CHEMICAL WEATHERING BY ECTOMYCORRHIZAL FUNGI UNDER WATER AND NUTRIENT-LIMITED CONDITIONS

Ectomycorrhizal fungi form symbiotic relationships with plants to exchange energy and nutrients. In response to water and nutrient limitations of the host plant, fungi chemically weather soil minerals, mobilizing the ions of necessary non-nitrogen nutrients, such as Fe, Ca, K, Mg, and P. Ponderosa pine forests are projected to become increasingly water-limited as a result of current climate change trends. This experiment seeks to characterize the effect that moisture limitation may have on mineral weathering by fungi associated with Ponderosa pine.

In a laboratory setting, water availability was regulated using 40, 60, and 80 percent relative humidity levels. Three fungal species were isolated from Ponderosa pine roots in Flagstaff, AZ. In the experiment, two strains of Suillus kaibabensis (S1 and S2) and one strain of Hebeloma crustilinforme (H1) were tested under sterile conditions. The non-nitrogen nutrients were provided by mineral plugs of biotite, apatite, and anorthite, while quartz served as a control in two agar mediums; with and without added cations. Each treatment was replicated five times and the experiment ran for ten weeks. Fungal growth was documented with digital images and colony diameter measurements. At the end of the 10 weeks, fungi and mineral samples were collected for analysis. Minerals were examined with scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy for surface morphology, fungal attachment, and chemical changes.

While three fungi were tested, only S2 strain data is shown. The largest fungal diameter was observed at 40% humidity, with the smallest diameter being recorded at 80% humidity. This trend of increased colonization with decreased humidity was observed regardless of the nutrient condition of the agar medium. Chemical changes are difficult to document over a short period, but all treatments showed hyphal attachment and mineral alterations. Biotite was weathered to the highest degree, followed by apatite and anorthite, with quartz being weathered the least. Similar results are expected from the other two fungal strains. This data will be used to help establish a future field study that investigates interactions between water and nutrient acquisition in increasingly moisture-limited Ponderosa pine forests.