Search

The article "Climate change mitigation through soil carbon sequestration in working lands: A reality check" questions the validity and realism of claims made by Almaraz et al. regarding the potential magnitude of soil carbon sequestration (SCS) in agricultural soils. The authors argue that Almaraz et al. overestimate the availability of organic matter for compost and biochar, inaccurately interpret carbon sequestration rates, and make premature assumptions about the applicability of enhanced rock weathering. They emphasize the need for realistic evaluations of the limitations and trade-offs of SCS practices and caution against overly optimistic estimates that can mislead policymakers. The authors declare no conflicts of interest. [Extracted from the article]

Soil and other Earth scientists who conduct research on C management found themselves, in the past decade within a swirl of efforts concerning climate mitigation, economic and business investments in carbon markets, and political aspirations. All these external pressures are issues with which soil science is largely unfamiliar. As a result, science has responded without deeply considering the landscape in which it finds itself, and some of the unanticipated challenges these issues present. Here, we suggest soil C scientists now consider and respond to these issues. The first order challenge is to transition from the concept of technical carbon sequestration potentials, made in the absence of social and policy contexts, to societally achievable sequestration estimates based on highly transdisciplinary teams of natural and social sciences and scientists. To achieve this will requires re-thinking national science funding programs, in which climate-relevant social science is under-funded. In addition, the science of soil C itself is in need of a priority shift. Presently, publications in soil C sequestration out-strip papers on soil feedbacks to climate change, and on how to adapt soil to climate change: two areas of research which may well be more societal important in the next few decades than sequestering C. Most seriously, given the urgent nature of our collective societal climate problem, our profession must not find itself a decade from now continuing the now 20-year-old narrative that soil C can potentially mitigate climate change and compensate for greenhouse gas emissions. We must consider the possibility that other options and expenditures of resources are more viable, and we must reframe our science's objectives to expand into the many other urgent needs that confront humanity. [ABSTRACT FROM AUTHOR]

At this point, it is maybe useful to examine what this new-found sexiness means, for soils and for Soil Science, and consider if, or how, we might better create our own science-based stories about soil and human society. Keywords: Carbon sequestration; Climate change; Regenerative agriculture EN Carbon sequestration Climate change Regenerative agriculture 127 130 4 12/14/21 20220101 NES 220101 When I was a young professor at Berkeley, my colleagues and I would frequently run into Jean Jenny, the wife of Hans Jenny, at various events on campus. The new wave of Hollywood soil documentaries, and particularly "Kiss the Ground", tell a story of hope, coupled with a "happy ending". [Extracted from the article]

Improved soil management is increasingly pursued to ensure food security for the world's rising global population, with the ancillary benefit of storing carbon in soils to lower the threat of climate change. While all increments to soil organic matter are laudable, we suggest caution in ascribing large, potential climate change mitigation to enhanced soil management. We find that the most promising techniques, including applications of biochar and enhanced silicate weathering, collectively are not likely to balance more than 5% of annual emissions of CO2 from fossil fuel combustion. Improved soil management is increasingly pursued to ensure food security for the world's rising global population, with the ancillary benefit of storing carbon in soils to lower the threat of climate change. While all increments to soil organic matter are laudable, we suggest caution in ascribing large, potential climate change mitigation to enhanced soil management. We find that the most promising techniques, including applications of biochar and enhanced silicate weathering, are not likely to balance more than 5% of annual emissions of CO2 from fossil fuel combustion. [ABSTRACT FROM AUTHOR]

The authors discuss the need to stabilize the concentration of greenhouse gas of the atmosphere as the great environmental challenge. They mention that humanity can lessen fossil fuel emissions and/or distinguish mechanisms to take greenhouse gases after they have been emitted. They say that atmospheric carbon dioxide sequestration as organic carbon in the biosphere drews in attention as alternate way to aid stem the growth rate of greenhouse gas and associated alterations in the climate.

The geomorphology and geochemistry data gathered by the MER Opportunity at Meridiani Planum is a rich data set relevant to soil research on Mars. Many of the data, particularly with respect to outcrops at Victoria Crater, have been only partially analyzed. Here, the previously published geochemical profile of Endurance Crater is compared to that of Victoria Crater, to understand aspects of the post-depositional aqueous and chemical alteration of the Meridiani land surface. The landsurface bears cracking patterns similar to those produced by multiple episodes of wetting and drying in expansive materials on Earth. The geochemical profiles at both craters are nearly identical, suggesting (using mass balance methods) that a very chemically homogenous sedimentary deposit has been engulfed by the apparent surficial addition of S, Cl, and Br (and associated cations) since exposure to the atmosphere. The chemistry and mineralogy at both locations is one where the most insoluble of the added components resides near the land surface (Ca sulfates), and the more soluble components are concentrated at greater depths in a vertical pattern consistent with their solubility in water. The profiles, when compared to those on Earth (and to physical constraints), are most similar those generated by the downward movement of meteoric water. When this aqueous alteration and soil formation occurred is not well constrained, but the processes occurred between late Noachian (?) to late Amazonian times. The exposure of the Victoria crater walls, which occurred likely less than 10 7 y ago (late Amazonian), shows the accumulation of dust as well as evidence for aqueous concentration of NaBr and/or CaBr, possibly by deliquescence. By direct comparison to Earth, the regional soil at Meridiani Planum is a Typic Petrogypsid (a sulfate cemented arid soil), bearing similarities to very ancient soils formed in the Atacama Desert of Chile. The amount of water required to produce the soils ranges from a very low (and physically unlikely) quantity of 2–4 m, to possibly (and more likely) kilometers of water that were added in small individual increments over long spans of geological time. [ABSTRACT FROM AUTHOR]

