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SBI Research Feature: Julie Pett-Ridge, new SBI faculty member in soil science
Posted: November 5, 2009
Dr. Julie Pett-Ridge became an assistant professor in the OSU Department of Crop and Soil Science in July 2009. Her position is partially funded by the Subsurface Biosphere Initiative. In this Web interview, Julie describes her research focus and introduces the expertise she brings to campus. Please contact Julie if you would like to learn more about her research and teaching.
Your research focuses on chemical weathering – what is it and what drives your interest in the topic?
Chemical weathering is the breakdown of rock as it interacts with air and water. One of the big reasons why I’m interested in chemical weathering is because it plays a key a role in the global carbon cycle. Rainwater naturally absorbs carbon dioxide in air to make carbonic acid. When carbonic acid reacts with silicate rocks, it releases cations and bicarbonate ions which are carried downstream into the oceans. Ultimately, carbonate rocks form on the ocean floor and there is a net removal of carbon dioxide from the atmosphere. By removing carbon dioxide from the atmosphere, chemical weathering controls global climate over geologic timescales.
I got interested in chemical weathering as an undergraduate when a professor explained this link. At the time, many earth scientists were excited about a hypothesis that the uplift of the Himalayas drove the planet into the last round of ice ages. When the warm, wet air coming over the Indian Ocean slams into the wall of rock created by the Himalayan uplift, it creates an enormous amount of rainfall – the monsoon system. The monsoon rainfall – reacting with the large area of silicate rocks uplifted in the Himalaya – creates high chemical weathering rates and that draws carbon dioxide out of the atmosphere and leads to its sequestration in the ocean. At that time, the hypothesis was that the accelerated chemical weathering from the tectonic uplift of the Himalaya caused late Cenozoic cooling of the Earth. Since then, alternative hypotheses to explain Cenozoic climate change have been proposed, but we are still trying to figure out good ways to test these ideas about chemical weathering and the carbon cycle.
Chemical weathering in soils is also a supply of nutrients to ecosystems. So that’s the other side of why I’m interested – I am trying to understand the controls on the supply of the main rock derived nutrients such as calcium, phosphorous and potassium.
What are some of your main research questions?
We don’t have a very good way of measuring chemical weathering rates. The weathering rate estimates we have rely on a number of assumptions which are not well-tested. A major goal of my research is to be able to quantify, for example, the mathematical relationship between chemical weathering rates and variables such as precipitation, topography, vegetation, or erosion. If we can quantify these controls, we can fundamentally improve our understanding of the connections between geology, long-term climate change, soils, and ecosystems.
Another research question I’m working on is quantifying mineral aerosol (dust) deposition to soils. Dust deposition fluxes are notoriously difficult to quantify accurately, but this is a necessary first step in evaluating the role of dust as a geochemical and biogeochemical pathway. For example, in my research in Puerto Rico, I used isotopes to quantify the deposition rate of Saharan dust from Africa, which in turn allows me to investigate the importance of that flux as a nutrient source to the forests in Puerto Rico.
How do you use isotopes in your research?
Isotopes are the way that I tackle these questions – they are my primary toolbox. With mass spectrometers, we are measuring small differences in isotope ratios, or the relative abundances of the different isotopes of an element. In some cases, the isotopic ratio is a signature of where that element has been processed – for example, has it been biologically cycled through plants? There are other isotopic systems where the ratio we measure is unique to a particular source so it tells us where the material came from. In Puerto Rico I used strontium isotope signatures to show that a significant component of the soil was actually African dust.
One of the main areas of my research areas right now is developing uranium and thorium isotopes as a weathering tracer. The fractionation for these isotopes is induced by chemical weathering. That fractionation, or ‘disequilibrium,’ can be used to quantify the timescale of weathering for soil or sediment because the radioactive decay of these isotopes provides a chronometer. I am also starting to use nontraditional stable isotopes, like iron, molybdenum and chromium.
Only recently has the mass spectrometry technology gotten to the point where we can measure these isotopes and it opens up a lot of new possibilities. For example, iron isotopes can be used to better understand redox cycling in soils, which in turn helps us understand soil microbial processes, phosphorus availability, and potential contaminant transport in soils.
What types of settings do you work in?
My main approach is going out to study natural systems– and particularly taking advantage of watershed studies like the H. J. Andrews Experimental Forest where you can do a mass balance calculation – all the precipitation and weathering inputs coming in, the biological cycling, and the outputs coming out the stream.
I also work on soil gradient studies. For example in Hawaii I am working on a three dimensional soil gradient study. One dimension is a chrono-sequence where the age of the soils varies based on the age of the underlying lava flow, but the other important soil forming variables, for example climate, parent material and slope remain constant. A second dimension is a climate sequence, where only precipitation varies. In the third dimension I look at a soil toposequence from a flat, stable ridge, to a shoulder with steep slopes and high erosion rates to the toe slope at the bottom where material is accumulating. This study design allows me to independently investigate the effects of time, climate and erosion on soil formation.
What is the link between your research and the subsurface biosphere?
Even though I look at soils from a geochemical perspective, I assume that the microbes are mediating all the soil forming reactions. It has been widely observed that the abundance of microbes is highest in surface of soils, and that it drops off in saprolite – the rotten rock at the base of the soil profile. But interestingly, a handful of studies have shown that when you examine the deepest part of the profile, where fresh rock is weathering into saprolite, microbial abundance increases significantly again. So even though it may be 20 or 30 feet below the surface, the weathering reactions are happening with the help of microbes. Additionally, there are many other ways that the subsurface biosphere affects chemical weathering in soils, such as simply producing carbon dioxide from respiration, and by the production of organic acids and chelates by roots and their symbionts.
What courses are you going to teach?
Principles of Soil Science (CSS 305) is going to be my primary undergraduate level course. I am also teaching the graduate student version of that course, Properties, Processes, and Functions of Soils (CSS 513). I will also teach another graduate level class.
What is exciting about being at OSU and in Corvallis?
Corvallis is a beautiful place to be with great access to outdoor activities. OSU’s faculty is very strong in soil science and the subsurface biosphere area and in ecology and geoscience as well. I can see a lot of people in multiple departments across campus that I want to collaborate with. The College of Oceanic and Atmospheric Sciences is also big draw for me – they have a lot of cutting-edge geochemistry research and they have the isotopic geochemistry lab that I’m going to be using. The lab is called the W. M. Keck Collaboratory for Plasma Spectrometry. It is called a collaboratory to emphasize that it’s open to the larger community of faculty at OSU and affiliated institutions.
Link to Julie Pett-Ridge's faculty Web page.
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