Browsing by Author "Richter, DD"
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Item Open Access A formal Anthropocene is compatible with but distinct from its diachronous anthropogenic counterparts: a response to W.F. Ruddiman’s ‘three flaws in defining a formal Anthropocene’(Progress in Physical Geography, 2019-06-01) Zalasiewicz, J; Waters, CN; Head, MJ; Poirier, C; Summerhayes, CP; Leinfelder, R; Grinevald, J; Steffen, W; Syvitski, J; Haff, P; McNeill, JR; Wagreich, M; Fairchild, IJ; Richter, DD; Vidas, D; Williams, M; Barnosky, AD; Cearreta, A© The Author(s) 2019. We analyse the ‘three flaws’ to potentially defining a formal Anthropocene geological time unit as advanced by Ruddiman (2018). (1) We recognize a long record of pre-industrial human impacts, but note that these increased in relative magnitude slowly and were strongly time-transgressive by comparison with the extraordinarily rapid, novel and near-globally synchronous changes of post-industrial time. (2) The rules of stratigraphic nomenclature do not ‘reject’ pre-industrial anthropogenic signals – these have long been a key characteristic and distinguishing feature of the Holocene. (3) In contrast to the contention that classical chronostratigraphy is now widely ignored by scientists, it remains vital and widely used in unambiguously defining geological time units and is an indispensable part of the Earth sciences. A mounting body of evidence indicates that the Anthropocene, considered as a precisely defined geological time unit that begins in the mid-20th century, is sharply distinct from the Holocene.Item Open Access Designing a network of critical zone observatories to explore the living skin of the terrestrial Earth(Earth Surface Dynamics, 2017-12-18) Brantley, SL; McDowell, WH; Dietrich, WE; White, TS; Kumar, P; Anderson, SP; Chorover, J; Ann Lohse, K; Bales, RC; Richter, DD; Grant, G; Gaillardet, JThe critical zone (CZ), the dynamic living skin of the Earth, extends from the top of the vegetative canopy through the soil and down to fresh bedrock and the bottom of the groundwater. All humans live in and depend on the CZ. This zone has three co-evolving surfaces: the top of the vegetative canopy, the ground surface, and a deep subsurface below which Earth's materials are unweathered. The network of nine CZ observatories supported by the US National Science Foundation has made advances in three broad areas of CZ research relating to the co-evolving surfaces. First, monitoring has revealed how natural and anthropogenic inputs at the vegetation canopy and ground surface cause subsurface responses in water, regolith structure, minerals, and biotic activity to considerable depths. This response, in turn, impacts aboveground biota and climate. Second, drilling and geophysical imaging now reveal how the deep subsurface of the CZ varies across landscapes, which in turn influences aboveground ecosystems. Third, several new mechanistic models now provide quantitative predictions of the spatial structure of the subsurface of the CZ.
Many countries fund critical zone observatories (CZOs) to measure the fluxes of solutes, water, energy, gases, and sediments in the CZ and some relate these observations to the histories of those fluxes recorded in landforms, biota, soils, sediments, and rocks. Each US observatory has succeeded in (i) synthesizing research across disciplines into convergent approaches; (ii) providing long-term measurements to compare across sites; (iii) testing and developing models; (iv) collecting and measuring baseline data for comparison to catastrophic events; (v) stimulating new process-based hypotheses; (vi) catalyzing development of new techniques and instrumentation; (vii) informing the public about the CZ; (viii) mentoring students and teaching about emerging multidisciplinary CZ science; and (ix) discovering new insights about the CZ. Many of these activities can only be accomplished with observatories. Here we review the CZO enterprise in the United States and identify how such observatories could operate in the future as a network designed to generate critical scientific insights. Specifically, we recognize the need for the network to study network-level questions, expand the environments under investigation, accommodate both hypothesis testing and monitoring, and involve more stakeholders. We propose a driving question for future CZ science and ahubs-and-campaigns
