Search

Soil Science The future of humanity is intertwined with the future of Earth's soil resources. Soil provides for agriculture, improves water quality, and buffers greenhouse gases in the atmosphere. Yet human activities, including agricultural soil erosion, are rapidly degrading soil faster than it is naturally replenished. At this rate, human security over the next century will be severely threatened by unsustainable soil management practices. Amundson et al. review recent advances in understanding global soil resources, including how carbon stored in soil responds to anthropogenic warming. Translating this knowledge into practice is the biggest challenge remaining. Science , this issue [10.1126/science.1261071][1] [1]: /lookup/doi/10.1126/science.1261071

Earth History The largest mass extinction in Earth's history occurred at the Permian-Triassic boundary 252 million years ago. Several ideas have been proposed for what devastated marine life, but scant direct evidence exists. Clarkson et al. measured boron isotopes across this period as a highly

Gas Formation Technologies such as hydraulic fracturing, or “fracking,” can now extract natural gas from underground reservoirs. Within the gas, the ratio of certain isotopes holds clues to its origins. Stolper et al. analyzed a wide range of natural gas, including samples from some of the most active fracking sites in the United States. Using a “clumped isotope” technique, the authors could estimate the high temperatures at which methane formed deep underground, as well as the lower temperatures at which ancient microbes produced methane. The approach can help to distinguish the degree of mixing of gas from both sources. Science , this issue p. [1500][1] [1]: /lookup/volpage/344/1500?iss=6191

Water Resources Soil evaporation and plant transpiration together contribute a substantial proportion of terrestrial freshwater fluxes. Land surface models are used to understand the partitioning of these fluxes on a continental scale; however, model outputs are often inconsistent with stable isotope observations. Maxwell and Condon incorporated dynamic groundwater flow into an integrated hydrologic model simulation for the entire United States. The model showed that water table depth and lateral flow strongly affect transpiration partitioning, thus explaining the inconsistencies between observations and models. Science , this issue p. [377][1] [1]: /lookup/doi/10.1126/science.aaf7891

Electric Vehicles![Figure][1] Roger Wallace in his electric car, 1899. PHOTO: © HERITAGE IMAGE PARTNERSHIP LTD / ALAMY STOCK PHOTO Shifting to electric passenger vehicles ideally will reduce the carbon footprint of the transportation sector. Two recent studies, however, show that the greenhouse gas emissions produced over the life cycle of electric vehicles, from production through use, may not always be less than those of gasoline-burning vehicles. Ellingsen et al. reveal that vehicle and battery size prohibit some larger electric vehicles from ever overcoming the high greenhouse gas emissions generated during production. Yuksel et al. show that regional factors in the United States such as electrical grid mix, temperature, and driving conditions strongly limit the potential of plug-in electric vehicles to out-perform high-efficiency gas vehicles. Blanket policies directed at the adoption of electric vehicles therefore could potentially fail to reduce the transportation sector's large carbon footprint. Environ. Res. Lett. 11, 054010, 044007 (2016). [1]: pending:yes

Gas Infrastructure The Aliso Canyon underground gas storage facility outside Los Angeles, CA, houses enormous natural gas reserves. One well at the site experienced a blowout in late October 2015 and began leaking gas until it was sealed in February 2016. Over the course of 13 flights in the region, Conley et al. sampled the air column and determined daily release rates of methane (a powerful greenhouse gas) and ethane throughout the leak. The methane release rates were nearly double that of the entire Los Angeles region combined. Thus, single vulnerabilities can have major implications for state and federal climate policy. Science , this issue p. [1317][1] [1]: /lookup/doi/10.1126/science.aaf2348

