12/11/00

Dawn Levy, News Service (650) 725-1944; e-mail: dawnlevy@stanford.edu

Scientists track phosphate to better understand global warming

Shake a snow globe and clear skies turn to a blizzard in your hand. The Earth is a similarly closed system, with chemical reactions that occur in soil affecting chemical reactions that occur in skies and seas. Chemistry is destiny.

Advanced technologies and methods are helping scientists take a new look at one of the Earth's most abundant elements -- phosphorus -- to better understand how it cycles through soil, sea and living organisms. Phosphorus is an essential nutrient for all organisms including plants, which through photosynthesis remove carbon dioxide -- the greenhouse gas most responsible for global warming -- from the atmosphere and bind it into organic matter. Phosphorus also plays a key role in the cycles of such biologically essential elements as nitrogen, oxygen and sulfur. Because of analytical limitations, little was known about phosphorus cycling until recently.

"We're at the stage where we can make another jump in our understanding of phosphate cycling because of the availability of new technology and new methods that can be applied to natural systems," says Adina Paytan, an assistant professor in Stanford's Department of Geological and Environmental Sciences. Paytan is leading a special session Dec. 15 with Stanford postdoctoral scholar Barbara Cade-Menun at this year's San Francisco meeting of the American Geophysical Union (AGU), an international scientific society with more than 35,000 members dedicated to advancing the understanding of Earth and its environment.

The new technologies and methods allow scientists to characterize phosphorus compounds in water and soil, estimate how fast phosphate cycles through natural reservoirs, estimate long-term phosphorus burial in the oceans through geologic time, pinpoint phosphorus sources in different ecosystems, determine fluctuations in phosphate concentrations at certain times and places, and study the effects of phosphorus pollution in estuaries and lakes.

In recent years, AGU meetings, traditionally devoted to geology and geophysics, have increasingly included biology-related topics. This year's meeting, however, marks the first at which "biogeosciences" is an official heading. The new heading describes an interdisciplinary field that treats the Earth system as a whole and attempts to understand connections and interactions between the atmosphere (air), hydrosphere (water), biosphere (life) and lithosphere (rock). It also attempts to evaluate human impact on the system.

Phosphorus is crucial to life as a building block of nucleic acids, proteins and lipids. It is a component of adenosine triphosphate (ATP) and nicotinamid adenosine dinucleotide phosphate (NADP), molecules that transfer energy in living systems. In photosynthesis, plants use ATP and NADP to harness the sun's energy to convert atmospheric carbon dioxide and water into sugar and oxygen. Through photosynthesis, organisms as tiny as algae and as mighty as redwoods become part of the global "sink" that absorbs excess carbon dioxide. In ecosystems that are relatively unaffected by human activities, plant growth may be limited by the availability of phosphorus and/or nitrogen.

Most phosphorus is found in rocks, where it is bound in minerals. In these minerals it usually exists as phosphate -- that's one phosphorus atom bound to four oxygen atoms. Weathering releases phosphorus into soil, where it is utilized and recycled by plants and bacteria. The phosphorus eventually travels through streams to oceans and into marine sediments.

A "feast or famine" element, phosphorus is unequally distributed throughout the Earth, with too much in North American and European soils and some coastal estuaries and not enough in soils of the tropics, sub-Saharan Africa, South America and in the open ocean.

Though phosphorus is necessary, too much of a good thing can be bad. Excess phosphate derived from fertilizers, detergents and other human sources makes its way into lakes and coastal waters by runoff, leaching or erosion, causing massive algae blooms that can affect taste and clarity of drinking water. Moreover, when algae die, thick pads sink to the bottom and oxidize, reducing dissolved oxygen and creating an environment inhospitable to fish.

Too little phosphorus also is a problem. In old, weathered soils and in ocean ecosystems, plants have enough nitrate (another nutrient whose scarcity limits growth) but not enough phosphorus. Crop harvesting can deplete phosphorus from agricultural lands, and old, weathered soils hold onto phosphate tightly, reducing its availability to plants. Most phosphate fertilizer is derived from rock phosphate ­ a nonrenewable resource. "Organic fertilizers could replace this source, but we need to know the forms of phosphorus available and how quickly they turn over," says Cade-Menun, who studies phosphorus as it cycles through soils.

Cade-Menun studies phosphorus cycling in temperate forests including warm, dry woods of the Sierra Nevada and cool, wet rainforests of the Pacific Northwest. In addition to traditional analytic techniques, she uses nuclear magnetic resonance (NMR) spectroscopy to identify the kinds and amounts of phosphorus compounds that are distributed throughout forests. She also is collaborating with Paytan to adapt techniques for soils to ocean sediment trap samples to track which compounds degrade first, releasing their phosphorus back into the water and making it available to organisms.