model to address that question and target the CZ as one unit. Only with such integrative efforts will we learn to steward the life-sustaining critical zone now and into the future.Item Open Access Estimates and determinants of stocks of deep soil carbon in Gabon, Central Africa(Geoderma, 2019-05-01) Wade, AM; Richter, DD; Medjibe, VP; Bacon, AR; Heine, PR; White, LJT; Poulsen, JR© 2019 Despite the importance of tropical forest carbon to the global carbon cycle, research on carbon stocks is incomplete in major areas of the tropical world. Nowhere in the tropics is this more the case than in Africa, and especially Central Africa, where carbon stocks are known to be high but a scarcity of data limits understanding of carbon stocks and drivers. In this study, we present the first nation-wide measurements and determinants of soil carbon in Gabon, a nation in Central Africa. We estimated soil carbon to a 2-m depth using a systematic, random design of 59 plots located across Gabon. Soil carbon to a 2-m depth averaged 163 Mg ha −1 with a CV of 61%. These soil carbon stocks accounted for approximately half of the total carbon accumulated in aboveground biomass and soil pools. Nearly a third of soil carbon was stored in the second meter of soil, averaging 58 Mg ha −1 with a CV of 94%. Lithology, soil type, and terrain attributes were found to be significant predictors of cumulative SOC stocks to a 2-m depth. Current protocols of the IPCC are to sample soil carbon from the surface 30 cm, which in this study would underestimate soil carbon by 60% and underestimate ecosystem carbon by 30%. A nonlinear model using a power function predicted cumulative soil carbon stocks in the second meter with an average error of prediction of 3.2 Mg ha −1 (CV = 915%) of measured values. The magnitude and turnover of deep soil carbon in tropical forests needs to be estimated as more countries prioritize carbon accounting and monitoring in response to accelerating land-use change.Item Open Access Game Changer in Soil Science. The Anthropocene in soil science and pedology.(Journal of Plant Nutrition and Soil Science, 2020-02-01) Richter, DD© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim The venerable science of pedology, initiated in the 19th century as the study of the natural factors of soil formation, is adapting to the demands of the Anthropocene, the geologic time during which planet Earth and its soils are transitioning from natural to human-natural systems. With vast areas of soils intensively managed, the future of pedology lies with a renewed science that can be called anthropedology that builds on the pedology of the past but proceeds from “human as outsider” to “human as insider.” In other words, the human in pedology must shift from being a soil-disturbing to soil-forming agent. Pedology is well prepared to respond to the challenges of the Anthropocene, given the decades of research on human-soil relations throughout human history and throughout the period of the Great Acceleration (Steffen et al., [76]). However, quantitative understanding of soil responses to the diversity of human forcings remains elementary and needs remedy.Item Open Access Ideas and perspectives: Strengthening the biogeosciences in environmental research networks(Biogeosciences, 2018-08-15) Richter, DD; Billings, SA; Groffman, PM; Kelly, EF; Lohse, KA; McDowell, WH; White, TS; Anderson, S; Baldocchi, DD; Banwart, S; Brantley, S; Braun, JJ; Brecheisen, ZS; Cook, CS; Hartnett, HE; Hobbie, SE; Gaillardet, J; Jobbagy, E; Jungkunst, HF; Kazanski, CE; Krishnaswamy, J; Markewitz, D; O'Neill, K; Riebe, CS; Schroeder, P; Siebe, C; Silver, WL; Thompson, A; Verhoef, A; Zhang, G© Author(s) 2018. Long-term environmental research networks are one approach to advancing local, regional, and global environmental science and education. A remarkable number and wide variety of environmental research networks operate around the world today. These are diverse in funding, infrastructure, motivating questions, scientific strengths, and the sciences that birthed and maintain the networks. Some networks have individual sites that were selected because they had produced invaluable long-term data, while other networks have new sites selected to span ecological gradients. However, all long-term environmental networks share two challenges. Networks must keep pace with scientific advances and interact with both the scientific community and society at large. If networks fall short of successfully addressing these challenges, they risk becoming irrelevant. The objective of this paper is to assert that the biogeosciences