Soil mineral surfaces linked to key atmospheric processes

Pictured are, from left, are David Bish, Melissa Donaldson and Jonathan Raff. Credit: Indiana University

Research by Indiana University scientists finds that soil may be a significant and underappreciated source of nitrous acid, a chemical that plays a pivotal role in atmospheric processes such as the formation of smog and determining the lifetime of greenhouse gases.

The study shows for the first time that the surface acidity of common minerals found in soil determines whether the gas nitrous acid will be released into the atmosphere. The finding could contribute to improved models for understanding and controlling air pollution, a significant public health concern.

"We find that the surfaces of minerals in the soil can be much more acidic than the overall pH of the soil would suggest," said Jonathan Raff, assistant professor in the School of Public and Environmental Affairs and Department of Chemistry. "It's the acidity of the soil minerals that acts as a knob or a control lever, and that determines whether nitrous acid outgasses from soil or remains as nitrite."

The article, "Soil surface acidity plays a determining role in the atmospheric-terrestrial exchange of nitrous acid," will be published this week in the journal Proceedings of the National Academy of Sciences. Melissa A. Donaldson, a Ph.D. student in the School of Public and Environmental Affairs, is the lead author. Co-authors are Raff and David L. Bish, the Haydn Murray Chair of Applied Clay Mineralogy in the Department of Geological Sciences.

Nitrous acid, or HONO, plays a key role in regulating atmospheric processes. Sunlight causes it to break down into nitric oxide and the hydroxyl radical, OH. The latter controls the atmospheric lifetime of gases important to air quality and climate change and initiates the chemistry leading to the formation of ground-level ozone, a primary component of smog.
Scientists have known about the nitrous acid's role in air pollution for 40 years, but they haven't fully understood how it is produced and destroyed or how it interacts with other substances, because HONO is unstable and difficult to measure.

"Only in the last 10 years have we had the technology to study nitrous acid under environmentally relevant conditions," Raff said.

Recent studies have shown nitrous acid to be emitted from soil in many locations. But this was unexpected because, according to basic chemistry, the reactions that release nitrous acid should take place only in extremely acidic soils, typically found in rain forests or the taiga of North America and Eurasia.

The standard method to determine the acidity of soil is to mix bulk soil with water and measure the overall pH. But the IU researchers show that the crucial factor is not overall pH but the acidity at the surface of soil minerals, especially iron oxides and aluminum oxides. At the molecular level, the water adsorbed directly to these minerals is unusually acidic and facilitates the conversion of nitrite in the soil to nitrous acid, which then volatilizes.

"With the traditional approach of calculating soil pH, we were severely underestimating nitrous acid emissions from soil," Raff said. "I think the source is going to turn out to be more important than was previously imagined."

The research was carried out using soil from a farm field near Columbus, Ind. But aluminum and iron oxides are ubiquitous in soil, and the researchers say the results suggest that about 70 percent of Earth's soils could be sources of nitrous acid.

Ultimately, the research will contribute to a better understanding of how nitrous acid is produced and how it affects atmospheric processes. That in turn will improve the computer models used by the U.S. Environmental Protection Agency and other regulatory agencies to control air pollution, which the World Health Organization estimates contributes to 7 million premature deaths annually.

"With improved models, policymakers can make better judgments about the costs and benefits of regulations," Raff said. "If we don't get the chemistry right, we're not going to get the right answers to our policy questions regarding air pollution."

Source: Indiana University

Live fast, die young: Soil microbes in a warmer world

Aerial view of the Northern Minnesota landscape including numerous conifer peatlands, deciduous uplands and lakes. Credit: USDA Forest Service Northern Research Station

Warmer temperatures shorten the lifespan of soil microbes and this may affect soil carbon storage, according to a new NSF-funded study published in Nature Climate Change this week.