Oil Spills![Figure][1] Airplane applying oil dispersant off the Louisiana coast PHOTO: PATRICK SEMANSKY/APIMAGES The 2010 Deepwater Horizon blowout released hundreds of millions of liters of oil into the Gulf of Mexico. One of the main mitigation strategies to prevent oil from reaching the shore—and potentially to stimulate natural bacterial communities capable of degrading oil and gas—was the application of 7 million liters of chemical dispersant. At the time, little was known about the effect these dispersants might have on hydrocarbon biodegradation rates. In a series of laboratory experiments, Kleindienst et al. observed that the same dispersant used in 2010 actually suppresses the major hydrocarbon-degrading groups in favor of dispersant-degrading bacteria. This confounding effect of dispersants therefore may explain why field data collected during the spill showed lower than expected biodegradation of oil and gas. Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.1507380112 (2015). [1]: pending:yes

Water Resources Some estimates suggest that humanity has already exceeded our sustainable global water footprint: the balance between fresh water use and supply. It seems that the situation may be more unsustainable than we realize. Jaramillo and Destouni analyzed hydroclimatic data for 100 large

Enviromental Science Excess nutrients such as nitrogen and phosphorus can negatively affect aquatic ecosystems. Nutrient-rich fertilizer runoff stimulates productivity, which can lead to harmful algal blooms or fish kills. Nolan and Weber show that such pollution has another wide-ranging consequence: increasing the mobility of uranium in groundwater. Geochemical data for two major aquifers in the United States, which in combination provide drinking water to millions and irrigation for one-sixth of U.S. agriculture, show that increased nitrate levels correlate strongly with the presence of uranium. Nitrate can oxidatively dissolve naturally occurring uranium minerals, which in turn can lead to potentially harmful levels of soluble uranium in groundwater, especially in shallow aquifers. Environ. Sci. Technol. Lett. 10.1021/acs.estlett.5b00174(2015).

Microbial Communities Natural microbial communities are frustratingly complex—even with the advent of powerful metagenomic tools. Overlooked abundant organisms often result in an incomplete view of the metabolic processes that affect major geochemical cycles. Hug et al. developed a subsampling metagenomic assembly technique which, when combined with metaproteomics, reveals a preponderance of highly abundant, novel microbial lineages in subsurface sediments at an old uranium mill tailings site. These organisms cycle carbon, nitrogen, and sulfur through unanticipated pathways. For example, despite low ammonium concentrations in the aquifer, the most abundant community member in the sediment is an ammonium-oxidizing archaeon that partially utilizes ammonium produced by nitrate-reducing bacteria. Environ. Microbiol. 17 , 10.1111/1462-2920.12930 (2015).

Mercury Microbiology Genes present in buried lake sediments can help reconstruct how microbial ecosystems responded to past exposure to mercury. When atmospheric mercury transported around the globe is eventually deposited in terrestrial or aquatic environments, microorganisms control its cycling and bioaccumulation. Poulain et al. isolated ancient microbial DNA from sediments in remote subarctic lakes in the Hudson Bay Lowlands of Canada. The distribution of genes coding for mercuric reductase (merA), the primary enzyme involved in mercury detoxification, correlates with historical mercury deposition, including a rapid population shift at the onset of the Industrial Revolution. ISME J. 10.1038/ismej.2015.86 (2015).

Environmental Science![Figure][1] Lake Erie as seen from space PHOTO: NASA/JEFF SCHMALTZ, LANCE/EOSDIS MODIS RAPID RESPONSE Nutrient-rich agricultural runoff or wastewater discharge can lead to the formation of harmful algal blooms or oxygen-depleted (hypoxic) “dead zones” in surface waters. But now, Zhou et al. show that low tributary river discharge was the largest contributor to a record-breaking hypoxic event in Lake Erie in 2012. Drought conditions across much of North America that year decreased water flow into the lake, exacerbating the effects of persistent nutrient runoff. Because drought and other extreme climate events are expected to increase with climate change, management strategies need to consider all factors that may degrade future water quality. Environ. Sci. Technol. 10.1021/es503981n (2014). [1]: pending:yes

Aquatic Microbiology Freshwater lakes are a major contributor of methane to the atmosphere—more so than the world's oceans combined. Some anaerobic microorganisms produce methane in sediments or deep anoxic water, but methane can also be produced biologically in oxic surface water. In the upper layers of methane-supersaturated Lake Matano, Indonesia, Yao et al. find that bacterial methane production is linked to phosphorus availability. Heterotrophic bacteria break down methylphosphonate as a phosphorus source, releasing methane in the process. Methane production decreases in culture when phosphate is added. Models for methane emissions from lakes should therefore incorporate nutrient availability in oxic water columns as another potential factor to help improve global methane predictions. Appl. Environ. Microbiol. 10.1128/AEM.02399-16 (2016).