offer environmental research networks a number of opportunities to expand scientific impact and public engagement. We explore some of these opportunities with four networks: the International Long-Term Ecological Research Network programs (ILTERs), critical zone observatories (CZOs), Earth and ecological observatory networks (EONs), and the FLUXNET program of eddy flux sites. While these networks were founded and expanded by interdisciplinary scientists, the preponderance of expertise and funding has gravitated activities of ILTERs and EONs toward ecology and biology, CZOs toward the Earth sciences and geology, and FLUXNET toward ecophysiology and micrometeorology. Our point is not to homogenize networks, nor to diminish disciplinary science. Rather, we argue that by more fully incorporating the integration of biology and geology in long-term environmental research networks, scientists can better leverage network assets, keep pace with the ever-changing science of the environment, and engage with larger scientific and public audiences.Item Open Access Limited carbon contents of centuries old soils forming in legacy sediment(Geomorphology, 2020-04) Wade, AM; Richter, DD; Cherkinsky, A; Craft, CB; Heine, PRItem Open Access Modifications of 2:1 clay minerals in a kaolinite-dominated Ultisol under changing land-use regimes(Clays and Clay Minerals, 2018-02-01) Austin, JC; Perry, A; Richter, DD; Schroeder, PA© 2018, Clay Minerals Society. All rights reserved. Chemical denudation and chemical weathering rates vary under climatic, bedrock, biotic, and topographic conditions. Constraints for landscape evolution models must consider changes in these factors on human and geologic time scales. Changes in nutrient dynamics, related to the storage and exchange of K+ in clay minerals as a response to land use change, can affect the rates of chemical weathering and denudation. Incorporation of these changes in landscape evolution models can add insight into how land use changes affect soil thickness and erodibility. In order to assess changes in soil clay mineralogy that result from land-use differences, the present study contrasts the clay mineral assemblages in three proximal sites that were managed differently over nearly the past two centuries where contemporary vegetation was dominated by old hardwood forest, old-field pine, and cultivated biomes. X-ray diffraction (XRD) of the oriented clay fraction using K-, Mg-, and Na-saturation treatments for the air-dried, ethylene glycol (Mg- EG and K-EG) solvated, and heated (100, 350, and 550ºC) states were used to characterize the clay mineral assemblages. XRD patterns of degraded biotite (oxidized Fe and expelled charge-compensating interlayer K) exhibited coherent scattering characteristics similar to illite. XRD patterns of the Mg-EG samples were, therefore, accurately modeled using NEWMOD2® software by the use of mineral structure files for discrete illite, vermiculite, kaolinite, mixed-layer kaolinite-smectite, illite-vermiculite, kaolinite-illite, and hydroxy-interlayered vermiculite. The soil and upper saprolite profiles that formed on a Neoproterozoic gneiss in the Calhoun Experimental Forest in South Carolina, USA, revealed a depth-dependence for the deeply weathered kaolinitic to the shallowly weathered illitic/vermiculitic mineral assemblages that varied in the cultivated, pine, and hardwood sites, respectively. An analysis of archived samples that were collected over a five-decade growth period from the pine site suggests that the content of illite-like layers increased at the surface within 8 y. Historical management of the sites has resulted in different states of dynamic equilibrium, whereby deep rooting at the hardwood and pine sites promotes nutrient uplift of K from the weathering of orthoclase and micas. Differences in the denudation rates at the cultivated, pine, and hardwood sites through time were reflected by changes in the soil clay mineralogy. Specifically, an increased abundance of illite-like layers in the surface soils can serve as a reservoir of K+.Item Open Access Persistent anthropogenic legacies structure depth dependence of regenerating rooting systems and their functions(Biogeochemistry, 2020-02-01) Hauser, E; Richter, DD; Markewitz, D; Brecheisen, Z; Billings, SA© 2020, Springer Nature Switzerland AG. Biotically-mediated weathering helps to shape Earth’s surface. For example, plants expend carbon (C) to mobilize nutrients in forms whose relative abundances vary with depth. It thus is