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Long-term or postglacial (~~12 000-year) net fluxes of Pb, Cu, Zn, Ni, Cr, and Co through three Spodosols, formed from different parent materials, were calculated using mass-balance modeling. Metal contentsin vegetation and O horizons were also estimated and measured, respectively. Total metal contents in mineral horizons were fractionated into different operationally defined pedogenic phases using a sequential extraction technique. With few exceptions, the three soils had significant leaching losses for all trace metals (from 7--508 kg ha{sup}- 1{end}), the greatest losses generally occurring in the E and upperB horizons. The magnitude of losses by leaching varied with parent material, as a result of differences in weatherabilities of mineral suites. Combined metal contents in vegetation and O horizons, which mayoriginate from anthropogenic additions as well as mineral weathering, are significantly large in some cases; however, these metal amountsare apparently not large enough to offset long-term leaching losses from the underlying mineral soil profiles. Some B and C horizons had net accumulations of extractable Pb, Cu, and/or Ni-bearing phases. Based on the sequential extraction procedure used, the extractable metal forms are probably pedogenic Fe oxide- and/or organic-bound phases.One soil apparently gained Cu (~~38 kg ha{sup}-1{end}), which may have been from atmospheric deposition. [ABSTRACT FROM AUTHOR]

Most terrestrial ecosystems support a similar suite of biogeochemical processes largely dependent on the availability of water and labile carbon (C). Here, we explored the biogeochemical potential of soils from Earth's driest ecosystem, the Atacama Desert, characterized by extremely low moisture and organic C. We sampled surface soil horizons from sites ranging from the Atacama's hyper-arid core to less-arid locations at higher elevation that supported sparse vegetation. We performed laboratory incubations and measured fluxes of the greenhouse gases carbon dioxide (CO), nitrous oxide (NO), and methane (CH) as indices of potential biogeochemical activity across this gradient. We were able to stimulate trace gas production at all sites, and treatment responses often suggested the influence of microbial processes. Sites with extant vegetation had higher C concentrations (0.13-0.68%) and produced more CO under oxic than sub-oxic conditions, suggesting the presence of aerobic microbial decomposers. In contrast, abiotic CO production appeared to predominate in the most arid and C-poor (<0.08% C) sites without plants, with one notable exception. Soils were either a weak source or sink of CH under oxic conditions, whereas anoxia stimulated CH production across all sites. Several sites were rich in nitrate, and we stimulated NO fluxes in all soils by headspace manipulation or dissolved organic matter addition. Peak NO fluxes in the most C-poor soil (0.02% C) were very high, exceeding 3 ng nitrogen g h under anoxic conditions. These results provide evidence of resilience of at least some soil biogeochemical capacity to long-term water and C deprivation in the world's driest ecosystem. Atacama soils appear capable of responding biogeochemically to moisture inputs, and could conceivably constitute a regionally-important source of NO under altered rainfall regimes, analogous to other temperate deserts. [ABSTRACT FROM AUTHOR]

In this study, we examined changes in isotopic (13C and 14C) and spectroscopic (UV and 13C NMR) properties of dissolved organic carbon (DOC) in relation to soil organic matter (SOM) to elucidate the sources and sinks of DOC as water percolates through the soils of two contrasting upland coastal California ecosystems—a redwood forest and a coastal prairie. Despite differences in the distribution of C stocks and litter chemistry at these two sites, we found similar shifts in DOC chemistry with soil depth. DOC concentrations dropped rapidly with increasing depth, with an accompanying decrease in the C:N ratio, an increase in the δ13C value and an decrease in specific UV adsorption. In the grassland soil, Δ14C values declined from current atmospheric values (+70‰) in the surface horizon to −75‰ at 100 cm. In the redwood soil, the Δ14C value of 111‰ in O horizon leachates was indicative of OM with a residence time of 8–10 yrs, with a decrease in Δ14C values to −80‰ at 100 cm. Solid-state CP/MAS 13C NMR spectra were generally most similar to highly humified OM, with a general decrease in the relative abundance of aromatic compounds and an increase in the alkyl C/O-alkyl C ratio with increasing depth. All of these trends are consistent with the shifts in SOM properties with increasing depth, which are interpreted to mean a shift from fresh plant material to older, highly altered OM. In this Mediterranean climate, we found distinct seasonal shifts in the quantity and composition of DOC found in soil solution during the winter rainy period that was also consistent with a shift from recent labile substrates to older, highly altered OM. These results fit in with a growing body of literature suggesting that the source of much of the DOC within mineral soils is the local soil OM, and the 14C data, in particular, indicate that DOC at depth is not simply the fraction of surficial leachates that have not been adsorbed or decomposed. Rather, exchange reactions with a portion of the more stabilized SOM pool exert the strongest control on both the concentration and composition of DOC found in these soils. [ABSTRACT FROM AUTHOR]