Pedogenic carbonate is common in arid and semiarid soils, and though its stable C and O isotope composition has been shown to reflect local environmental conditions, questions remain about the quantitative nature of this relationship and the implications for paleoclimate applications. To further address these questions, a climosequence of four Holocene soils in Fish Lake Valley, Nevada (western United States), was instrumented and examined over the course of more than a year. The annual precipitation along the transect ranges from ~80 mm yr-1 to ~220 mm yr-1. Results show that the C and O stable isotope compositions of soil CO2, H2O, and carbonate change systematically with elevation and climate. However, there was considerable temporal variability in the conditions that affected carbonate isotope values. At the lowest elevation, CO2 δ13C and H2O δ18O values were similar to that in equilibrium with carbonate nearly year-round. At the midelevation sites, spring through summer CO2 δ13C and H2O δ18O values appeared to most closely match the δ13C and δ18O values of pedogenic carbonate. At the highest elevation, the C and O isotopes of carbonate did not reflect the soil CO2 δ13C and H2O δ18O values measured during the period of study, but they did appear to reflect soil respiration rates during the late spring. The transect shows that arid soil carbonate δ13C values strongly reflect variations in soil respiration rates and the resulting concentrations of soil CO2. These results also indicate that depth profile sampling may be required to adequately constrain respiration rates, which affect the interpreted atmospheric CO2 concentrations. [ABSTRACT FROM AUTHOR]

The Cenozoic paleoclimate of the Atacama Desert is not well known. We examined 14 early to mid-Miocene paleosols exposed in the El Tesoro Mine, near Calama, Chile. The paleosols developed on an aggrading alluvial fan system, and lie above the mineralized gravel horizons that host a copper ore body. Soil-forming conditions that oscillated between chemical weathering and clay production (humid: analogous to modern Alfisols) to environments favoring the accumulation of pedogenic carbonate (arid to semi-arid: analogous to modern Aridisols) are indicated. In contrast, the region is presently hyperarid, and soils accumulate sulfates, chlorides, and nitrates. While total chemical analyses clearly show the accumulation of Ca by the carbonate-rich paleosols, none of the soils exhibit significant losses of elements by leaching. The δ 18 O values of the carbonates range from -8.79‰ to − 3.16‰ (VPDB). The O isotope data, when combined with published data from the region, reveal a significant divergence in the O isotope composition of precipitation in the eastern and western margins of the Andean plateau since the early Miocene, suggesting that simple interpretations of declining δ 18 O values of carbonate with increasing elevation may not be appropriate. These paleosols clearly indicate that wetter conditions prevailed in what is now the Atacama Desert during the early to mid-Miocene. [ABSTRACT FROM AUTHOR]

An abstract of an article within the issue and at "Science" online, titled Soil and Human Security in the 21st Century," by Ronald Amundson, Asmeret Asefaw Berhe, Jan W. Hopmans, Carolyn Olson, A. Ester Sztein, and Donald L. Sparks is presented.

Assessing how vegetation controls hillslope soil processes is a challenging problem, as few abiotic landscapes exist as observational controls. Here we identify five avenues to examine how actively eroding hillslope soils and processes would differ without vegetation, and we explore some potential feedbacks that may result in landscape resilience on vegetated hillslopes. The various approaches suggest that a plant-free world would be characterized by largely soil-free hillslopes, that plants may control the maximum thickness of soils on slopes, that vegetated landforms erode at rates about one order of magnitude faster than plant-free outcrops in comparable settings, and that vegetated hillslope soils generally maintain long residence times such that both N and P sufficiency for ecosystems is the norm. We conclude that quantitatively parameterizing biota within process-based hillslope models needs to be a priority in order to project how human activity may further impact the soil mantle. [ABSTRACT FROM AUTHOR]

We examined the terrestrial sulfur (S) cycle in the wet tropical Luquillo Experimental Forest (LEF), Puerto Rico. In two previously instrumented watersheds (Icacos and Bisley), chemical and isotopic measurements of carbon (C), nitrogen (N) and S were used to explore the inputs, in-soil processing, and losses of S through comparison to the N cycle. Additionally, the impact of soil forming factors (particularly climate, organisms, topography and parent material) on S cycling in this system was considered. Atmospheric inputs (δ 34 S values of 16.1 ± 2.8‰), from a mixture of marine and anthropogenic sources, delivered an estimated 2.2 g S/(m 2 yr) at Icacos, and 1.8 g S/(m 2 yr) at Bisley. Bedrock N and S inputs to soil were minimal. We estimated a hydrologic export of 1.7 ± 0.1 g S/(m 2 yr) at Icacos, and 2.5 ± 0.2 g S/(m 2 yr) at Bisley. Stream baseflow S isotope data revealed significant bedrock S in the hydrologic export at Bisley (with a distinctive δ 34 S values of 1.6 ± 0.7‰), but not at Icacos. Pore water data supported the co-occurrence of at least three major biological S-fractionating processes in these soils: plant uptake, oxidative degradation of organic S and bacterial sulfate reduction. The rates and relative importance of these processes varied in time and space. Vegetation litter was 3–5‰ depleted in 34 S compared to the average pore water, providing evidence for fractionation during uptake and assimilation. Out of all abiotic soil forming factors, climate, especially the high rainfall, was the main driver of S biogeochemistry in the LEF by dictating the types and rates of processes. Topography appeared to impact S cycling by influencing redox conditions: C, N and S content decrease downslope at all sites, and the Bisley lower slope showed strongest evidence of bacterial sulfate reduction. Parent material type did not impact the soil S cycle significantly. To compare the fate of S and N in the soil, we used an advection model to describe the isotopic fractionation of total S and N associated with downward movement of organic matter in both dissolved and solid fractions. This model worked well for N, but the assumption of a constant fractionation factor α with depth failed to describe S transformations. This result revealed a fundamental difference between N and S cycling in these soils, indicating an apparent greater sensitivity of S isotopes to fluctuating redox conditions. [ABSTRACT FROM AUTHOR]