likely that trees’ nutrient acquisition strategies—their investment in rooting systems and exudates—may function differently following disturbance-induced changes in depth of rooting zones and soil nutrient stocks. These changes may persist across centuries. We test the hypothesis that plant C allocation for nutrient acquisition is depth dependent as a function of rooting system development and relative abundances of organic vs. mineral nutrient stocks. We further posit that patterns of belowground C allocation to nutrient acquisition reveal anthropogenic signatures through many decades of forest regeneration. To test this idea, we examined fine root abundances and rooting system C in organic acid exudates and exo-enzymes in tandem with depth distributions of organically- and mineral-bound P stocks. Our design permitted us to estimate C tradeoffs between organic vs. mineral nutrient benefits in paired forests with many similar aboveground traits but different ages: post-agricultural mixed-pine forests and older reference hardwoods. Fine roots were more abundant throughout the upper 2 m in reference forest soils than in regenerating stands. Rooting systems in all forests exhibited depth-dependent C allocations to nutrient acquisition reflecting relative abundances of organic vs. mineral bound P stocks. Further, organic vs. mineral stocks underwent redistribution with historic land use, producing distinct ecosystem nutritional economies. In reference forests, rooting systems are allocating C to relatively deep fine roots and low-C exudation strategies that can increase mobility of mineral-bound P stocks. Regenerating forests exhibit relatively shallower fine root distributions and more diverse exudation strategies reflecting more variable nutrient stocks. We observed these disparities in rooting systems’ depth and nutritional mechanisms even though the regenerating forests have attained aboveground biomass stocks similar to those in reference hardwood forests. These distinctions offer plausible belowground mechanisms for observations of continued C sink strength in relatively old forests, and have implications for soil C fates and soil development on timescales relevant to human lifetimes. As such, depth-dependent nutrient returns on plant C investments represent a subtle but consequential signal of the Anthropocene.Item Open Access Quantification of Mixed-Layer Clays in Multiple Saturation States Using NEWMOD2: Implications for the Potassium Uplift Hypothesis in the SE United States(Clays and Clay Minerals, 2020-02-01) Austin, JC; Richter, DD; Schroeder, PA© 2020, The Clay Minerals Society. Quantification of mineral assemblages in near-surface Earth materials is a challenge because of the often abundant and highly variable crystalline and chemical nature of discrete clay minerals. Further adding to this challenge is the occurrence of mixed-layer clay minerals, which is complicated because of the numerous possible combinations of clay layer types, as defined by their relative proportions and the ordering schemes. The problem of ensuring accurate quantification is important to understanding landscape evolution because mineral abundances have a large influence on ecosystem function. X-ray diffraction analysis of the variable cation-saturated clay fraction in soil and regolith from the Calhoun Critical Zone observatory near Clinton, South Carolina, USA, was coupled with modeling using NEWMOD2 to show that mixed-layer clays are often dominant components in the mineral assemblages. Deep samples in the profile (>6.5 m) contain mixed-layer kaolinite/smectite, kaolinite/illite-like, kaolinite-vermiculite, illite-like/biotite, and illite-like/vermiculite species (with ‘illite-like’ defined herein as Fe-oxidized 2:1 layer structure with a negative layer charge of ~0.75 per unit formula, i.e. weathered biotite). The 2:1 layers in the mixed-layer structures are proposed to serve as exchange sites for K+, which is known to cycle seasonally between plant biomass and subsurface weathering horizons. Forested landscapes have a greater number of 2:1 layer types than cultivated landscapes. Of two nearby cultivated sites, the one higher in landscape position has fewer 2:1 layer types. Bulk potassium concentrations for the forested and two cultivated sites show the greatest abundances in the surface forested site and lowest abundance in the surface upland cultivated site. These observations suggest that landscape use and landscape position are factors controlling the