Organic carbon storage and turnover were altered in soils formed from volcanic ash (Andisols) as a result of conversion of tropical forest to pasture and sugarcane cropland. Changes in soil carbon storage after approximately a century of each land use were estimated using stable carbon isotope values and carbon contents. Total organic carbon stored in soils varied as a result of management, with pasture soils showing net carbon gain and sugarcane soils showing net carbon loss. In pasture soils, increases in carbon at depth (40 to 80 cm) are below the rooting zone of the introduced (C4) vegetation, and have stable carbon isotopic values indicative of forest (C3) plants. Within the pasture rooting zone (0–40 cm) the isotopic data reveals that additions of pasture (C4) organic matter have been offset by losses of C3 carbon. The concentration of Fe/Al oxides (soil minerals that bind with organic matter to form oxide-humus complexes) appear to control the quantity of carbon stored in soils, as well as the difference in the depth and magnitude of carbon storage changes that occur with each type of land use change. Sugarcane land use appears to induce dissociation of Fe/Al oxide-humus complexes and loss of oxide-associated organic matter from the profile. In pastures, Fe/Al oxide-humus complexes are translocated to deeper horizons in the soils, resulting in greater profile carbon storage and longer apparent turnover time of carbon stored below 50 cm depth. In this high precipitation region, carbon losses from the soil appear to occur via downward transport, either as colloids or in solution, in addition to the generally assumed pathway of flux to the atmosphere as CO2. [ABSTRACT FROM AUTHOR]

Climate and topography have been widely recognized but studied separately as important factors controlling soil organic carbon (SOC) dynamics. Subsequently, the significance of their interplay in determining SOC storages and their pools is not well understood. Here we examined SOC storages and SOC-mineral interactions along two hillslope transects in differing climate zones (MAP = 549 mm in semi-arid eucalyptus savannah vs. 816 mm in temperate eucalyptus forest) in southeastern Australia. On eroding slopes, SOC inventories were twice as large at the wetter site (4.5 ± 0.6 vs. 2.3 ± 0.9 kg m −2 ), whereas depositional soils had similar SOC inventories at both locations (7.5 ± 2.0 vs.7.0 ± 2.2 kg m −2 ). On eroding slopes, carbon concentrations of the mineral-associated SOC fraction (<250 μm and >2.0 g cm −3 ) increased by ~50% with increasing rainfall, which was also positively correlated to abundances of clay minerals and pedogenic iron oxides. Within individual hillslopes, carbon concentrations of the mineral-associated SOC fraction doubled from eroding to depositional soils at the drier site, but no topographic trend was observed at the wetter site. The effects of topography on SOC inventories and mineral-associated SOC were more strongly expressed under the drier climate, where vegetation was sparser and soil erosion involved mineral grain size sorting. Our results demonstrate that SOC pools and their interactions with minerals are dependent on topographic locations, emphasizing the need to include geomorphic data when assessing climatic controls of SOC. [ABSTRACT FROM AUTHOR]

In the high plateau region of central Iran, there is both a lack of information on calcic soil processes and aspects of Quaternary paleoclimate. Thus, the research discussed here was undertaken as a first step to assess the utility of calcic soil research in the region. The properties and isotope composition of calcic soils was studied on a complex of alluvial fans, located about 50 km SE of the city of Isfahan. At least three geomorphic surfaces have formed during an interval from the middle Pleistocene to the Holocene. These geomorphic surfaces contain calcareous soils and paleosols. Interpretation of the pedogenic evidence indicates that there have been multiple periods of clay formation and carbonate accumulation over time, with an overall trend of increasing environmental aridity. Microscopy shows that overprinting is a major factor responsible for the accumulation of calcrete, suggesting the impacts of climate oscillations on the calcrete formation. Stable isotope composition of pedogenic carbonates in soils of differing ages is suggestive of a decrease in plant density and an increase in evaporation as soils become younger. This research highlights the utility of the morphology and isotope chemistry of calcic soils for constraining environmental change during the Quaternary. [ABSTRACT FROM AUTHOR]

