California’s crops are creating some noxious air.

The Golden State is at the vanguard in the United States in reducing auto emissions of nitrogen oxide gases, which help produce toxic smog and acid rain. But the NOx pollution problem isn’t limited to auto exhaust. California’s vast agricultural lands — particularly soils heavily treated with nitrogen fertilizers — are now responsible for as much as 51 percent of total NOx emissions across the state, researchers report January 31 in Science Advances.
The catchall term “NOx gases” generally refers to two pollution-promoting gases: nitric oxide, or NO, and nitrogen dioxide, or NO2. Those gases react with incoming sunlight to produce ozone in the troposphere, the lowest layer of the atmosphere. At high levels, tropospheric ozone can cause respiratory problems from asthma to emphysema.

Between 2005 and 2008, regulations issued by the California Air Resources Board on transportation exhaust reduced NOx levels in cities such as Los Angeles, San Francisco and Sacramento by 9 percent per year. However, the U.S. Environmental Protection Agency has increasingly recognized nitrogen fertilizer use as a significant source of NOx gases to the atmosphere.

NOx gases are produced in oxygen-poor soils when microbes break apart nitrogen compounds in the fertilizer, a process called denitrification. The release of those gases from fertilized soils increases at high temperatures due to increased microbial activity, says Darrel Jenerette, an ecologist at the University of California, Riverside, who was not involved in the new study.

Jenerette and others have studied local NOx emissions from soils in California, but no statewide assessment existed. So Maya Almaraz, an ecologist at the University of California, Davis, and her colleagues designed a study to examine the question — both from above and below.
Using a plane equipped with scientific instruments including a chemiluminescence analyzer to detect NOx gases in the atmosphere, the researchers measured the concentrations of the gases above the San Joaquin Valley, an area of California’s fertile Central Valley, over six days at the end of July and beginning of August. The team also simulated NOx emissions from soils across the state, using the San Joaquin Valley data to ensure that the simulations gave accurate results. Finally, the researchers compared those data with nitrogen fertilizer inputs, as estimated by crop type and U.S. Department of Agriculture fertilizer consumption data.

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Croplands are contributing 20 to 51 percent of the total NOx in California’s air, Almaraz’s team reports. In the simulations, those soil emissions were particularly sensitive to two factors: climate, especially temperature, and rates of nitrogen input. That findings suggests that regions with greater inputs of nitrogen fertilizer will also see greater soil emissions — and that the emission of NOx gases from the soils will also increase as temperatures rise in the region due to climate change.

Although food demands — and the need for fertilizer for crops — are likely to increase in the future, there are numerous possible ways to limit unwanted nitrogen fertilizer spillover, the researchers note. For example, farmers can use more efficient fertilization strategies such as adjusting how much fertilizer is used depending on specific growing stages, or planting what are called cover crops along with the target crops that enrich soils and consume the excess nitrogen.

Almaraz’s team has produced an important finding, Jenerette says. “The combination of bottom-up soil emission measurements and top-down airborne measurements provide strong evidence for their emission assessments,” he says. The finding that NOx emission rates will increase with warming temperatures also highlights the urgency of taking steps to better manage nitrogen fertilizer use in a warming world, he says.

Female polar bears prowling springtime sea ice have extreme weight swings, some losing more than 10 percent of their body mass in just over a week. And the beginnings of bear video blogging help explain why.

An ambitious study of polar bears (Ursus maritimus) in Alaska has found that their overall metabolic rate is 1.6 times greater than thought, says wildlife biologist Anthony Pagano of the U.S. Geological Survey in Anchorage. With bodies that burn energy fast, polar bears need to eat a blubbery adult ringed seal (or 19 newborn seals) every 10 to 12 days just to maintain weight, Pagano and his colleagues report in the Feb. 2 Science. Camera-collar vlogs, a bear’s-eye view of the carnivores’ diet and lifestyle secrets, show just how well individual bears are doing.
The study puts the firmest numbers yet on basic needs of polar bears, whose lives are tied to the annual spread and shrinkage of Arctic sea ice, Pagano says. As the climate has warmed, the annual ice minimum has grown skimpier by some 14 percent per decade (SN Online: 9/19/16), raising worries about polar bear populations. These bears hunt the fat-rich seals that feed and breed around ice, and as seal habitat shrinks, so do the bears’ prospects.
Pagano and colleagues used helicopters to search for polar bears on ice about off the Alaska coast in the Beaufort Sea. It’s “a lot of grueling hours looking out the window watching tracks and looking at whiteness,” he says.
After tracking down female bears without cubs, the researchers fitted the animals with a camera collar. A full day’s doings of bears on the sea ice have been mostly a matter of speculation, Pagano says. Collar videos showed that 90 percent of seal hunts are ambushes, often by a bear lurking near a hole in the ice until a seal bursts up for a gulp of air. Videos also caught early glimpses of the breeding season and what passes for courtship among polar bears. Males, Pagano says, “pretty much harass the female until she’ll submit.”

The researchers also injected each bear with a dose of water with extra neutrons in both the hydrogen and oxygen atoms. Eight to 11 days later, the team caught the same bear to check what was left of the altered atoms. Lower traces of the special form of oxygen indicated that the bear’s body chemistry had been very active, and that the bear had exhaled lots of carbon dioxide. (The unusual form of hydrogen let scientists correct results for oxygen atoms lost in H2O, for instance when the bear urinated.)