mixed-layer mineral assemblages in Kanhapludults typical of the S.E. United States Piedmont. These mixed-layer clays are key components of the proposed mechanism for K+ uplift concepts, whereby subsurface cation storage may occur in the interlayer sites (with increased negative 2:1 layer charge) during wetter reduced conditions of the winter season and as biomass decay releases cation nutrients. Cation release from the mixed-layer clays (by decreased 2:1 layer charge) occurs under drier oxidized conditions during the growing seasons as biota utilize cation nutrients. The types and abundances of mixed layers also reflect long-term geologic factors including dissolution/alteration of primary feldspar and biotite and the subsequent transformation and dissolution/precipitation reactions that operate within the soil horizons. Thus, the resulting mixed-layer clay mineral assemblages are often complex and heterogeneous at every depth within a profile and across landscapes. X-ray diffraction (XRD) assessment, using multiple cation saturation state and modeling, is essential for quantifying the clay mineral assemblage and pools for cation nutrients, such as potassium, in the critical zone.Item Open Access Redoximorphic Bt horizons of the Calhoun CZO soils exhibit depth-dependent iron-oxide crystallinity(Journal of Soils and Sediments, 2019-02-12) Chen, C; Barcellos, D; Richter, DD; Schroeder, PA; Thompson, A© 2018, Springer-Verlag GmbH Germany, part of Springer Nature. Purpose: Iron (Fe) oxyhydroxides and their degree of ordering or crystallinity strongly impact the role that Fe plays in ecosystem function. Lower crystallinity phases are generally found to be more reactive than higher crystallinity phases as sorbents for organic matter and chemical compounds, as electron acceptors for organic matter mineralization or as electron donors for dysoxic respiration. We investigated Fe solid phase speciation as a function of soil depth in a redoximorphic upland soil profile. Materials and methods: We examined a redoximorphic upland soil profile, which displayed alternating Fe-enriched and Fe-depleted zones of the Bt horizons with platy structure from 56 to 183 cm depth at the Calhoun Critical Zone Observatory in South Carolina, USA. Redoximorphic Fe depletion and enrichment zones were sampled to enable a detailed investigation of Fe mineralogy during redox transformations. All samples were characterized by total elemental analysis, X-ray diffraction, and 57 Fe Mössbauer spectroscopy. Results and discussion: Total Fe in the Fe-enriched and Fe-depleted zones was 26.3 – 61.2 and 15.0 – 22.7 mg kg −1 soil, respectively, suggesting periodic redox cycling drives Fe redistribution within the upland soil profile. The Mössbauer data clearly indicated goethite (56 – 74% of total Fe) and hematite (7 – 31% of total Fe) in the Fe-enriched zones, with the proportion of hematite increasing with depth at the expense of goethite. In addition, the overall crystallinity of Fe phases increased with depth in the Fe-enriched zones. In contrast to Fe-enriched zones, Fe-depleted zones contained no hematite and substantially less goethite (and of a lower crystallinity) but more aluminosilicates-Fe(III) (e.g., hydroxy-interlayered vermiculite, biotite, kaolinite) with XRD and Mössbauer data suggesting a shift from oxidized biotite-Fe(III) at depth to hydroxy-interlayered vermiculite plus low-crystallinity goethite in the Fe-depleted zones in the upper Bt. Conclusions: Our data suggest the varied crystalline states of hematite and goethite may be important for Fe reduction over long-term time scales. The persistence of low-crystallinity Fe phases in Fe depletion zones suggests that both dissolution and re-precipitation events occur in the Fe-depleted layers. These variations in Fe phase abundance and crystallinity within similar redoximorphic features suggest that Fe likely shifts ecosystem roles as a function of soil depth and likely has more rapid Fe cycling in the upper Bt horizons in upland soils, while serving as a weathering engine at depth.Item Open Access Soil production and the soil geomorphology legacy of Grove Karl Gilbert(Soil Science Society of America Journal, 2020-01-01) Richter, DD; Eppes, MC; Austin, JC; Bacon, AR; Billings, SA; Brecheisen, Z; Ferguson, TA; Markewitz, D; Pachon, J; Schroeder, PA; Wade, AM© 2019 The Authors. Soil Science Society of America published by Wiley Periodicals, Inc. on behalf of Soil Science Society of America Geomorphologists are quantifying the rates of