The hyperarid Atacama Desert contains numerous local basins with surficial salt crusts, known as salars, where evaporation of shallow groundwater drives the major soil processes. We examine chemical and isotopic profiles in two soils of differing ages from the Salar Llamara to determine the geochemical processes involved in their formation. Evaporation, which provides salts to the soils through mineral precipitation, decreases with increasing salt crust thickness, and average ~ 0.03 mm m − 2 d − 1 over geological time frames. Salt distribution varies predictably with depth and soil age, with the most soluble compounds concentrated nearest to the land surface, indicating the direction of fluid flow. δ 34 S values of mineral sulfate tend to decrease with decreasing soil depth, following a pattern indicative of Rayleigh-like fractionation as solute-rich waters migrate toward the land surface. δ 13 C and δ 18 O values of carbonate suggest that the uppermost halite layers, which contain very small amounts of carbonate, have a strong biological signature. In contrast, carbonate-rich layers deeper in the profiles consist of largely unmodified lacustrine carbonate that formed in highly evaporitic lake conditions. The continuous upward evaporation of water and dissolved solutes creates a rugged and physically dynamic halite crust composed of rounded salt nodules. The crust undergoes deliquescence as atmospheric relative humidity rises from marine air intrusions, and we found that the halite nodules on the surface of the Salar Llamara are nearly always at or above deliquescence relative humidity. The interiors of these nodules are therefore able to buffer the large diurnal changes in atmospheric relative humidity allowing for the survival of halophilic microbial communities in an otherwise very dry environment. Radiocarbon measurements of occluded organic C in the surface crusts indicate that C cycling occurs at differing rates depending on local micrometeorological conditions, and that a given salt crust feature may persist for thousands of years once formed. [ABSTRACT FROM AUTHOR]

A relict soil on a late Tertiary/early Quaternary gravelly alluvial fan in the southern Atacama Desert was studied to determine if evidence of Quaternary climate change was evident in the profile. Stratigraphic relations of soil morphological features indicated that the initial phase of soil development was extensive chemical weathering with the loss of Si, Na, etc. and the formation of secondary clay. This was then followed by a prolonged period of carbonate accumulation that has discontinuously impregnated the earlier formed argillic horizons. The carbonate accumulation occurred both as fine-grained accumulations that engulfed and expanded the gravelly soil matrix, and as carbonate that formed dense and continuous coatings on gravel bottoms. Overlying the carbonate is a Holocene calcareous sand unit. Stable C and O isotope profiles of the disseminated carbonate show that this material appears to have formed under conditions of low to modest plant cover and significant soil water evaporation, very similar to soils presently found at higher latitudes and altitudes in modern Argentina. Seven transects of the carbonate laminations on the gravels at 10-μm scale reveal that all show a nearly 4‰ increase in δ 13 C values with time over distances of approximately 150 mm, and variable δ 18 O values. The shift in C isotope values, which is unlikely to be due to significant changes in C3 vs. C4 vegetation, reflects a profound and prolonged aridification with a corresponding lowering of plant density and soil respiration. The precise beginning of the aridification is unknown due to a lack of carbonate dating methods amenable to the time frame involved. When all changes are considered, this remarkable soil indicates that local rainfall declined from somewhere between 500 to 1000 mm y − 1 in the late Tertiary/early Quaternary, to the present climate of about 25 mm y − 1 . Future work will focus on developing more precise geochronological controls, but this initial study reveals the enormous potential that carbonate-bearing relict soils have for understanding climate change. [ABSTRACT FROM AUTHOR]

Our understanding of climatic conditions, and therefore forcing factors, in North America during the past two glacial cycles is limited in part by the scarcity of long, well-dated, continuous paleoclimate records. Here, we present the first, to our knowledge, continuous, millennial-resolution paleoclimate proxy record derived from millimeter-thick pedogenic carbonate clast coatings (pedothems), which arewidely distributed in semiarid to arid regions worldwide. Our new multiisotope pedothem record from the Wind River Basin inWyoming confirms a previously hypothesized period of increased transport of Gulf of Mexico moisture northward into the continental interior from 70,000 to 55,000 years ago based on oxygen and carbon isotopes determined by ion microprobe and uranium isotopes and U-Th dating by laser ablation inductively coupled plasma mass spectrometry. This pronounced meridional moisture transport, which contrastswith the dominant zonal transport of Pacific moisture into the North American interior by westerly winds before and after 70,000-55,000 years ago, may have resulted from a persistent anticyclone developed above the North American ice sheet during Marine Isotope Stage 4. We conclude that pedothems, when analyzed using microanalytical techniques, can provide high-resolution paleoclimate records that may open new avenues into understanding past terrestrial climates in regions where paleoclimate records are not otherwise available. When pedothem paleoclimate records are combined with existing records they will add complimentary soil-based perspectives on paleoclimate conditions. [ABSTRACT FROM AUTHOR]