A research team led by Shannon Hagerty and Paul Dijkstra from Northern Arizona University measured two key characteristics of soil microbes that determine their role in the soil carbon cycle: how efficiently they use carbon to grow and how long they live. "Higher temperatures make microbes grow faster, but they also die faster," said Hagerty, who conducted the research as part of her master's degree and was lead author on the study.

Soil microbes consume organic carbon compounds in soil, use some of it to make more microbes and release the rest to the atmosphere as carbon dioxide. The efficiency with which microbes use their food to make new microbes affects how much carbon remains in soil, and how much is released back to the atmosphere. The accepted idea before this study was that microbes would become less efficient at warmer temperatures.

The scientists incubated soil from a peatland and a forest in Minnesota at different temperatures and measured the efficiency with which microbes grew. They used a new method to measure microbial efficiency: they added small amounts of sugar and tracked how individual atoms in this sugar were turned into carbon dioxide.

"Microbes process sugars in similar ways as we do," says Paul Dijkstra. "We know very well how these processes work in laboratory studies, and can predict which carbon atoms in sugar molecules end up as carbon dioxide, and which are used to build new microbes. We applied this knowledge to the microbes living in soil."

The researchers found, contrary to expectation, that temperature had no effect on how microbes utilized their food, but instead boosted microbial death. "We don't yet know why microbes are dying faster at higher temperatures. Maybe they are eaten by nematodes or mites, or they die because of viruses," said Hagerty. "We need to know more about how temperature affects microbial death."

To explore what these new findings could mean for soil carbon storage in a warming world, the team compared output from a soil model that includes the effect of temperature on microbial lifespan to models unaffected by temperature change. "Models are used to predict how soil processes change, for example, in response to climate change," said Steve Allison, coauthor from the University of California, Irvine. "If we want to predict the future correctly, we'd better use models that accurately describe these microbial processes."

Including a temperature-dependent lifespan to the model increased the amount of carbon retained in soils at warmer temperatures compared to estimates from traditional models. The study concludes that incorporating this new insight into soil models will improve our understanding of how soils influence atmospheric carbon dioxide levels and global climate.

Does this mean that with climate change, more carbon will stay in the soil? "Too early to tell," said Bruce Hungate, Director of the Center for Ecosystem Science and Society at NAU. 

"The results suggest that the biochemistry of the microbes remains the same with warmer temperature, but that predation and death become more important. This laboratory study is just the first step, identifying a potential mechanism. Now we need to study how, in the real world, and in the long-term, the processes of biochemical efficiency and lifespan will change. And nobody has done that yet."

Source: Northern Arizona University

Improving forecasts for rain-on-snow flooding

Flooding in January 2009 closed a section of Interstate 5 south of Seattle.Washington State Dept. of Transportation Credit: Image courtesy of University of Washington

Many of the worst West Coast winter floods pack a double punch. Heavy rains and melting snow wash down the mountains together to breach riverbanks, wash out roads and flood buildings.

These events are unpredictable and difficult to forecast. Yet they will become more common as the planet warms and more winter precipitation falls as rain rather than snow.

University of Washington mountain hydrology experts are using the physics behind these events to better predict the risks.

"One of the main misconceptions is that either the rain falls and washes the snow away, or that heat from the rain is melting the snow," said Nicholas Wayand, a UW doctoral student in civil and environmental engineering. He will present his research Dec. 18 at the annual meeting of the American Geophysical Union.

Most of the largest floods on record in the western U.S. are associated with rain falling on snow. But it's not that the rain is melting or washing away the snow.

Instead, it's the warm, humid air surrounding the drops that is most to blame for the melting, Wayand said. Moisture in the air condenses on the cold snow just like water droplets form on a cold drink can. The energy released when the humid air condenses is absorbed by the snow. The other main reason is that rainstorms bring warmer air, and this air blows across the snow to melt its surface. His work support previous research showing that these processes provide 60 to 90 percent of the energy for melting.