Abstract: Halite (NaCl) and gypsum or anhydrite (CaSO4) are water-soluble minerals found in soils of the driest regions of Earth, and only modest attention has been given to the hydrological processes that distribute these salts vertically in soil profiles. The two most notable chloride and sulfate-rich deserts on earth are the Dry Valleys of Antarctica and the Atacama Desert of Chile. While each is hyperarid, they possess very different hydrological regimes. We first show, using previously published S and O isotope data for sulfate minerals, that downward migration of water and sulfate is the primary mechanism responsible for depth profiles of sulfate concentration, and S and O isotopes, in both deserts. In contrast, we found quite different soluble Cl concentration and Cl isotope profiles between the two deserts. For Antarctic soils with an ice layer near the soil surface, the Cl concentrations increase with decreasing soil depth, whereas the ratio of 37Cl/35Cl increases. Based on previous field observations by others, we found that thermally driven upward movement of brine during the winter, described by an advection/diffusion model, qualitatively mimics the observed profiles. In contrast, in the Atacama Desert where rare but relatively large rains drive Cl downward through the profiles, Cl concentrations and 37Cl/35Cl ratios increased with depth. The depth trends in Cl isotopes are more closely explained by a Rayleigh-like model of downward fluid flow. The isotope profiles, and our modeling, reveal the similarities and differences between these two very arid regions on Earth, and are relevant for constraining models of fluid flow in arid zone soil and vadose zone hydrology. [Copyright &y& Elsevier]

The Atacama Desert has experienced a long and protracted period of hyperaridity that has resulted in what may be the most unusual biome on Earth, but the duration of this aridity is poorly constrained. We reconstructed aspects of the fluvial and geochemical history of this region using integrated landscape features (alluvial fans, hillslope soils, soil chemistry, river profiles) in the southern portion of the present desert. Topographic reconstructions of a large watershed (11,000 km²) show deep incision and sediment removal between the late Miocene and the end of the Pliocene, and modest to negligible incision in post-Pliocene times. These changes in incision suggest an ∼50--280x reduction in river discharge, which should reflect corresponding changes in precipitation. Changes in the nature of hillslope soils in the Atacama Desert indicate that in the Pliocene or earlier, hillslopes were mantled with silicate-derived soil. This mantle was stripped off and locally deposited as alluvial fans (late Pliocene to early Pleistocene) that now block or otherwise cause a rearrangement of Pliocene and earlier river channels. Finally, the hillslopes have largely accreted a soil mantle of dust and salt since the apparent late Pliocene stripping, suggesting a decline in annual precipitation of at least 125 mm yr-1 or more (mean annual precipitation [MAP] is now <3 mm yr-1). Embedded in the long post-Pliocene era of salt accumulation, there are a variety of features suggesting overland flow on hillslopes (rills, striped gravel deposits, piping, and water spouts) and large, infrequent storms that infiltrated gentle alluvial fans (due to the depth of salt-rich horizons). Despite evidence for episodes that punctuate the hyperaridity, the magnitude and duration of these pluvial events have been insufficient to remove the regional accumulations of sulfate, chloride, and nitrate. The late Pliocene cessation of many fluvial features is coincident with recent research on the tropical Pacific, which shows that the Pacific was in a permanent El Niño state until ca. 2.2 Ma, at which time sea-surface temperatures offshore of South America declined greatly relative to those of the western Pacific, in turn setting up the present El Niño--Southern Oscillation (ENSO) climate system. These observations indicate that the latest period of aridity has been prolonged and largely continuous, and it appears to have occurred in step with the onset of the ENSO climate system, beginning ∼2 m.y.ago. [ABSTRACT FROM AUTHOR]

Despite the wealth of information available on respiratory fluxes of CO2 from the soil surface, relatively few studies have examined the depth-related flux and production of CO2 within the soil. We developed a depth-integrated CO2 flux model based on soil profile CO2 measurements to explore both the temporal and spatial patterns of, and controls on, CO2 production in two contrasting California coastal ecosystems: a redwood [Sequoia sempervirens (D. Don) EndI.] forest and a seasonal grassland ecosystem. In the forest, CO2 production increased exponentially with temperature but the dominant control on CO2 efflux was due to changes in soil diffusivity with water content. Here, ∼67% of the total efflux was produced in the top 20 cm, with little seasonal variation. In the grassland, soil temperature and water content both exerted strong positive controls on CO2 production, with CO2 efflux highest in early spring (8 g Cm-2 d-1) and lowest in late summer (0.4 g Cm-2 d-1). Here, the relative proportion of total respiration in the top 20 cm varied from 5% at the end of the dry season to 70% during the wet winter months. Soil respiration in the redwood soil appeared to be well buffered from climate extremes due to the stable microclimate created by the forest canopy. The grassland soil, however, lacking such buffering, was subject to large and rapid changes in CO2 production throughout the year. These results indicate that despite some uncertainty in the calculation of diffusion rates, significantly more information on soil biological processes is gained from the CO2 production-diffusion approach than from surface flux measurements alone. [ABSTRACT FROM AUTHOR]

For soil carbon to be effectively sequestered beyond a timescale of a few decades, this carbon must become incorporated into passive reservoirs or greater depths, yet the actual mechanisms by which this occurs is at best poorly known. In this study, we quantified the magnitude of dissolved organic carbon (DOC) leaching and subsequent retention in soils of a coniferous forest and a coastal prairie ecosystem. Despite small annual losses of DOC relative to respiratory losses, DOC leaching plays a significant role in transporting C from surface horizons and stabilizing it within the mineral soil. We found that DOC movement into the mineral soil constitutes 22% of the annual C inputs below 40 cm in a coniferous forest, whereas only 2% of the C inputs below 20 cm in a prairie soil could be accounted for by this process. In line with these C input estimates, we calculated advective transport velocities of 1.05 and 0.45 mm year−1 for the forested and prairie sites, respectively. Radiocarbon measurements of field-collected DOC interpreted with a basic transport-turnover model indicated that DOC which was transported and subsequently absorbed had a mean residence time of 90–150 years. Given these residence times, the process of DOC movement and retention is responsible for 20% of the total mineral soil C stock to 1 m in the forest soil and 9% in the prairie soil. These results provide quantitative data confirming differences in C cycles in forests and grasslands, and suggest the need for incorporating a better mechanistic understanding of soil C transport, storage and turnover processes into both local and regional C cycle models. [ABSTRACT FROM AUTHOR]