Highlights: [•] Smectite clays in soil are not isotopically inert with respect to the soil water. [•] Mg, Ca, or K adsorbed to smectite fractionate the bulk water in clay-water mixtures. [•] Isotopic measurement by direct equilibration may reflect only portions of soil water. [•] This isotope effect may be present in natural, high-clay soils and sediments. [•] These findings are relevant to pedomineralogy, ecohydrology, and climatology. [Copyright &y& Elsevier]

Climate controls erosion and weathering on soil-mantled landscapes through diverse processes that have remained difficult to disentangle due to their complex interactions. We quantify denudation, soil and saprolite weathering, and soil transport near the base and crest of the western slope of the Sierra Nevada to examine how large differences in climate affect these processes. Depth profiles of fallout radionuclides and field observations show relative differences in erosion and weathering processes at these two climatically diverse sites, and our data suggest fundamentally different patterns of soil production and transport mechanisms: biotically driven soil transport at low elevation, and surface erosion driven by overland flow at high elevation. Soil production rates from cosmogenic 10Be decrease from 31.3 to 13.6 m/Ma with increasing soil depth at low elevation, but show uncertain depth dependence at the high elevation site. Our data also show a positive correlation between physical erosion and saprolite weathering at both sites. Highly weathered saprolites are overlain by weakly weathered and rapidly eroding soils, while chemically less depleted saprolites are overlain by slowly eroding, more weathered soils. Our data are among the first to quantify the critical role of saprolite weathering in the evolution of actively eroding upland landscapes, and our results provide quantitative constraints on how different climates can shape hillslopes by driving processes of erosion and weathering. [ABSTRACT FROM AUTHOR]

Abstract: We measured Ca stable isotope ratios (δ44/40Ca) in an ancient (2My), hyperarid soil where the primary source of mobile Ca is atmospheric deposition. Most of the Ca in the upper meter of this soil (3.5kmolm−2) is present as sulfates (2.5kmolm−2), and to a lesser extent carbonates (0.4kmolm−2). In aqueous extracts of variably hydrated calcium sulfate minerals, δ44/40CaE values (vs. bulk Earth) increase with depth (1.4m) from a minimum of −1.91‰ to a maximum of +0.59‰. The trend in carbonate-δ44/40Ca in the top six horizons resembles that of sulfate-δ44/40Ca, but with values 0.1–0.6‰ higher. The range of observed Ca isotope values in this soil is about half that of δ44/40Ca values observed on Earth. Linear correlation among δ44/40Ca, δ34S and δ18O values indicates either (a) a simultaneous change in atmospheric input values for all three elements over time, or (b) isotopic fractionation of all three elements during downward transport. We present evidence that the latter is the primary cause of the isotopic variation that we observe. Sulfate-δ34S values are positively correlated with sulfate-δ18O values (R 2 =0.78) and negatively correlated with sulfate δ44/40CaE values (R 2 =0.70). If constant fractionation and conservation of mass with downward transport are assumed, these relationships indicate a δ44/40Ca fractionation factor of −0.4‰ in CaSO4. The overall depth trend in Ca isotopes is reproduced by a model of isotopic fractionation during downward Ca transport that considers small and infrequent but regularly recurring rainfall events. Near surface low Ca isotope values are reproduced by a Rayleigh model derived from measured Ca concentrations and the Ca fractionation factor predicted by the relationship with S isotopes. This indicates that the primary mechanism of stable isotope fractionation in CaSO4 is incremental and effectively irreversible removal of an isotopically enriched dissolved phase by downward transport during small rainfall events. [Copyright &y& Elsevier]