Places that experience rain-on-snow flooding are cities on rivers that begin in the mountains, such as Sacramento, California, and Centralia, Washington. In the 1997 New Year's Day flood in Northern California, melting snow exacerbated flooding, which broke levees and caused millions of dollars in damage. The biggest recent rain-on-snow event in Washington was the 2009 flood in the Snoqualmie basin. And the Calgary flood in summer of 2013 included snow from the Canadian Rockies that caused rivers to overflow their banks.
The UW researchers developed a model by recreating the 10 worst rain-on-snow flooding events between 1980 and 2008 in three regions: the Snoqualmie basin in Washington state, the upper San Joaquin basin in central California and the East North Fork of the Feather River basin in southern California.

Their results allow them to gauge the risks for any basin and any incoming storm. The three factors that matter most, they found, are the shape of the basin, the elevation of the rain-to-snow transition before and during the storm, and the amount of tree cover. Basins most vulnerable to snowmelt are treeless basins with a lot of area within the rain-snow transition zone, where the precipitation can fall as snow and then rain.

Trees reduce the risk of flooding because they slow the storm's winds.

"If you've ever been in a forest on a windy day, it's a lot calmer," Wayand said. That slows the energy transferred from condensation and from contact with warm air to the snowpack.
Simulations also show that meltwater accounted for up to about a quarter of the total flooding. That supports earlier research showing that snow is not the main contributor to rain-on-snow floods, but cannot be neglected since it adds water to an already heavy winter rainstorm.

The complexity of mountain weather also plays a role.

"The increase in precipitation with elevation is much greater than usual for some of these storms," said Jessica Lundquist, a UW associate professor of civil and environmental engineering. "Higher flows can result from heavier rainfall rates at higher elevations, rather than from snowmelt."

In related work, Lundquist's group has developed a tennis-ball snow sensor and is measuring growth and melt of the snowpack in the foothills east of Seattle. The scientists aim to better understand how changes in climate and forestry practices might affect municipal water supplies and flood risks.

Wayand and another student in the group have developed a high school curriculum for Seattle teachers to explain rain-on-snow events and the physics behind why they occur. They hope to begin teaching the curriculum sometime next year.

The other collaborator on the work being presented in San Francisco is Martyn Clark at the National Center for Atmospheric Research in Colorado.

Source: University of Washington

Groundwater patches play important role in forest health, water quality

This is Kevin McGuire, associate director of the Virginia Water Resources Research Center. Credit: Virginia Tech

Even during summer dry spells, some isolated patches of soil in forested watersheds remain waterlogged.

These patches act as hot spots of microbial activity that remove nitrogen from groundwater and return it to the atmosphere, researchers from several institutions, including Virginia Tech, report in a leading scientific journal.

The discovery provides insight into the health of a forest. Nitrogen is an important nutrient for plant growth and productivity, but in streams, it can be a pollutant.

"The importance of these fragmented patches of saturated soil and their role in the fate of nitrogen in forested watersheds has been underappreciated until recently," said Kevin McGuire, an associate director of the Virginia Water Resources Research Center based in Virginia Tech's College of Natural Resources and Environment, co-author of the article to be published in the Proceedings of the National Academy of Sciences.

"We were able to determine the importance of denitrification in patches of shallow groundwater, which have largely been overlooked control points for nitrogen loss from temperate forested watersheds," McGuire said.

Most nitrogen is deposited by rain. Temperate forests receive much larger inputs of nitrogen from the atmosphere than they export to streams. Once nitrogen leaves the forest in streams, it can become a water pollutant.

"In some ecosystems, there have been long-term declines in stream water export of nitrogen when inputs have remained elevated," said co-author Christine Goodale, an associate professor of ecology and evolutionary biology at Cornell University.

"Understanding the fate of this nitrogen has been a challenge because denitrification -- a gaseous loss of nitrogen to the atmosphere -- is notoriously difficult to measure," said co-author Peter Groffman, an expert on denitrification at the Cary Institute of Ecosystem Studies.

Denitrification removes nitrogen from water and can therefore improve water quality in downstream lakes and estuaries.