"Unlike nitrogen, little is known about the role of specific organisms in soil phosphorus cycling," says Cade-Menun. "I think it would be exciting to link the phosphorus forms revealed by NMR spectroscopy back to the soil organisms producing them, and to link specific enzyme production to the transformation of these phosphorus forms."

Cade-Menun also is interested in the effect of land management practices in forestry, such as clear cutting and slash burning, on soil nutrient dynamics: "My Ph.D. work in the coastal forests of British Columbia showed that during slash burning, fire, as a strong mineralizing agent, converted much of the soil phosphorus from organic to inorganic forms. In this high-rainfall environment, where natural forest fires rarely occur, the phosphorus became occluded and unavailable to replanted trees, leading to phosphorus deficiencies despite high soil total phosphorus concentrations. But in drier ecosystems, such as the Sierra Nevada where fire regularly occurs, it may be an important means by which phosphorus is released back to plant-available forms."

While Cade-Menun studies land systems, Paytan, a marine biogeochemist, investigates the intimate interactions between the solid Earth and the oceans through biogeochemical cycles that affect global environment and climate.

Funded by the National Science Foundation, her work focuses on phosphorus cycles of both the present and the past. To study cycling in today's oceans, she boards research vessels during different seasons and collects water samples from various ocean regions and depths. She also obtains water samples from Professor Rob Dunbar (geological and environmental sciences) and Assistant Professor Kevin Arrigo (geophysics), faculty colleagues in Stanford's new ocean sciences program.

Studying the phosphate cycles of the past may yield valuable clues about the future. To learn how phosphorus cycled through oceans millions of years ago, Paytan boards research vessels with Monterey Bay Aquarium Research Institute scientists to collect samples of phosphate preserved in marine sediments. She also obtains samples from the core repository of the Ocean Drilling Program, an international program partially funded by the National Science Foundation. Postdoctoral scholar Kristina Faul will join Paytan's lab in the spring to further these studies.

"From the chemistry of marine sediments, you can tell a lot about the chemistry of seawater, about where substances precipitated, about the biology, about circulation," Paytan says. "All the information about past oceans is basically retrieved from the sediments. If you're looking at shorter time scales, of course you have other archives, such as coral, tree rings, ice cores and cave and lake deposits. But for the longer time scales you have to go to marine sediments."

Reading the chemical signatures of the sediments, Paytan tries to correlate phosphate burial in the ancient ocean with carbon dioxide levels in the ancient atmosphere. She would like to know, for example, if the weathering of the Himalayas, which potentially transferred lots of phosphate from continents to oceans, led to an increase in photosynthesis, and if that, in turn, helped reduce the levels of carbon dioxide in the atmosphere. By understanding the consequences of natural fluctuations in the ancient phosphorus cycle, Paytan hopes to gain an understanding of how similar fluctuations today might affect modern climate.

With Stanford graduate student Karen McLaughlin and researchers Carol Kendall and Steve Silva of the U.S. Geological Survey in Menlo Park, Calif., Paytan analyzes phosphates using advanced techniques, including oxygen isotope analysis. Isotopes are atoms of a chemical element with the same atomic number and nearly identical chemical behavior but with different atomic masses. Oxygen has several naturally occurring stable isotopes -- ¹⁶O, ¹⁷O and ¹⁸O. Because of the differences in mass, the individual isotopes of oxygen in phosphate tend to participate at different rates in biochemical reactions, resulting in isotopic fractionation (partitioning of the isotopes between reactants and products).

The phosphorus-oxygen bond is so strong that it is resistant to breakage over the range of the Earth's surface temperatures. But when a phosphorus-containing compound finds itself in a biological environment, such as a cell, enzymes easily mediate breakage of that bond, and the oxygen isotopes in the phosphate exchange with the oxygen in the surrounding water. This exchange is temperature dependent. That means scientists can look at changes in oxygen isotope ratios and gain information about phosphate cycling and the water temperature in which an ancient sea-dwelling organism lived. In terms of their isotopic ratios, organisms are not what they eat but rather what they drink, as well as the temperature of their drink.

Eventually, Paytan and Dr. Ken Caldeira at Lawrence Livermore National Laboratory would like to use lab and field data to build a model that describes phosphorus cycling in the ocean. And someday phosphorus scientists studying marine cycling may collaborate with those studying terrestrial cycling to create a "big picture" of planet-wide cycling of phosphorus and even linked elements, such as carbon and nitrogen.

This AGU session marks the first official meeting between marine and terrestrial phosphorus scientists. But this small step may lead to giant leaps in understanding that shape public policy in areas ranging from forestry to global warming.

A better scientific understanding of Earth's complex chemical cycles couldn't come too soon. Consider that on Nov. 25, 2000, negotiations broke down between officials trying to put the finishing touches on the Kyoto Protocol, a 1997 treaty drafted by more than 170 countries to cut greenhouse gases. A major reason: persistent disagreement over the role of trees and managed farmland as "sinks" to absorb carbon dioxide.

-30-

By Dawn Levy