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.
In agricultural practices, including forestry, the productivity has been maximized by the use of large quantities of fertilizers and pesticides. To produce the chemicals needed in these practices high levels of energy is consumed, mainly from fossil fuels that emit large amounts of carbon dioxide, increase ground- and surface water pollution and soil degradation all over the world. Part of the problem is a lack of fundamental understanding about mineral-derived nutrient dynamics in the root zone of plants (rhizosphere), the release of base cation nutrients by microbial activity including bacteria (biofilm) and fungi, and the storage and transport of these nutrients in soils and roots while maintaining high, but sustainable, plant production. The overarching concept of this study was that plant-driven weathering rates and mechanisms vary, depending on geologic setting and ecosystem phase; and here the focus is on a primary-successional setting, where plants must extract cation nutrients from soil minerals by chemical weathering processes. The PIs hypothesized that the plants adapt to the calcium and potassium limitations by developing rhizospheric biofilms that help to attach the fungal hyphal network to the mineral grains and micro-localize the biology, chemistry, and hydrology of weathering and nutrient uptake processes at the mineral-fungus-bacteria interface. Biofilm functions as a medium for the dissolution and transport processes and at the same time it isolates the bulk soil water from the mineral surfaces, thus increases efficiency of weathering and nutrient uptake. Specifically, varying degrees of calcium and potassium limitation influence biofilm development and weathering and nutrient uptake functions. To address these hypotheses a replicated column growth experiment with ectomycorrhizal fungus, bacteria and pine trees was set up and the components of this laboratory column growth experiment were investigated to study the interface, morphology of mineral surfaces, associated microbes and biofilm when the availability of calcium and potassium in bulk soil water and primary minerals were varied by manipulating irrigation solutions and initial mineral composition. Subsets of columns were destructively sampled after 3, 6 and 9 months of growth. Biotite (potassium source) and anorthite (calcium source) minerals were analyzed from the rhizosphere of each tree and also from the bulk material with helium ion microscopy (HeIM), various modes of scanning electron microscopy (SEM) equiped with energy dispersive x-ray spectroscopy (EDS), and transmission electron microscopy (TEM) also equipped with EDS. The collaborating team analyzed drainage and pore water chemistry, change in biomass and soil composition. This study have shown that symbiotic fungi can promote chemical dissolution as well as applying physical forces to "manufacture" fungi diameter, channel shaped features and that they take up base cation nutrients during this process, based on laboratory observations. On the other hand, abiotic processes can also contribute to similarly sized and shaped channels on biotite surfaces as seen in the abiotic controls, because mica is soft, easily scratched by sharp and hard objects such as sand grains and sampling tools. However, calcium and potassium stress did not promote biofilm formation and root development, which contradicts our hypotheses and under investigation. Also, there were no significant differences observed regarding biological coverage and etching patterns and percentages among the varying degree of calcium and potassium limited treatments. Calcium deficiency was not apparent, but K deficiency occurred in most experimental conditions and biotite minerals alone were not sufficient to sustain plant growth. Overall, the results supports that symbiotic relationship between plants, fungi and bacteria can facilitate increased weathering of mineral surfaces, but there are no clear relationship between the degree of supplied calcium and potassium, and biofilm development around roots and fungal hyphae. However, this study provided insight to better understand microbial attachment to mineral surfaces and the morphology of the mineral-fungi-bacteria interface. Ultimately, this type of work has a potential to provide information for improving plant nutrition and soil sustainability, and better understanding terrestrial and hydrospheric carbon sequestration.