Abstract: Early (>3Gy) wetter climate conditions on Mars have been proposed, and it is thus likely that pedogenic processes have occurred there at some point in the past. Soil and rock chemistry of the Martian landing sites were evaluated to test the hypothesis that in situ aqueous alteration and downward movement of solutes have been among the processes that have transformed these portions of the Mars regolith. A geochemical mass balance shows that Martian soils at three landing sites have lost significant quantities of major rock-forming elements and have gained elements that are likely present as soluble ions. The loss of elements is interpreted to have occurred during an earlier stage(s) of weathering that may have been accompanied by the downward transport of weathering products, and the salts are interpreted to be emplaced later in a drier Mars history. Chemical differences exist among the sites, indicating regional differences in soil composition. Shallow soil profile excavations at Gusev crater are consistent with late stage downward migration of salts, implying the presence of small amounts of liquid water even in relatively recent Martian history. While the mechanisms for chemical weathering and salt additions on Mars remain unclear, the soil chemistry appears to record a decline in leaching efficiency. A deep sedimentary exposure at Endurance crater contains complex depth profiles of SO4, Cl, and Br, trends generally consistent with downward aqueous transport accompanied by drying. While no model for the origin of Martian soils can be fully constrained with the currently available data, a pedogenic origin is consistent with observed Martian geology and geochemistry, and provides a testable hypothesis that can be evaluated with present and future data from the Mars surface. [Copyright &y& Elsevier]

For soil carbon to be effectively sequestered beyond a timescale of a few decades, this carbon must become incorporated into passive reservoirs or greater depths, yet the actual mechanisms by which this occurs is at best poorly known. In this study, we quantified the magnitude of dissolved organic carbon (DOC) leaching and subsequent retention in soils of a coniferous forest and a coastal prairie ecosystem. Despite small annual losses of DOC relative to respiratory losses, DOC leaching plays a significant role in transporting C from surface horizons and stabilizing it within the mineral soil. We found that DOC movement into the mineral soil constitutes 22% of the annual C inputs below 40 cm in a coniferous forest, whereas only 2% of the C inputs below 20 cm in a prairie soil could be accounted for by this process. In line with these C input estimates, we calculated advective transport velocities of 1.05 and 0.45 mm year−1 for the forested and prairie sites, respectively. Radiocarbon measurements of field-collected DOC interpreted with a basic transport-turnover model indicated that DOC which was transported and subsequently absorbed had a mean residence time of 90–150 years. Given these residence times, the process of DOC movement and retention is responsible for 20% of the total mineral soil C stock to 1 m in the forest soil and 9% in the prairie soil. These results provide quantitative data confirming differences in C cycles in forests and grasslands, and suggest the need for incorporating a better mechanistic understanding of soil C transport, storage and turnover processes into both local and regional C cycle models. [ABSTRACT FROM AUTHOR]

We estimated the soil organic carbon (SOC) and soil inorganic carbon (SIC) inventory for the conterminous USA using the State Soil Geographic Database (STATSGO). The relative contribution of each soil order and Land Resource Region (LRR) to the national SOC and SIC inventory was determined. There are 302 to 1499 x 108 Mg of SOC and 226 to 937 X 108 Mg of SIC in the upper 2 m of soil in the conterminous USA. About 30 and 80% of the upper 2-m SOC is in the 0- to 0.20- and 0- to 1.0-m soil layers, respectively. For SIC, only about 8% of the upper 2-m SIC is in the upper 0.2 m, and about 50% is in the top 1.0-m layer. The relative spatial variability of SOC increases dramatically as soil depth increases while the largest relative variability of SIC is in the surface layer. Because of its large area (27% of the soil area in the conterminous USA), Mollisols are the largest contributors both to the SOC stock (about 31 to 39%) and to the SIC stock (about 43 to 44%) in the conterminous USA. The results of this study provide a view of soil C partitioning by taxonomic group and land resource area, information that may be useful for assessing the impact of land use and climatic change on SOC and SIC pools. [ABSTRACT FROM AUTHOR]

Abstract: Significant portions of the global soil organic carbon (SOC) pool must reside on sloping terrains where the spatial distribution of SOC reflects the combined effects of geomorphic processes and biological C cycling. Using a newly developed soil C mass balance model that explicitly includes soil production and sediment transport, we investigated the relative roles of sediment production/transport vs. biological C cycling in creating the observed spatial patterns of SOC storage within two grass-covered hillslopes in California. The study sites differed in bedrock geology, climate, and erosion rates. Measurements of SOC, soil texture, plant C inputs, and soil thickness were combined with topographic surveys and published soil erosion and production rates in the analysis. Soil thickness was found to be the key control on SOC storage, and soil thickness is balance between soil production and curvature-dependent erosional losses. Additionally, topographically varying rates of plant C inputs, decomposition rates, and SOC erosional losses or depositional inputs were found to only partially explain the observed SOC storage patterns. We used the measured relationships between SOC storage, soil thickness, and topographic curvature to create SOC storage maps of the two watersheds. At both sites, about 70% of the hillslope SOC is stored in depositional areas that are susceptible to episodic mass wasting. At the drier site, there was a larger SOC storage despite the lower soil C % because the clay-rich bedrock resulted in the development of relatively thick soils for a given slope curvature. We conclude that the geomorphic processes driving soil thickness provide fundamental mechanisms that control the spatial SOC patterns on vegetated hillslopes. [Copyright &y& Elsevier]

Burrowing organisms assist in shaping earth surfaces and are simultaneously affected by the environment they inhabit; however, a conceptual framework is not yet available to describe this feedback. We introduce a model that connects the population density of soil-burrowing animals to sediment transport via energy. The model, combined with avail- able data from California hillslopes where soil erosion is driven by pocket gophers (Thomomys bottae), suggests that a gopher annually expends ∼9 kJ of energy, or ∼1% of reported burrowing energy expenditure, in generating sediment transport. The model is used to evaluate the case that gophers prefer to populate thicker soils. The results suggest that this behavior may drastically dampen the spatial and temporal variations of soil thickness and gopher populations, implying that burrowing organisms may create landscapes distinct from those affected by abiotic processes. [ABSTRACT FROM AUTHOR]