However, nitrogen is also an important nutrient for plant growth in the forest so removals of nitrogen by natural processes can reduce the productivity of the forest.
The research, led by Sarah Wexler while she was a postdoctoral associate in hydrology and stable isotope geochemistry at Cornell University, took place in the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire, where the atmosphere annually deposits five to seven pounds of nitrogen per acre.

The Hubbard Brook Experimental Forest is part of the National Science Foundation's Long Term Ecological Research Network.

McGuire, also an associate professor of hydrology in the Department of Forest Resources and Environmental Conservation, led another National Science Foundation-funded project at the site, which developed an organizing framework to describe and map variations of soil in the watershed that explain shallow groundwater occurrence and frequency.
Groundwater wells from this earlier study were used in the new research to monitor soils that may have had the right conditions to function as hot spots for denitrification.

At sites throughout the forest, the research team measured the presence of nitrate, a form of nitrogen that is highly mobile and reactive in the environment, determined whether the nitrate is a result of atmospheric deposition or microbial conversion, and discovered the nitrogen loss to the atmosphere.

"We were able to differentiate sources of nitrate and show that some of the nitrate was lost to the atmosphere by looking at nitrate at the atomic level, that is, at the isotopic composition of the nitrogen and oxygen in nitrate," said Wexler, who is now at the School of Environmental Sciences at the University of East Anglia in the United Kingdom. "The isotopic composition of nitrate provides a natural way to directly track the details of nitrogen cycling."

McGurie said, "Some work remains to be done, but the aim is to be able to develop a better sense of where and how nitrogen is processed in the environment and be in a position to predict how changes in climate, for example warmer and wetter conditions, affect nitrogen cycling and water quality in forested ecosystems."

Source: Virginia Tech

'Aquatic osteoporosis' jellifying lakes

A handful of Holopedium capsules which are replacing the water flea Daphnia due to declining calcium levels in many lakes.
Credit: Image courtesy of Queen's University

A plague of "aquatic osteoporosis" is spreading throughout many North American soft-water lakes due to declining calcium levels in the water and hindering the survival of some organisms, says new research from Queen's University.

Researchers from Queen's, working with colleagues from York University and the University of Cambridge, as well as other collaborators, have identified a biological shift in many temperate, soft-water lakes in response to declining calcium levels after prolonged periods of acid rain and timber harvesting. The reduced calcium availability is hindering the survival of aquatic organisms with high calcium requirements and promoting the growth of nutrient-poor, jelly-clad animals.

In the study, researchers looked at the microscopic organisms (~1 mm) Daphnia and Holopedium -- the latter whose size is greatly increased by its jelly capsule.

"Calcium is an essential nutrient for many lake-dwelling organisms, but concentrations have fallen so low in many lakes that keystone species can no longer survive," says Adam Jeziorski, one of the lead authors of the study and a postdoctoral fellow in the Department of Biology at Queen's.

The research team found that when calcium levels are low, the water flea Daphnia, which has high calcium requirements, becomes less abundant. Importantly, this keystone species is being replaced by its jelly-clad competitor, Holopedium.

"Conditions now favour animals better adapted to lower calcium levels, and these changes can have significant ecological and environmental repercussions," says Dr. Jeziorski.

Tiny fossils from lake sediments were studied to determine the pre-impact conditions of the lakes as the calcium decline began before monitoring programs were in place. Using this technique, the team was able to examine the environmental trends from the past approximately 150 years.

"Lake sediments act like a history book of past changes in a lake, recording what happened before the problem was identified," says John Smol (Biology), Canada Research Chair in Environmental Change. "Jelly-clad invertebrates have been increasing in an alarming number of lakes. This is likely a long-term effect of acid rain on forest soils, logging and forest regrowth."

The increase in jelly-clad invertebrates can have important implications for lake biology, altering food webs, but can also clog water intakes.

"Many lakes we investigated have passed critical thresholds," says Dr. Smol. "We have been reduced to the role of spectator as these changes continue to unfold. Once again we see there are many unexpected consequences of our actions, most of which are negative."

