Plants and microorganisms modify the local weathering environment by exuding organic compounds, CO2 and protons, altering the composition of geomedia surfaces, redirecting soilwater and solute fluxes, and immobilizing lithogenic nutrients. The goal of this study is to better understand the effects of plants on weathering and denudation. The PIs propose to measure how plant-microbe interactions affect the initial weathering of four distinct rock types (basalt, granite, schist, and rhyolite) and the extent to which this weathering results in chemical denudation versus biomass accumulation or re-precipitation of dissolution products. Two types of higher plants will be used in a replicated plant-microbe-rock mesocosms experiment: a grass species (with vesicular arbuscular mycorrhizal association) and a tree species (with ectomycorrhizal association). The experiments will also include plant-free (but microbially-colonized) and abiotic (sterile) controls. One hypothesis is that the presence of plants will increase weathering but decrease denudation (i.e., we will see more mineral transformation but less element loss from system in the presence versus absence of plants). This implies important feedbacks to soil fertility and C sequestration. A series of environmentally-controlled, greenhouse experiments will be conducted that involve measuring plant uptake, mineral transformation and chemical denudation in basalt, granite, rhyolite, and schist, as affected by presence and growth of microbiota and vascular plants. Weathering will be estimated based on denudation, bio-uptake, sorption, and secondary mineral precipitation, each of which will be quantified by detailed characterization of biomass, aqueous solution and effluxes, and solid phase changes. Experimental results will be used to parameterize a biogeochemical model.
Broader Impacts: This project will enable the training of two collaborating Ph.D. students whose thesis research will focus on the vibrant interface between biogeochemistry, microbial ecology, and plant physiological ecology. Research will be closely coordinated with broader efforts associated with the National Critical Zone Observatory (CZO) system, and also with Biosphere2 (B2), a unique research instrument now managed by the University of Arizona. Conceptual and practical linkages with B2 and CZO will place the well-controlled experiments into a broader context of parallel field and "macrocosm" studies that are unable to address these questions unambiguously. Interaction with B2 and CZO will also create a collaborative environment where the Ph.D. students will be required to integrate their mechanistic findings into an evolving understanding manifested at larger (pedon to hillslope to catchment) scales. B2, which has fifty thousand visitors a year, will provide a compelling environment and powerful venue for research translation to the general public.
Plants obtain mineral nutrients from soils, where they come from dissolution of the rocks. This project was looking at how biological organisms, including plants and microorganisms influence dissolution of rocks and formation of soils. We wanted to know how interactions between plants and microorganisms influence amount of elements released from the rock and distribution of released elements between water that would drain into rivers and streams, plant biomass and newly formed minerals. We used four ground rocks: basalt, rhyolite, granite, and schist. Basalt was collected from Miriam Crater (Flagstaff, AZ, USA), rhyolite from Valles Caldera National Preserve (Jemez Springs, NM, USA), granite from W. Santa Catalina Mountains and schist from E. Santa Catalina Mountains (Coronado National Forest, Tucson, AZ, USA). Biological treatments included two plants, Buffalo grass (Bouteloua dactyloides) and Ponderosa pine (Pinus ponderosa), grown with and without mycorrhizal fungi which can enhance nutrient uptake and rock dissolution; microbial treatment containing microbial community collected from basalt in natural environment, and control without living organisms. Planted treatments also had basalt microbial community. Rock with plants and microorganisms was placed in 30 cm long and 5 cm in diameter Plexiglas columns inside transparent boxes to prevent outside contamination. Columns were regularly watered and solution collected to determine how much of the dissolved elements leached with the drainage. About every 4 month columns were removed to evaluate plant growth and nutrient uptake and changes to the chemical and mineralogical composition of the rock. Since we used purified water for irrigation and filtered air, all other elements in the water and plants came from rock dissolution. In general, it was observed that both microbes alone and plants in association with microbial communities increase element mobilization compared to control. Over time differences between treatments became more pronounced signifying cumulative biota effect over time that is important part of soil formation. We found that there were large individual differences in effect of biota between elements due to their chemical properties and biological importance, and between rocks related to different chemical and mineralogical composition of the rocks. Plants developed large root systems and for some elements, such as P, Fe, Mn, Al, and Ti, plant biomass presented a greater pool of mobilized metals than soil solution indicating that monitoring drainage concentrations in streams and rivers (a preferred current method due to easier access to these samples) may not allow accurately determining element mobilization from the rock due to biota. Presence of mycorrhiza influenced metal concentrations both in solution and in the plants. Over two years of the experiment rocks that had only sand-sized particles (250-500 μm) in the beginning as a result of weathering processes developed a measurable amount of finer material, an integral part of soil formation. Composition of easily available pools of metals in the soil also changed during experiment. This work used the same rocks and thematically is closely related to two larger-scale inter-disciplinary earth science projects currently underway, the Santa Catalina – Jemez Critical Zone Observatory (SCM-JRB CZO, http://criticalzone.org/catalina-jemez/) and the Landscape Evolution Observatory at UA’s Biosphere 2 (LEO, http://leo.b2science.org/). Findings from this laboratory-based research on element cycling, weathering rates, plant growth and incipient soil formation are being applied to help interpret the field and ‘macrocosm’ scale studies being conducted at the CZO and LEO. Project provided opportunities for research training to two postdoctoral scientists and a number of undergraduate students drawn from populations underrepresented in science. Postdoctoral scientists gained experience in highly integrated area of Earth sciences, learning techniques and methods in geochemistry and mineralogy, microbial ecology and plant physiology. They also had ample opportunities for student mentoring and public outreach. Students learned technical aspects of their projects, but also more general professional skills in how to ask questions, plan research, summarize obtained data and present results, talk to the public about their work and its importance. Project location at Biosphere 2 on the tour route provided extensive outreach opportunities with close to 300 thousand people learning about the work and its implications for Earth sciences and society in general.