The occurrence of hypolithic cyanobacteria colonizing translucent stones was quantified along the aridity gradient in the Atacama Desert in Chile, from less arid areas to the hyperarid core where photosynthetic life and thus primary production reach their limits. As mean rainfall declines from 21 to ≤2 mm year−1, the abundance of hypolithic cyanobacteria drops from 28 to <0.1%, molecular diversity declines threefold, and organic carbon residence times increase by three orders of magnitude. Communities contained a single Chroococcidiopsis morphospecies with heterotrophic associates, yet molecular analysis revealed that each stone supported a number of unique 16S rRNA gene-defined genotypes. A fivefold increase in steady-state residence times for organic carbon within communities in the hyperarid core (3200 years turnover time) indicates a significant decline in biological carbon cycling. Six years of microclimate data suggest that the dry limit corresponds to ≤5 mm year−1 rainfall and/or decadal periods of no rain, with <75 h year−1 of liquid water available to cyanobacteria under light conditions suitable for photosynthesis. In the hyperarid core, hypolithic cyanobacteria are rare and exist in small spatially isolated islands amidst a microbially depauperate bare soil. These findings suggest that photosynthetic life is extremely unlikely on the present-day surface of Mars, but may have existed in the past. If so, such microhabitats would probably be widely dispersed, difficult to detect, and millimeters away from virtually lifeless surroundings. [ABSTRACT FROM AUTHOR]

The Atacama Desert contains the driest regions on Earth, with significant rain occurring only a few times per century, based on sparse historical records. However, the frequency and magnitude of rainfall remains speculative. On March 24–26 of 2015, an unusual storm caused rainfall rates and quantities to exceed many historical records. Of interest is whether this storm was able to activate geomorphic processes whose impacts are evident on numerous landscape features. Here, the results of a reconnaissance from N to S transecting through the plant-free expanse of the Atacama Desert, between 22 and 26° S, are examined in relation to evidence of past runoff activity coupled with soil architecture and soil hydraulic properties. The results suggest the rain initiated some minor runoff processes on the upper hillslopes. However, the rainfall was too small to reactivate many features that appear to be driven by larger, less frequent storms. The field evidence suggests that larger scale rainfalls have occurred throughout the Quaternary, and that there are fossilized (or infrequently active) features in various stages of "repair" that provide evidence of rainfall re-occurrence. The landscapes largely escaped overland flow alteration due to the high infiltration rate capacity caused by the salt-rich soils, which we estimated to average 78 mm h−1 for hillslopes and 244 mm h−1 for alluvial soils, based on disc infiltrometer measurements. This gives a resilience, and potential rainfall threshold, to alteration by intensive rainfall events. Published paleoclimatic records coupled with evidence from soil examined at the arid/hyperarid periphery of the desert show evidence of a cessation of carbonate formation since ∼11 ka, a time of aridification similar to the drying of lakes and marshlands in the hyperarid region. Thus, the past fluvial alteration features are likely to be, at least partially, remnant Pleistocene features which have been largely unaffected by Holocene events, whose magnitudes were similar to that of 2015. • On March 2015 an unusual storm caused rainfall rates and quantities to exceed many historical records in the Atacama Desert. • Rainstorm seemed to be sparsely affected at the driest area of the desert. • Evidence suggests that the soil has an important influence on the landscape's response to rainfall in the Atacama Desert. • The absolute Atacama Desert has a resiliency to impact from rainfall due to high rates of water infiltration into soil. [ABSTRACT FROM AUTHOR]

Ground-based monitoring and remote sensing of extreme rain events in the hyperarid Atacama Desert, Chile, reveal a complex relationship between precipitation, soil types and interferometric synthetic aperture radar (InSAR) coherence. These integrated analyses allow examination of temporal and spatial variations of the soil moisture response between locations dominated by sulfate soils and those with immature, silicate-mineral soils. The radar dataset captures at least four separate rain events within the 2015–2017 timeframe, two of which were regionally devastating. The lack of vegetation in this region allows us to discriminate between contributions to the InSAR coherence from permanent changes of the landscape (e.g., erosion or deposition) and transient changes associated with soil moisture variability. The spatial distribution and character of the transient InSAR response depends strongly on soil type, and is remarkably repeatable between rain events. The areas that experienced permanent changes included river channels, steep slopes, playas, and sites of anthropogenic activity, such as roads, mines, or telescope construction. Ground-based observations of soil moisture after each event also exhibit a strong dependence on soil type. The observations presented here demonstrate how InSAR data can constrain variations in soil moisture with high spatial resolution over large regions, complementing the higher-sensitivity but sparser field sites and enabling discrimination of inter-event variability and analysis of longer-term changes in soil mineralogy in arid regions. Unlabelled Image • Multi-year SAR timeseries allow separation of factors contributing to coherence. • Soil moisture changes strongly impact transient InSAR coherence signal. • CaSO 4 soils have stronger transient signals than do silicate mineral soils. • Ground-based observations are consistent with the magnitude/timescale from InSAR. [ABSTRACT FROM AUTHOR]