The accuracy of paleoenvironmental reconstructions based on isotope analyses of equid teeth is currently uncertain because the exact relationship between the isotope composition of modern feral equids and their environment has not been thoroughly studied. We analyzed the carbon and oxygen isotope values (δ13C and δ18O) of tooth enamel carbonate and the δ13C values of fecal samples from modern feral horses. We compared those values with the δ13C values of local vegetation and the δ18O values of local waters. Herds were studied in two contrasting localities: eastern Oregon, where grasslands consisted of 100% C3 species, and New Mexico, where >95% of the grasses were C4 species. Carbon isotope analyses of fecal material and tooth enamel suggest that horses consumed primarily grass, but some New Mexico horses also consumed significant amounts of shrubs and/or forbs. Microhistological analyses of fecal samples show that Oregon horses consumed 95% grass, and Oregon enamel δ13C values are consistent with a diet containing 100% C3 plants. Microhistological analyses of fecal samples from New Mexico indicate a diet averaging 75% grass, while enamel δ13C values suggest that diets averaged 85% C4 plants (range=72–97%). Thus, reconstructions of the C3/C4 ratio of grasses in ancient grasslands that are based on the δ13C values of fossil equid teeth may underestimate the abundance of C4 grasses. The mean δ18O values of tooth enamel paralleled the trends observed in the mean δ18O values of precipitation. However, the mean δ18O values of enamel carbonate from Oregon and New Mexico differed by only 3.3‰, which is less than the difference in the mean δ18O values of precipitation (6.5‰). In addition, the range of δ18O values within New Mexico enamel samples (6.5‰) was greater than the difference between mean enamel δ18O values at each site. Calculated values for the δ18O of water ingested by horses are 2–3‰ more positive than mean δ18O values for corresponding precipitation, suggesting that horses consumed waters that were enriched in 18O due to evaporation. While our results confirm that local climatic and hydrological conditions can influence the δ18O values of equid enamel, they also show that the δ18O values of equid teeth are not always a direct proxy for the isotope ratios of precipitation. [Copyright &y& Elsevier]

Here, we investigate the response of soil organic carbon (SOC) and soil organic nitrogen (SON) to cultivation within two different climatic regimes by comparing large soil data sets from India and the Great Plains. Multiple regression models for both regions show that SOC and SON, as well as C/N ratios, increase with decreasing temperatures and increasing precipitation, trends also noted in soil data organized by Holdridge life zones. The calculated difference between natural and cultivated soils in India revealed that the greatest absolute SOC and SON losses occurred in regions of low temperatures and high precipitation, while the C/N ratio decreased during cultivation only with decreasing temperature. In India, the fractional loss of SOC relative to undisturbed soils increases with decreasing temperature whereas, in the Great Plains, it increases with increasing precipitation. Also, the fractional loss of SOC increased in India with increasing amounts of original C, whereas no relationship between fractional loss and original C was noted for the Great Plains. The differential response of each region to cultivation is hypothesized to be due to differences in both climate and management practices (crop cycles, fertilization). These findings suggest that estimates of soil C loss due to cultivation should be based on an array of factors, and that it is unlikely that a constant relative C loss occurs in any region. [ABSTRACT FROM AUTHOR]

Soils are dynamic components of terrestrial ecosystems that historically have been viewed as economic resources by government and private interests. The large-scale conversion of many sections of the United States to agriculture and urban land uses, combined with the growing awareness of the role of soils in global biogeochemistry and ecology, ultimately requires an assessment of the remaining distribution of undisturbed soils in the country. Here we conduct the first quantitative analysis of disturbed and undisturbed soil distribution in the USA using a GIS-based approach. We find that a sizable fraction (4.5%) of the nation’s soils are in danger of substantial loss, or complete extinction, due to agriculture and urbanization. In the agricultural belt of the country, up to 80% of the soils that were naturally of low abundance are now severely impacted (greater than 50% conversion to agricultural/urban uses). Undisturbed soils provide ecosystem services that warrant their preservation, including a somewhat complex relationship with rare or endangered plants. The known and unknown attributes of undisturbed soils suggests the need for an integrated biogeodiversity perspective in landscape preservation efforts. [ABSTRACT FROM AUTHOR]

In this paper, we report the taxonomic structure, the spatial distribution, and relative abundance of soils in the USA. In the analysis, we used the STATSGO (1997 version) database, which contains information on 11 orders, 52 suborders, 232 great groups, 1175 subgroups, 6226 families, and 13 129 series. The analysis of taxonomic structure showed that the numbers of taxa in any category were distributed unevenly in relation to taxa present at the next higher category. This uneven distribution becomes more pronounced in the lower categories of the hierarchy. In addition, there is a trend for taxa to produce only one, or a very small number, of taxa in the next lower category at the lower categories of the system. The analysis of the spatial distribution of taxa showed that 10 662 (51.2%) out of 20 825 taxa exist only 1 Major Land Resource Area (MLRA), indicating that most soil taxa are not widely spread, and are specific to particular combinations of state factors. The area abundance of soil taxa is asymmetric with most taxa in a category having relative small area extent. Five (2.1%) greatgroups, 37 (2.1%) subgroups, 417 (6.7) families, and 827 (6.3%) series had total areas less than 10 km², and were defined as rare taxa. Among the rare taxa, four (80%) rare great groups, 36 (97.3%) rare subgroups, 378 (90.6%) rare families, and 750 (90.7%) rare series were found in only 1 MLRA. The portion of rare soils might be much higher because not all the soils are included in STATSGO because of their too limited area. The spatial and area abundance analyses of the soils provides a perspective useful for discussions on the preservation of soil resources in the USA, a topic whose importance is likely to grow in conjunction with increasing interest in global biodiversity and more intense uses of the world's soil resources. [ABSTRACT FROM AUTHOR]