an important component of bedrock's weathering in research that needs wide discussion among soil scientists. By using cosmogenic nuclides, geomorphologists estimate landscapes’ physical lowering, which, in a steady landscape, equates to upward transfers of weathered rock into slowly moving hillslope-soil creep. Since the 1990s, these processes have been called “soil production” or “mobile regolith production”. In this paper, we assert the importance of a fully integrated pedological and geomorphological approach not only to soil creep but to soil, regolith, and landscape evolution; we clarify terms to facilitate soil geomorphology collaboration; and we seek a greater understanding of our sciences’ history. We show how the legacy of Grove Karl Gilbert extend across soil geomorphology. We interpret three contrasting soils and regoliths in the USA's Southern Piedmont in the context of a Gilbert-inspired model of weathering and transport, a model of regolith evolution and of nonsteady systems that liberate particles and solutes from bedrock and transport them across the landscape. This exercise leads us to conclude that the Southern Piedmont is a region with soils and regoliths derived directly from weathering bedrock below (a regional paradigm for more than a century) but that the Piedmont also has significant areas in which regoliths are at least partly formed from paleo-colluvia that may be massive in volume and overlie organic-enriched layers, peat, and paleo-saprolite. An explicitly integrated study of soil geomorphology can accelerate our understanding of soil, regoliths, and landscape evolution in all physiographic regions.Item Open Access The effect of accelerated soil erosion on hillslope morphology(Earth Surface Processes and Landforms, 2019-12-01) Bonetti, S; Richter, DD; Porporato, A© 2019 John Wiley & Sons, Ltd. Intensive agricultural land use can have detrimental effects on landscape properties, greatly accelerating soil erosion, with consequent fertility loss and reduced agricultural potential. To quantify the effects of such erosional processes on hillslope morphology and gain insight into the underlying dynamics, we use a twofold approach. First, a statistical analysis of topographical features is conducted, with a focus on slope and gradient distributions. The accelerated soil erosion is shown to be fingerprinted in the distribution tails, which provide a clear statistical signature of this human-induced land modification. Theoretical solutions are then derived for the hillslope morphology and the associated creep and runoff erosion fluxes, allowing us to distinguish between the main erosional mechanisms operating in disturbed and undisturbed areas. We focus our application on the landscape at the Calhoun Critical Zone Observatory in the US Southern Piedmont, where severe soil erosion followed intensive cotton cultivation, resulting in highly eroded and gullied hillslopes. The observed differences in hillslope morphologies in disturbed and undisturbed areas are shown to be related to the disruption of the natural balance between soil creep and runoff erosion. The relaxation time required for the disturbed hillslopes to reach a quasi-equilibrium condition is also investigated. © 2019 John Wiley & Sons, Ltd.Item Open Access The Formation of Clay-Enriched Horizons by Lessivage(Geophysical Research Letters, 2018-08-16) Calabrese, S; Richter, DD; Porporato, A©2018. American Geophysical Union. All Rights Reserved. Soils with argillic horizons comprise more than 25% of the Earth's surface. Although their origin is still debated, lessivage is often invoked to explain them, but the long timescales involved hinder its direct experimental verification. We present a parsimonious model of clay transport, formulated for long timescales over which lessivage is modeled stochastically, complemented by detailed field observations. This probabilistic description allows us to predict the clay profile, the depth of the Bt horizon from the surface, and the mean clay residence time. The results are tested with field measurements at different locations in the Calhoun Critical Zone Observatory. Dimensional analysis unveils two dimensionless parameters governing lessivage dynamics, leading to a classification based on erosion rates and lessivage characteristics. We identify static and eluviated regimes, in which erosion or eluviation prevails, and an illuviated regime, in which the balance between lessivage and erosion brings about the formation of a Bt horizon.