Using CO₂ data from nine females, Pagano and his colleagues calculated the field metabolic rates for polar bears going about their springtime lives. The team found that female bears need to eat a bit more than 12,000 kilocalories (or what human dieters call calories) a day just to stay even. That estimate adds some 4,600 kilocalories a day to the old estimate. But merely maintaining weight isn’t enough for a polar lifestyle. To survive lean times, polar bears typically pack on extra weight in spring.

To get a broader view of the bears’ energy needs, similar metabolic measurements for other seasons would be useful, says physiological ecologist John Whiteman of the University of New Mexico in Albuquerque. That could help resolve whether and how much bear metabolism drops when there’s little food, a response that might protect bears during hard times. Using temperature loggers to estimate metabolic rates, he has seen only a gradual decline in metabolic rates in summer as food gets tougher to find. Winter metabolic rates remain a mystery.

Hunting success and bear activity are only part of the picture of polar bear health, says ecotoxicologist Sabrina Tartu, of the Norwegian Polar Institute, which is based in Tromsø. Tartu coauthored a 2017 paper showing that toxic pollutants such as polychlorinated biphenyls, or PCBs, can build up in bear fat. Such “pollutants could, by direct or indirect pathways, disrupt metabolic rates,” she says. So changing the climate is far from the only way humankind could affect polar bear energy and hunting dynamics.

Ultrasounds during pregnancy can be lots of fun, offering peeks at the baby-to-be. But ultrasounds aren’t just a way to get Facebook fodder. They are medical procedures that involve sound waves, technology that could, in theory, affect a growing fetus.

With that concern in mind, some researchers have wondered if the rising rates of autism diagnoses could have anything to do with the increasing number of ultrasound scans that women receive during pregnancy.

The answer is no, suggests a study published online February 12 in JAMA Pediatrics. On average, children with autism were exposed to fewer ultrasounds during pregnancy, scientists found. The results should be “very reassuring” to parents, says study coauthor Jodi Abbott, a maternal fetal medicine specialist at Boston Medical Center and Boston University School of Medicine.
To back up: Autism rates have risen sharply over the last several decades (though are possibly plateauing). Against this backdrop, researchers are searching for the causes of autism — and there are probably many. Autism is known to run in families, and scientists have found some of the particular genetic hot spots that may contribute. Other factors, such as older parents and maternal obesity, can also increase the risk of autism.

Scientists suspect that in many cases, autism is caused by many factors, all working together. Could prenatal ultrasounds, which have become more routine and more powerful, be one of those factors? These scans use sound waves that penetrate mothers’ bodies, and then collect the waves that bounce back, forming a picture of fetal tissues. During this process, the waves may be able to heat up the tissue they travel through.

Work on animals has suggested that ultrasounds can in fact interfere with fetal brain development, derailing the normal movements of cells that populate the brain. Mice exposed to 30 or more minutes of ultrasound in utero had abnormal brain development, for instance. But it’s not at all clear whether a similar thing might happen in humans, and if so, whether such effects might contribute to autism.
The new study compared ultrasound exposure among three groups: 107 children diagnosed with autism spectrum disorder, 104 children diagnosed with a developmental delay, and 209 typically developing children. On average, the children with autism were exposed to 5.9 ultrasound scans over the course of pregnancy. Children with developmental delays were exposed to 6.1 scans, and typically developing children were exposed to 6.3 scans, the researchers found. (For all groups, these numbers are way above the one to two scans per low-risk pregnancy recommended by the American College of Obstetricians and Gynecologists.)

For all three groups, the duration of the scans was similar. So was the thermal index, an indication of how much warming might have happened. “In almost every parameter we looked at, ultrasound seemed perfectly safe,” says study coauthor N. Paul Rosman, a pediatric neurologist at Boston Medical Center and Boston University School of Medicine.

One measure was different, the researchers found: During the first trimester, mothers who had children with autism had slightly deeper ultrasounds than women who had typically developing children and children with developmental delays. Ultrasound depth measures the distance from the transducer paddle that emits the waves to the spot that’s being imaged. The measure “has a lot to do with the size of the mother and the distance between her skin, where the ultrasound transducer is, and where the baby is,” Abbott says.

Lots of questions remain about whether — and how — ultrasound depth, or other aspects of the technology, might affect fetuses. “The study certainly wasn’t perfect,” Rosman says. It combed back through medical records of women instead of following women from the beginning. And it didn’t control for certain traits that may influence autism, such as smoking.

The results suggest that on their own, ultrasounds don’t cause autism spectrum disorder, says Sara Jane Webb of Seattle Children’s Research Institute and the University of Washington, who cowrote a JAMA Pediatrics companion piece. “At this time, there is no evidence that ultrasound is a primary contributor to poor developmental outcomes when delivered within medical guidelines,” she says.

While there’s more science to sort out here, the news is reassuring for women who might be worried about getting scanned. Women should follow their doctors’ guidance on ultrasounds, Rosman says. “We don’t think there’s anything in this study to recommend otherwise.”