The global soil C reservoir, ∼1500 Gt of C (1 Gt = 10[sup 12] kg of C), is dynamic on decadal time scales and is sensitive to climate and human disturbance. At present, as a result of land use, soil C is a source of atmospheric CO2 in the tropics and possibly part of a sink in northern latitudes. Here I review the processes responsible for maintaining the global soil C reservoir and what is known about how it responds to direct and indirect human perturbations. “I am fire and air; my other elements I give to baser life” —William Shakespeare, Antony and Cleopatra [ABSTRACT FROM AUTHOR]

• Factors of Soil Formation is experiencing an exponential increase in scientific citations. • The book, patterned on thermodynamics, reformulated ideas introduced by Dokuchaev. • The State Factor approach is an organizing principle for the Critical Zone research program. • The State Factor theory reveals important remaining research questions. • The least examined State Factor is humans, a glaring need for societally important issues. The year 2021 marks the 80th anniversary of the publication of Factors of Soil Formation. A System of Quantitative Pedology by Hans Jenny. Given the exponential surge in annual citations of the book over the past 30 years, and this important milestone in its history, it is an opportune time to examine what the book presents, and how it is relevant to transdisciplinary research in the 21st Century. The book did not originate the concept of soil forming factors, which were by then already widely accepted in soil science, but instead translated it into a quantitative scientific framework through rigorous definition of the soil system, and the identification and separation of dependent and independent variables. The initial formulation of the theoretical framework was inspired by thermodynamics, but with pedologically-relevant state variables. Subsequent formulations of the model by Jenny articulated the connection and utility to examining energy and mass fluxes through the soil system, a point of departure for recent efforts to bring open system, non-equilibrium thermodynamic methods to bear on soil research. The State Factor theory is a powerful lens to examine a number of academic and societally relevant issues in the 21st century, such as the role of the biotic factor and its interplay with the physical soil environment. However, the most critical, and understudied, state factor is the effect of humans on soil systems. Earth science must embrace the challenge, and obligation to society, to explore the human footprint on the planet, and devise ways to remediate its impact. [ABSTRACT FROM AUTHOR]

Reviews the structure and attributes of the state factor model, briefly identifying its origin, tracing its evolution and illustrating its use during the twentieth century, with the goal of bringing it to a wider audience in the ecological community. Structure and state factor model; Development of the model; The state factor model as a historical science; The model in the study of terrestrial ecosystems.

Observes that the oxygen isotope composition of precipitation on continents in middle to polar latitudes is a function of mean annual temperature. Evidence of precipitation seasonality and storm sources; Interpretation of oxygen isotope ratios on pre-Holocene soils; Identification of past storm sources.

• The governance of soil to achieve global solutions is a long-standing challenge. • Binding international soil policy is lacking. • A complex web of institutions exhibit emergent properties that may lead to change. • Soil management problems are chronic problems requiring natural/social science partnerships. The global soilscape is a resource commons that is essential to society. Social science has been engaged in theoretical and empirical research as to how commons can be effectively managed, especially at a global scale, for more than 50 years. This has provided some rules and guideposts for their sustainable management, rules relevant to the soil commons. Presently, there is an international soil policy vacuum. The absence of binding policy helps explain the inability to implement scientific tools, such as soil C climate mitigation strategies, at significant levels over the past two decades. At national and subnational scales, especially in the USA which is considered here, there are a multitude of public and private soil health policies and programs, with vastly different scales and funding resources. These are largely not well coordinated, but according to polycentric management theory, may be viewed as a complex system, with evolving emergent properties and resulting impacts. Soil science must connect with business leaders, social scientists, policy implementers, politicians and others to develop a more coherent and effective policy web, one which might eventually lead to binding international agreements. This is likely to be a multi-generational effort, but will provide intellectually stimulating challenges for present and future generations of scholars. [ABSTRACT FROM AUTHOR]

The article highlights soil management for climate mitigation and climate adaptation strategy. Topics discussed include soil scientists on rolling for both climate mitigation and adaption for soil carbon sequestration; the U.S. farm bill on supporting to adoption of soil conservation; and the U.S. Conservation Reserve Program for shifting in land management due to climate change.

• On the California coast N is tightly cycled by biological activity whereas S appears to be in excess. • Depth profiles of C, N, and their isotopes provides insights into rates of C and N cycling. • Soil age in this setting seemed to have a limited effect on C, N, and S cycling. Carbon (C), nitrogen (N), and sulfur (S) are elements strongly influenced by biological cycling and redox reactions in soils, but few comparative analyses have investigated the behaviors of these elements with time. Thus, we examined changes in content and isotope composition of soil profiles along a chronosequence (58–212 kyr) of marine terraces on the central California coast, in an area with significant background geochemical research. Unlike in other chronosequences in more humid locations, the total C, N, and S in these soils did not vary strongly with age, possibly due to the retention of phosphorus (P). The total pools of soil N and S cannot be explained by wet deposition of NO 3 and SO 4 alone, suggesting other sources of atmospheric inputs such as NH 4 + and dimethyl sulfide. Total C and N declined in a characteristic logarithmic pattern with depth, while S did not. The ratio of extractable soil nitrate (NO 3) to total N declined with depth, suggesting strong biological demand via various avenues. In contrast, the ratio of extractable sulfate (SO 4) to total S increased with depth, suggesting that S was in biological excess. We used a simple reactive transport model to integrate the depth profiles of total C and N and their isotope values. The depth trends of total concentrations suggested one-pool residence times of approximately 500 to 1000 y, consistent with turnover times calculated by mass balance. Depth trends of stable isotope values indicated that N is isotopically fractionated at a magnitude twice that of C, consistent with observed 15N-depleted nitrous oxide (N 2 O) emissions during the dry summer months. The isotope composition of S (total and SO 4) suggests some isotope enrichment during biogeochemical cycling, but far less than observed for N. Thus, despite significant chemical weathering and elemental loss over time, the biogeochemical cycles of C, N, and S remain relatively unaffected by soil age in this climatic setting. [ABSTRACT FROM AUTHOR]