Hydrology![Figure][1] Seepage of coastal groundwater rich in iron and other dissolved constituents PHOTO: MATTHIJS WETTERAUW/ALAMY STOCK PHOTO Not all groundwater ends up flowing into rivers. Some is discharged directly into the ocean along the coast. Although much lower in volume than water transported by rivers, such submarine groundwater discharge can be a hidden source of dissolved ions, nutrients, or contaminants from human activities. Sawyer et al. performed a high-resolution continental-scale analysis of fresh groundwater discharge along the coastline of the United States. In total, more than one-fifth of coastal waters are vulnerable to groundwater-borne contamination. Science , this issue p. [705][2] [1]: pending:yes [2]: /lookup/doi/10.1126/science.aag1058

Energy-Water Nexus![Figure][1] Thermoelectric power plants require huge volumes of water for cooling. PHOTO: © FRANS LEMMENS/ALAMY STOCK PHOTO High energy demands lead to the burning of prodigious amounts of fossil fuels and also require huge amounts of water. Cooling water for

Green Infrastructure![Figure][1] Green potable water systems can harbor elevated levels of pathogens PHOTO: © NORDICPHOTOS/ALAMY STOCK PHOTO Strategies to reduce water consumption in buildings may have some unintended consequences. Rainwater cisterns, solar water heaters, and other similar efforts minimize potable water use and reduce energy consumption, but they may also degrade water quality. In a survey of buildings with various green infrastructure, Rhoads et al. found that water age, which affects residual chlorine levels, pipe corrosion, and even taste, was significantly greater than that in a conventional home. Consistent with those findings, there were also orders of magnitude more opportunistic pathogens such as Legionella species in the green plumbing systems. Reducing water age by eliminating unnecessary storage and regularly flushing pipes may help green water efforts become more widely adopted. Environ. Sci. Water Res. Technol. 10.1039/C5EW00221D (2016). [1]: pending:yes

Biogeochemistry![Figure][1] Lake Powell, seen from Alstrom Point in Arizona, USA. PHOTO: JOHN SHAW/SCIENCE SOURCE Lakes of all sizes are sensitive to local water and pollution management strategies. Excess nutrients in lakes can induce a series of unexpected consequences for water quality or greenhouse gas emissions. Based on previously collected data from over 8000 lakes across six continents, Chen et al. compiled a global estimate of carbon, nitrogen, and phosphorus in lakes. These trace nutrients have intertwined fates in lakes, often related to morphological and climatic factors that change over time. Perturbations of climate or land use by humans will therefore have wide-ranging effects on biogeochemical cycling of nutrients within lakes across the globe. Sci. Rep. 10.1038/srep15043 (2015). [1]: pending:yes

Induced Earthquakes The number of earthquakes is increasing in regions with active unconventional oil and gas wells, where water pumped at high pressure breaks open rock containing natural gas, leaving behind wastewater in need of disposing. Keranen et al. show that the steep rise in earthquakes in Oklahoma, USA, is likely caused by fluid migration from wastewater disposal wells. Twenty percent of the earthquakes in the central United States could be attributed to just four of the wells. Injected fluids in high-volume wells triggered earthquakes over 30 km away. Science , this issue p. [448][1] [1]: /lookup/volpage/345/448?iss=6195

Water Resources Mobile surface waters and soil waters are relatively disconnected on a global scale. Water on land is eventually lost by surface runoff into the oceans or is ultimately sent back to the atmosphere through evapotranspiration processes. Good et al. determined that 65% of continental water evaporation is from soils, which includes water taken up and transpired by plants (see the Perspective by Brooks). Although just a small fraction of global surface waters pass through soils, individual stream ecosystems may be affected by water quality changes in nearby soils. Science , this issue p. [175][1]; see also p. [138][2] [1]: /lookup/doi/10.1126/science.aaa5931 [2]: /lookup/doi/10.1126/science.aac4742

