Intellectual merit: Recent advances across several fields set the stage for process-based research into the biogeochemical agency of vascular plants -- in particular, how their physiologies drive Earth?s ?weathering engine? to extract mineral matter from regolith to build soils, chemically denude the continents, and set the chemistry of the ocean / atmosphere on geologic timescales. Such research is timely and needed to interpret pedo-geologic records of global change, and to forecast the effects of terrestrial C sequestration on the global CO2 cycle and soil sustainability in a human-altered world. The PIs overarching concept is that plant-driven weathering rates and mechanisms vary, depending on geologic setting and ecosystem phase. They focused on primary-successional settings, where plants must extract nutrients from soils by chemical weathering. The premise is that a key adaptation of many plants to these conditions is development of mycorrhizospheric biofilms, which attach the root system to mineral surfaces and micro-localize the biology, chemistry, and hydrology of weathering and nutrient uptake at the root system-mineral interface. At this micron scale, dissolution and biological mass transfers occur over very small distances and in relative isolation from bulk soil water, thereby increasing macroscopic nutrient acquisition efficiency and decreasing nutrient loss in drainage. The central hypothesis is that varying degrees of nutrient limitation (i.e., the need to extract base cations from mineral sources) influence biofilm development and weathering/uptake function. To address this hypothesis, the PIs propose to use replicated ectomycorrhizal seedling systems in a growth experiment, and vary the availability of Ca and K in bulk soil water and primary minerals by manipulating irrigation solutions and initial mineral composition. This research will provide insights into the mechanisms that link micron-scale processes of mineral weathering to ecosystem-scale processes of nutrient acquisition and ultimately global-scale processes of continental denudation.
Broader Impact: Eight undergraduate students will work on this project. Because of the unique combination of researchers, students will be drawn from a community college, a four-year undergraduate college, and two research universities. The proposed research will foster a collaborative network of scientists that includes Pacific Northwest National Laboratory, the US Forest Service, the Agricultural Research Service, and academic institutions. The results will be disseminated to science networks including the Critical Zone Exploration Network and the Hubbard Brook Ecosystem Study, and introduced to the general public through teaching and learning modules designed for middle and high school classrooms. Ultimately, this work will serve as a foundation for improving plant nutrition and soil sustainability, and better understanding terrestrial and hydrospheric carbon sequestration.
Without mineral weathering, life as we know it could not exist. In nature, mineral weathering is the only source of key nutrients like calcium (Ca) and potassium (K) needed in every cell of all life in soils and the ecosystems built on those soils. Mineral weathering stores atmospheric carbon in soils, in fresh and ocean waters, and in rocks, thereby regulating Earth’s atmospheric greenhouse and making our planet habitable. Despite the fundamental importance of mineral weathering, much about the phenomenon remains poorly understood. In particular, although we know that rooted plants exchange photosynthetically fixed carbon and acid for mineral-derived nutrients (Figure 1) and thus drive the weathering process, we do not know why all these dissolved chemicals are not simply washed away as soils are flushed with percolating rainwater (hydrologic loss). Our concept is that microbial communities living symbiotically with plant roots, i.e. mycorrhizal communities, are the key. These microbes build a biofilm (slime) habitat on mineral surfaces and around attached rootlets (Figure 2), where they can feed on C exuded by the rootlets, grow and reproduce, and generate acids that dissolve the surface. Nutrients released by dissolution into the biofilm are then taken up directly by the rootlets. Thus our idea is that mycorrhizal biofilms make mineral weathering and nutrient uptake more efficient by microlocalizing and coupling them at root-mineral-microbe interfaces beneath biofilms. This also isolates and protects the chemical exchanges from hydrologic loss. In our research we have developed methods to control and monitor the growth of replicated seedlings of a selected tree species and their root systems (Figure 3). We have also developed methods to microscopically measure the development of microbes, biofilms, and weathering on mineral surfaces (Figure 4). To date we have found that trees without mycorrhizal symbioses acidify percolating soil water and develop severe K deficiencies which can be partially remedied by supplying K in water used to irrigate the plants. Our experiments also show that soil water percolating from trees with the symbioses is less acidified, and washes away less Ca and K. Our experiments lay conceptual and some of the technical groundwork for further research into how rooted plants and their microbial partners accumulate and protect nutrient capital for the construction of terrestrial ecosystems. This is practically important as chemical fertilizers become increasingly expensive in terms of fossil fuel requirements and pollution, and as we move toward sustainable agricultural systems based on nutrients from mineral sources. Although we did not consider this when we started, our research is also applicable to understanding how root-exuded carbon (Figure 1) is transformed into carbon compounds that do not break down quickly. These transformations are key to increasing the storage of carbon in soils, and are of considerable interest to a range of research partners including collaborators at the Pacific Northwest National Laboratories. The carbon transformations have long been suspected to be mediated by both microbes and mineral surfaces, and our methods can be used to study the processes.