In the past decade, pedological research in hyperarid environments worldwide has revealed landscapes and soil features that are regionally prevalent and distinctive, but not well captured in the current soil taxonomy. Hyperarid soils bear genetic features that can differ considerably from those in more humid desert environments. In particular, they often contain horizons cemented with halite (NaCI) or heavily enriched with nitratine (NaNO3). Unlike soils commonly found in more humid regions, they may also lack most vascular plants, pedogenic carbonates, and biotic mixing processes. Based on the global occurrence of indurated salic horizons and nitric horizons in hyperarid soils, we propose that the soil taxonomy be amended to include a petrosalic diagnostic subsurface horizon, a nitric diagnostic subsurface horizon, and Petrosalids great group within the Aridisols order. Additionally, we suggest the definition and establishment of a Hyperaridic Soil Moisture Regime (SMR). In this paper, we use soils from the Atacama Desert to illustrate how pedogenesis in extreme hyperarid environments differs from that in more humid environments, and we review the current literature regarding soils in other hyperarid locations with indurated soluble salt horizons. The changes proposed here would create a more encompassing classification system for the Earth's desert regions, improving our ability to clearly communicate relevant genetic, ecological, and economic information to both land-managers and researchers. [ABSTRACT FROM AUTHOR]

Abstract: There is significant debate over the rates and types of fluvial activity at the Plio-Pleistocene boundary in the hyperarid Atacama Desert of Chile. To quantify fluvial processes and help resolve this debate, we measure terrestrial cosmogenic nuclide (TCN) (10Be and 21Ne) concentration depth profiles in three settings representing a chronosequence: (1) a late Pliocene alluvial fan representative of major regional deposits, (2) a modern, active channel and (3) an adjacent low terrace inset into the Pliocene alluvium. Late Pliocene deposits that are widely preserved in the region contain TCN profiles consistent with relatively rapid stripping of upland sediment at the Plio-Pleistocene boundary. Deposits inset into these Late Pliocene features record cut and fill cycles that rework sediment throughout the Quaternary. The TCN profile in the modern channel is best explained by sediment aggradation at 2.1mMyr−1 during the last 250,000yr. Similarly, the adjacent low terrace sediments contain TCN concentration profiles consistent with aggradation of 2.0mMyr−1 over a period of 250,000–750,000yr prior to the last 250,000yr of stability. In summary, depth profiles of two TCNs provide constraints on the rates of sediment deposition, sources of sediment and transport history, as well as the subsequent exposure conditions of the sediment following deposition. Our results are consistent with early Quaternary initiation of hyperaridity for the region. During the Quaternary, winter precipitation events experienced at our sites' latitude (24°S) drive active erosion–deposition cycles. The northward migration of the subtropical front during Quaternary glacial cycles may have enhanced precipitation at 24°S, leading to more active fluvial processes during cooler periods. [Copyright &y& Elsevier]

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]

Maps of soil organic carbon (SOC) and inorganic carbon (SIC) were generated from the State Soil Geographical database (STATSGO) and were overlain with land-cover, topography (elevation and slope), mean annual precipitation (MAP), and mean annual temperature (MAT) databases to study the effects of environmental driving factors (or ‘state factors’) in the conterminous USA. In the USA, human disturbance has significantly reduced SOC and SIC in the surface layer. The SOC decreases as elevation increases. Level topography has twice the SOC content of other slope classes. Soil organic C increases as MAP increases up to values of 700–850 mm yr-1. There is no obvious pattern in SIC as MAP increases until MAP exceeds 1000 mm, at which point on SIC drops dramatically. For natural vegetation with MAP < 1000 mm, SOC decreases as MAT increases (within restricted ranges of elevation and topography). The relationship between SOC versus MAT varies with MAP, and SOC is most sensitive to temperature in low and moderate MAP regions and in the upper soil layers. This GIS-based analysis is a relatively coarse evaluation given the nature of the database, but does provide a first quantitative assessment of soil C to ‘state factors’ for the USA as a whole using commonly available digital databases. [ABSTRACT FROM AUTHOR]

Abstract: We analyzed the isotopic patterns found in the tooth enamel of modern feral horses from Shackleford Banks, North Carolina (USA), which has a temperate climate and supports primarily C4 grasslands. Enamel δ 13C values averaged −4.1‰ with a standard deviation (1σ) of 1.7‰, which corresponds to an average diet of 66 ± 12% C4 plants. Our results differ from dietary reconstructions from 1978 to 1981, which found that horses consumed 91% C4 plants. This suggests that horses have increased their consumption of C3 forbs, likely as a result of the removal of cattle, sheep, and goats from the island. Shackleford surface waters had δ 18O values that averaged −3.3 ± 0.5‰ and −1.3 ± 1.8‰ on the western and eastern ends of the island, respectively. Tooth enamel samples averaged 27.3 ± 1.5‰ and displayed the same range of δ 18O values as surface waters. The variability of both δ 18O and the δ 13C values among individuals within this population demonstrates that horses from relatively homogenous temperate environments can display a wide range of isotopic values. Given the observed range of isotopic values for modern horses, we suggest that researchers use the mean values of multiple (≥9) equids when attempting to reconstruct average paleodiets and/or paleoenvironmental conditions. [Copyright &y& Elsevier]

Little attention has been paid to analyses of pedodiversity. In this study, quantitative aspects of pedodiversity were explored for the USA based on the State Soil Geographic database (STATSGO). First, pedodiversity indices for the conterminous USA were estimated. Second, taxa–area relationships were investigated in each soil taxonomic category. Thirdly, differences in pedodiversity between the USDA-NRCS geographical regions were compared. Fourth, the possible mechanisms underlying the observed relative abundance of soil taxa were explored. Results show that as the taxonomic category decreases from order to series, Shannon''s diversity index increases because taxa richness increased dramatically. The relationship between the number of taxa (S) and area (A) is formulated as S=cAz. The exponent z reflects the taxa-richness of soil ‘communities’ and increases constantly as taxonomic categories decrease from order to series. The “West” USDA-NRCS geographical region has the highest soil taxa richness, followed by the “Northern Plains” region. The “South Central” region has the highest taxa evenness, while taxa evenness in the “West” region is the lowest. The “West” or the “South Central” regions have the highest overall soil diversity in the four highest taxonomic categories, while the “West” or “Northern Plains” regions have the highest diversity in the two lowest taxonomic levels. The high diversity index in the “West” region results from high taxa richness while the high diversity index in the “South Central” region results from an evenness of taxa. As the taxonomic level decreases, the pattern of taxa abundance approaches a lognormal distribution. One of the key findings of this research is that at lower levels of soil taxonomic divisions (especially the series level), soil taxa increase continuously with increasing area, indicating considerable soil endemism in the USA (and likely around the world), a key consideration in conservation and preservation planning. [Copyright &y& Elsevier]