Biogeochemistry![Figure][1] Soil chlorine inventories depend on the species of trees they host PHOTO: © IMAGEBROKER/ALAMY The density and type of trees in forested ecosystems control the large organic chlorine pool of soils. Chlorine is a trace nutrient for plant growth; however, its distribution is highly variable, even in instances where it was deposited regionally as chloride from the atmosphere. In an experimental forest ecosystem in eastern France, Montelius et al. show that tree species controlled total soil chlorine fluxes over 30 years. Coniferous trees, especially Norway spruce, retained organic chlorine and chloride ions in nearby soil more than deciduous trees. The preferential retention or transformation of certain chlorine compounds beyond microbial processes complicates the terrestrial chlorine cycle. Environ. Sci. Technol. 49 , 4921 (2015). [1]: pending:yes

Water Quality Modern wastewater treatment processes still leave behind trace amounts of pharmaceuticals, personal care products, and other potentially harmful or toxic compounds. For example, over 10 metric tons of the antimicrobial compound triclosan are released from wastewater treatment plants into U.S. waterways each year. Hensley et al. detected trace contaminants in the effluent from four U.S. wastewater treatment plants, including triclosan, chlorinated triclosan derivatives, and hydroxylated by-products of polybrominated diphenyl ethers (PBDEs)—recalcitrant compounds used as flame retardants. Chlorinated triclosan derivatives were present at the same low but appreciable levels as PBDE by-products, both of which produce toxic dioxins during photodegradation. Environ. Sci. Water Res. Technol. 10.1039/c4ew00102h (2015).

Freshwater Ecology![Figure][1] Forest-derived leaves and wood support stream functions but are lost with nutrient pollution IMAGE: PHILLIP M. BUMPERS Excess nutrients added to streams result in net carbon loss from aquatic ecosystems. Nitrogen and phosphorus are known to fuel increases in algal carbon. Now, Rosemond et al. show that nutrients stimulate losses of terrestrially derived carbon (e.g., from twigs and leaves). The authors monitored several multiyear experiments on headwater forest streams in the United States. Some of these streams had extra nitrogen and phosphorus added at levels that are now common in many streams and lakes. To successfully manage river ecosystems, we need to take into account nutrient pollution effects on multiple carbon pathways. Science , this issue p. [1142][2] [1]: pending:yes [2]: /lookup/doi/10.1126/science.aaa1958

Organic Geochemistry The remnants of microbial lipids preserved in sediments provide reliable records of past aquatic environments. Extracting trace amounts of these molecules from sediment cores is often tricky and requires a large amount of material, which causes the time resolution of these records to be rather coarse. Wormer et al. used a high-resolution mass spectrometer coupled to a laser ionization source to track variations of nanogram levels of archaeal tetraethers at a resolution of ∼4 years. Tiny variations in lipid structure and abundance in sediments from the Mediterranean Sea with ages of over 10,000 years show that planktonic archaea there were strongly influenced by ∼200-year-long solar cycles. Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.1405237111 (2014).

Earth History Oxygen levels in Earth's early atmosphere had an important influence on the evolution of complex life. Planavsky et al. analyzed the isotopic signature of chromium in sedimentary rocks from across the globe—a proxy for past oxygen levels. Oxygen levels in the mid-Proterozoic (1.6

Nitrogen Cycling Much of the ammonia fertilizer we use ends up in surface or coastal waters as nitrate. This runoff lowers water quality and damages ecosystems. Although microbial nitrogen respiration influences the fate of nitrate, it is unclear which environmental factors exert the most control. Kraft et al. performed multiple long-term incubation studies of microbial communities from marine sediments. The relative supply of nitrate and nitrite, as well as total carbon and nitrogen, provided selective pressures that drove communities toward denitrification or ammonification. The average generation time of the community also strongly influenced which respiration pathway dominated. Science , this issue p. [676][1] [1]: /lookup/doi/10.1126/science.1254070

