Elemental and isotopic perspectives on the impact of arbuscular mycorrhizal and ectomycorrhizal fungi on mineral weathering across imposed geologic gradients

• Field experiments were conducted to assess arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) weathering.
• Mesh in-growth bags with different lithologies were deployed in temperate forest stands dominated by AM- or EcM-symbioses.
• Mg, P, and Ca contents and Pb and Sr isotopic compositions were measured in resulting leachates after 100-day deployment.
• Only Ca concentrations were significantly different between treatments, with AM plot bags yielding higher concentrations.
• Major element concentrations paired with isotopic ratios suggest similar dissolution reactions occurring in both sites.

Symbiotic mycorrhizal fungi are thought to play a role in weathering by targeting minerals that contain required plant nutrients, such as phosphorus (P), magnesium (Mg) and calcium (Ca). Field experiments coupled with geochemical analyses can provide insights into nutrient cycling in forest landscapes dominated by arbuscular-mycorrhizal (AM) and ectomycorrhizal (EcM) fungi. In-growth bags containing different rock types of varying nutrient content (granite, tonalite) were fabricated to create an in-situ geologic gradient for 100-day deployment in two forest stands dominated by different mycorrhizal symbioses in Hubbard Brook Experimental Forest in New Hampshire, USA. Mineral dissolution within the substrate bags under different biologic conditions was assessed via analysis of major elemental nutrients (Ca, Mg, and P) and isotope ratios of lead (Pb) and strontium (Sr) on leachates representing both the easily exchangeable and future available weatherable mineral stores. We hypothesized that the two types of mycorrhizal fungi would differ in their weathering ability and would show significant variations in tracers of mineral weathering from the same rock type in the two stand but only Ca concentrations in the exchangeable solution of the tonalite and the future weatherable stores solutions of both tonalite and granite varied significantly between the two fungal types. The other measured nutrient concentrations (Mg, P) and Pb and Sr isotopic compositions of the exchangeable and weathering solutions were not significantly different within lithology. Taken together, the results suggest that weathering fluxes in AM- and EcM-dominated forest stands may be more comparable than previously argued.