Bacteriophages typically have small genomes 1 and depend on their bacterial hosts for replication 2 . Here we sequenced DNA from diverse ecosystems and found hundreds of phage genomes with lengths of more than 200 kilobases (kb), including a genome of 735 kb, which is-to our knowledge-the largest phage genome to be described to date. Thirty-five genomes were manually curated to completion (circular and no gaps). Expanded genetic repertoires include diverse and previously undescribed CRISPR-Cas systems, transfer RNAs (tRNAs), tRNA synthetases, tRNA-modification enzymes, translation-initiation and elongation factors, and ribosomal proteins. The CRISPR-Cas systems of phages have the capacity to silence host transcription factors and translational genes, potentially as part of a larger interaction network that intercepts translation to redirect biosynthesis to phage-encoded functions. In addition, some phages may repurpose bacterial CRISPR-Cas systems to eliminate competing phages. We phylogenetically define the major clades of huge phages from human and other animal microbiomes, as well as from oceans, lakes, sediments, soils and the built environment. We conclude that the large gene inventories of huge phages reflect a conserved biological strategy, and that the phages are distributed across a broad bacterial host range and across Earth's ecosystems.

Competing Interests: The authors declare no conflict of interest.

Although once thought to be devoid of biology, recent studies have identified salt deposits as oases for life in the hyperarid Atacama Desert. To examine spatial patterns of microbial species and key nutrient sources, we genomically characterized 26 salt crusts from three sites along a fog gradient. The communities are dominated by a large variety of Halobacteriales and Bacteroidetes, plus a few algal and Cyanobacterial species. CRISPR locus analysis suggests the distribution of a single Cyanobacterial population among all sites. This is in stark contrast to the extremely high sample specificity of most other community members. Only present at the highest moisture site is a genomically characterized Thermoplasmatales archaeon (Marine Group II) and six Nanohaloarchaea, one of which is represented by a complete genome. Parcubacteria (OD1) and Saccharibacteria (TM7), not previously reported from hypersaline environments, were found at low abundances. We found no indication of a N 2 fixation pathway in the communities, suggesting acquisition of bioavailable nitrogen from atmospherically derived nitrate. Samples cluster by site based on bacterial and archaeal abundance patterns and photosynthetic capacity decreases with increasing distance from the ocean. We conclude that moisture level, controlled by coastal fog intensity, is the strongest driver of community membership.

The tree of life is one of the most important organizing principles in biology(1). Gene surveys suggest the existence of an enormous number of branches(2), but even an approximation of the full scale of the tree has remained elusive. Recent depictions of the tree of life have focused either on the nature of deep evolutionary relationships(3-5) or on the known, well-classified diversity of life with an emphasis on eukaryotes(6). These approaches overlook the dramatic change in our understanding of life's diversity resulting from genomic sampling of previously unexamined environments. New methods to generate genome sequences illuminate the identity of organisms and their metabolic capacities, placing them in community and ecosystem contexts(7,8). Here, we use new genomic data from over 1,000 uncultivated and little known organisms, together with published sequences, to infer a dramatically expanded version of the tree of life, with Bacteria, Archaea and Eukarya included. The depiction is both a global overview and a snapshot of the diversity within each major lineage. The results reveal the dominance of bacterial diversification and underline the importance of organisms lacking isolated representatives, with substantial evolution concentrated in a major radiation of such organisms. This tree highlights major lineages currently underrepresented in biogeochemical models and identifies radiations that are probably important for future evolutionary analyses.

Human security has and will continue to rely on Earth's diverse soil resources. Yet we have now exploited the planet's most productive soils. Soil erosion greatly exceeds rates of production in many agricultural regions. Nitrogen produced by fossil fuel and geological reservoirs of other fertilizers are headed toward possible scarcity, increased cost, and/or geopolitical conflict. Climate change is accelerating the microbial release of greenhouse gases from soil organic matter and will likely play a large role in our near-term climate future. In this Review, we highlight challenges facing Earth's soil resources in the coming century. The direct and indirect response of soils to past and future human activities will play a major role in human prosperity and survival., (Copyright © 2015, American Association for the Advancement of Science.)