Subsurface Microbes The sea floor is teeming with microbes, whose sheer numbers produce a major effect on the global biogeochemical cycles of carbon, sulfur, and other important nutrients. Bowles et al. constructed a map showing how deeply sulfates penetrate marine sediments worldwide and how

Geochemistry The sulfur isotope signatures of sediments and aerosol particles provide a rich record of biological and chemical processes in modern environments and in the geologic record. For example, the ratio of two or more of sulfur's several stable isotopes can indicate whether it was

Environmental Science![Figure][1] CREDIT: ISTOCKPHOTO As freshwater resources dwindle, purified wastewater is increasingly discussed as a supplement for such high-volume uses as crop irrigation. In a 15-year field-scale test of this prospect, Assouline and Narkis studied the soil properties of an irrigated avocado farm in Israel; one side of the farm received natural fresh water, while the other received treated domestic wastewater that by most accounts was quite similar to the natural source but had increased dissolved organic matter, salinity, and suspended solids. Over the study period, several hydraulic properties of the clay-rich soil degraded in the plots that received treated wastewater, to the point that irrigation events created a smaller but more saturated volume of wetted soil. These conditions may have contributed to smaller root zones that ultimately led to lower crop yields. Because some soil horizons responded differently to the treated wastewater, it is not yet clear if treated wastewater will have this effect on other soil types or if precipitation patterns across different regions buffer against soil degradation. Water Resour. Res. 10.1029/2011WR010498 (2011). [1]: pending:yes

Geochemistry The scarcity of bioavailable iron [Fe(II)aq] in the oceans limits primary productivity. In most marine environments, Fe(II)aq becomes available to phytoplankton only upon the reduction of Fe(III)-oxide particles added to the ocean from wind-blown dust or river outflows; however, in

Geochemistry![Figure][1] CREDIT: [JUPITERIMAGES.COM][2] High levels of nitrates released from agricultural fertilizers pose a serious threat to groundwater quality. Ameliorating this problem is not easy, but some help may be available from the natural soil bacterial communities that reduce nitrate into inert N2 gas. The rate of this denitrification step typically depends on the nature and concentration of the compounds that feed electrons into the process, providing the bacteria with energy. Most often bacteria use abundant organic matter, but in certain environments the iron(II) in minerals such as pyrite (FeS2) can also be oxidized. By monitoring the groundwater below a stretch of farmland in the Netherlands, Zhang et al. found that pyrite-coupled denitrification was the dominant nitrate-consuming reaction. Due at least in part to this reaction, nitrate levels in the groundwater were lower than levels measured 10 years ago; however, groundwater below a nearby unfertilized forest actually became enriched in nitrate over this same period. The transport of nitrate away from the farmland suggests that despite abundant pyrite remaining in the aquifer, complete denitrification occurs more slowly than nitrate's residence time. Complicating matters is the fact that levels of trace elements associated with pyrite (e.g., As, Zn, Ni) increased below the forest as well, signifying that these bacteria may be mobilizing heavy metals in the subsurface despite their benevolence in removing some of the nitrates. Geochim. Cosmochim. Acta 73 , 6716 (2009). [1]: pending:yes [2]: http://JUPITERIMAGES.COM

Researchers are now discovering that naturally occurring environmental nanoparticles can play a key role in important chemical characteristics and the overall quality of natural and engineered waters. The detection of nanoparticles in virtually all water domains, including the oceans, surface waters, groundwater, atmospheric water, and even treated drinking water, demonstrates a distribution near ubiquity. Moreover, aquatic nanoparticles have the ability to influence environmental and engineered water chemistry and processes in a much different way than similar materials of larger sizes. This review covers recent advances made in identifying nanoparticles within water from a variety of sources, and advances in understanding their very interesting properties and reactivity that affect the chemical characteristics and behaviour of water. In the future, this science will be important in our vital, continuing efforts in water safety, treatment, and remediation.