This research was funded by the Natural Sciences and Engineering Research Council of Canada and the Ontario Ministry of the Environment and Climate Change.
The study is published in Proceedings of the Royal Society B.

Source: Queen's University

The Loss of biodiversity limits toxin degradation

You might not think of microbes when you consider biodiversity, but it turns out that even a moderate loss of less than 5% of soil microbes may compromise some key ecosystem functions and could lead to lower degradation of toxins in the environment.

Research published today in the SfAM journal, Environmental Microbiology, reports that without a rich diversity of soil bacteria, specialized functions such as the removal of pesticide residues are not as effective.

Dr Brajesh Singh of the University of Western Sydney led the work, he said "If the ability of the ecosystem to remove toxins from the environment is reduced, there will be higher toxicity risks in the environment and for non-target organisms, including humans, from agricultural chemicals. It is likely that these contaminants will remain at higher levels in surface and underground water, as well. It is vital to gain a better understanding of the extent to which soil bacteria are involved in the removal of contaminants."

The reasons for, and extent of, the decline in microbial diversity in agricultural soils is likely to be complex. The team has looked specifically at long-term heavy metal pollution where metals such as cadmium, zinc, and copper build up in the environment, usually as a result of industrial use. Another source is from digested sewage sludge, which is spread in agriculture fields to supply nutrients to crops and improve soil fertility; the sludge has historically contained some heavy metals, which can become concentrated in the soil.

Although the concentration of heavy metal used this study was higher than the current EU limit, this study has confirmed that long-term exposure to such contaminants does reduce the diversity of bacteria in the soil.

With the global population set to reach nine billion by 2050, we face a challenge to feed an extra two billion mouths using the same resources that we have at present. Crop losses to pests and disease account for a large percentage of under-production and so giving up pesticides will be difficult. Similarly, the use of sludge as a fertilizer is likely to become more prevalent. Research like this allows us to understand better how to use important agrichemicals and waste products in a sustainable way and so will contribute to future food and environmental security.

Source: Wiley

Are ants the answer to carbon dioxide sequestration?

A 25-year-long study provides the first quantitative measurement of in situ calcium-magnesium silicate mineral dissolution by ants, termites, tree roots, and bare ground.

A 25-year-long study published in Geology on 14 July provides the first quantitative measurement of in situ calcium-magnesium silicate mineral dissolution by ants, termites, tree roots, and bare ground. This study reveals that ants are one of the most powerful biological agents of mineral decay yet observed. It may be that an understanding of the geobiology of ant-mineral interactions might offer a line of research on how to "geoengineer" accelerated CO2 consumption by Ca-Mg silicates.

Researcher Ronald Dorn of Arizona State University writes that over geological timescales, the dissolution of calcium (Ca) and magnesium (Mg) bearing silicates has led to the graduate drawdown of atmospheric carbon dioxide (CO2) through the accumulation of limestone and dolomite. Many contemporary efforts to sequester CO2 involve burial, with some negative environmental consequences.

Dorn suggests that, given that ant nests as a whole enhance abiotic rates of Ca-Mg dissolution by two orders of magnitude (via biologically enhanced weathering), future research leading to the isolation of ant-based enhancement process could lead to further acceleration. If ant-based enhancement could reach 100 times or greater, he writes, this process might be able to geo-engineer sequestration of CO2 from the atmosphere. Similarly, ants might also provide clues on geoengineering efficient pathways of calcium carbonate precipitation to sequester atmospheric CO2.

Earth's climate has cooled significantly over the past 65 m.y., likely from hydrologic regulation, vegetation change, and interactions related to tectonism, in part mediated by Ca-Mg silicate mineral dissolution that draws down CO2. Although speculative, says Dorn, the timing of the expansion in the variety and number of ants in the Paleogene and the Neogene suggests that biologically enhanced weathering by ants could potentially be a part of the puzzle of Cenozoic cooling.

Source: Geological Society of America