without the orange retardant using Google Earth (Picture 2). Logs and rocks remained orange even one year after aerial drops (Picture 3). We sampled several native and exotic plants and will estimate mycorrhizal colonization of roots and measure soil nitrogen and phosphorus concentrations. This will tell us about long-term influences of fire retardants on plants and their associated soil microbes.
Picture 3. The orange fire retardant was still visible on rocks and logs, which made it easy to locate last year’s application areas (a). Abigail Marshall samples plants (b).
References
Besaw LM et al. 2011. Disturbance, resource pulses and invasion: short-term shifts in competitive effects, not growth responses, favor exotic annuals. Journal of Applied Ecology 48, 998-1006.
Smith, S.E. & Read, D.J. (2008). Mycorrhizal Symbiosis. Academic Press, Cambridge.