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]

Rationale: Nitrate is an oxyanion similar to CO3 - and thus should undergo stable N and O isotope fractionation during dissolution or precipitation. This process should dominate abiotic soil nitrate processes in hyperarid regions of Earth and possibly Mars. Methods: The N and O isotope fractionations during the precipitation of nitrate salt from saturated solutions at 20C were determined by two methods: rapid precipitation by antisolvent crystallization and slow uninhibited precipitation in a desiccator. In the antisolvent crystallization procedure, increasing volumes of acetone were added to samples of saturated sodium and strontium nitrate solutions to instantaneously precipitate nitrate salt. In the slow procedure (requiring weeks), slow evaporative water loss drove the process. Results: There was little difference between the two procedures. Using a Rayleigh model, the calculated N fractionation (15eproduct-residual) ranged from 1.69?to 2.77?, whereas for O, the 18eproduct-residual values were between 1.27? and 4.61?. The N isotope fractionation between NO3 - and the metal solid is similar to that between C in dissolved CO3 -2 and carbonates. We found that O versus N isotope plots of soil nitrate in a cold/dry Antarctic chronosequence had slopes similar to those from the experiments, revealing abiotic transport. In the Atacama Desert, where the soil nitrates are a mix of biological and tropospheric nitrate, there is an inverse relationship between soil N and O isotopes. These two relationships were compared with the isotope composition of nitrate from Martian meteorite EETA79001. Conclusions: While the N and O isotope composition of the Martian nitrate is remarkably similar to that of the present Atacama Desert, the interpretation of the slope of the O versus N isotopes remains ambiguous due to the limited number of samples. Additional NO3 samples from Martian meteorites are needed to address the question of abiotic versus biotic alteration of NO3 - on Mars. [ABSTRACT FROM AUTHOR]

We discovered permanently hydrated CaCl2-rich soils in Earth's driest region, the Atacama Desert. The soils contain up to ~15% CaCl2. X-ray diffraction indicates the rare minerals sinjarite, schoenite, and tachyhydrite. When water is added, the CaCl2 crust immediately turns white due to an apparent mineralogical phase change from sinjarite to a brine. The surfaces are nearly continuously wet due to the salt's hygroscopicity. The Ca-enriched soils occur in rare exposures, possibly from shallow groundwater. Unlike the surface of adjacent abundant halite crusts, the CaCl2 outcrops remain continuously wet, with up to 12% water under modern, and essentially rainless, climatic conditions. The wet surface stabilizes the land surface and acts as a dust trap. The sediment began accumulating at ca. 14 ka, contains trace quantities of organic carbon, and has total nitrogen that isotopically reflects significant biologically mediated gaseous losses. These deliquescent salts are unique habitats for life within the climatic limits of life on Earth, and are a potential analog for transient liquid- water sources for microorganisms in Martian soils. [ABSTRACT FROM AUTHOR]

Abstract The halite-encrusted salt pans (salars) present at low elevations in the hyperarid core of the Atacama Desert in northern Chile are unique features of one of the driest and possibly oldest deserts on Earth. Here we show that these landscapes were shallow freshwater lakes and wetlands during the last glacial period and formed periodically between ∼46.9 ka and 7.7 ka. The moisture appears to have been sourced from increased Andean runoff and most of our chronologies for these deposits were coeval with the Central Andean Pluvial Event (17.5–14.2 ka and 13.8–9.7 ka), but we also find evidence for older as well as slightly younger wet phases. These environments supported a diverse hygrophytic-halophytic vegetation, as well as an array of diatoms and gastropods. Using a regional hydrological model, we estimate that recharge rates from 1.5 to 4 times present were required to activate and maintain these wetlands in the past. Activation in the late Pleistocene was part of a regional enhancement of water resources, extending from the Andes, downstream and through riparian corridors, to the lowest and most arid portions of the desert itself. This fundamentally unique environment was encountered by the earliest human explorers in the region, and most likely facilitated migration and encampments on a landscape that at present lacks macroscopic life on its surface. Highlights • Lakes and wetlands formed in the hyperarid core of the Atacama during the late Quaternary. • Groundwater highstands formed a network of wetlands and paleolakes from the Andes to the Pacific. • Most of our findings are coeval with the Central Andean Pluvial Event (CAPE) 17.5–9.7 ka. • Pre-CAPE enhanced periods of recharge have occurred since ∼46.9 ka. [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]

This paper reports a previously unidentified paleoenvironmental record found in sand dunes of the Atacama Desert, Chile. Long-term aeolian deflation by prevailing onshore winds has resulted in the deposition of sand on the irregular surface of a Miocene-aged anhydrite outcrop. Two deposits ~25 km apart, along the prevailing wind trajectory, were hand excavated then analyzed for vertical (and temporal) changes in physical and chemical composition. Radiocarbon ages of organic matter embedded within the deposits show that rapid accumulation of sediment began at the last glacial maximum and slowed considerably after the Pacific Ocean attained its present post-glacial level. Over this time period, grain sizes are seen to increase while accumulation rates simultaneously decrease, suggesting greater wind speeds and/or a change or decrease in sediment supply. Changes in δ34S values of sulfate in the sediment beginning ~10 ka indicate an increase in marine sources. Similarly, δ2H values from palmitic acid show a steady increase at ~10 ka, likely resulting from aridification of the region during the Holocene. Due to the extreme aridity in the region, these sand dunes retain a well-preserved chemical record that reflects changes in elevation and coastal proximity after the last glacial maximum. [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]