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Mineral specific surface area (SSA) increases as primary minerals weather and restructure into secondary phyllosilicate, oxide, and oxyhydroxide minerals. SSA is a measurable property that captures cumulative effects of many physical and chemical weathering processes in a single measurement and has meaningful implications for many soil processes, including water-holding capacity and nutrient availability. Here we report our measurements of SSA and mineralogy of two 21 m deep SSA profiles at two landscape positions, in which the emergence of a very small mass percent (

Stone and earthen architecture is nearly ubiquitous in the archaeological record of Pacific islands. The construction of this architecture is tied to a range of socio-political processes, and the temporal patterning of these features is useful for understanding the rate at which populations grew, innovation occurred, and social inequality emerged. Unfortunately, this temporal patterning is poorly understood for many areas of the region, including the Sāmoan archipelago. Here, we describe a project directed toward establishing a robust chronology for the construction of these earthen and stone terraces and linear mounds on Ta‘ū Island. Using recent methodological improvements, we highlight the tempo at which different architectural types were constructed on the island and the implications for understanding demographic expansion and changing land tenure practices in the last 1500 years. This research suggests the construction of architecture was largely confined to the 2nd millennium AD with a small number of terraces plausibly built in the 1st millennium AD. This temporal patterning suggests that a reconfiguration of settlement patterns occurred within West Polynesia as people there moved into other regions of Oceania.

Over the last decade, an increasing number of studies have used soundscapes to address diverse ecological questions. Sound represents one of the few sources of information capable of providing in situ insights into processes occurring within opaque soil matrices. To date, the use of soundscapes for soil macrofauna monitoring has been experimentally tested only in controlled laboratory environments. Here we assess the validity of laboratory predictions and explore the use of soil soundscape proxies for monitoring soil macrofauna (i.e., earthworm) activities in an outdoor context. In a common garden experiment in northern Sweden, we constructed outdoor mesocosm plots (N = 36) containing two different Arctic vegetation types (meadow and heath) and introduced earthworms to half of these plots. Earthworms substantially altered the ambient soil soundscape under both vegetation types, as measured by both traditional soundscape indices and frequency band power levels, although their acoustic impacts were expressed differently in heath versus meadow soils. While these findings support the as-of-yet untapped promise of using belowground soundscape analyses to monitor soil ecosystem health, direct acoustic emissions from earthworm activities appear to be an unlikely proxy for tracking worm activities at daily timescales. Instead, earthworms indirectly altered the soil soundscape by 're-engineering' the soil matrix: an effect that was dependent on vegetation type. Our findings suggest that long-term (i.e., seasonal) earthworm activities in natural soil settings can likely be monitored indirectly via their impacts on soundscape measures and acoustic indices. Analyzing soil soundscapes may enable larger-scale monitoring of high-latitude soils and is directly applicable to the specific case of earthworm invasions within Arctic soils, which has recently been identified as a potential threat to the resilience of high-latitude ecosystems. Soil soundscapes could also offer a novel means to monitor soils and soil-plant-faunal interactions in situ across diverse pedogenic, agronomic, and ecological systems.

The interaction of soil organic matter (SOM) and minerals is a critical mechanism for retaining SOM in soil and protecting soil fertility and long-term agricultural sustainability. The chemical speciation of carbon (C) and nitrogen (N) in mineral-associated SOM can be sensitive to both anthropogenic management practices and landscape positions, but these two aspects are rarely examined in tandem. Here we examined the effects of long-term (>100 years) agricultural management and erosion on mineral-associated SOM along grassland and agricultural hillslope transect. The mineral-associated SOM was obtained using particle size and density fractionation approaches. Chemical speciation of C and N in mineral-associated SOM was characterized using micro X-ray absorption near-edge fine structure (XANES) spectroscopy. The extent of SOM coverage and contribution of iron oxyhydroxides (Fe oxides) to the total specific mineral surface area (SSA) were determined using the BET–N2 adsorption method of soil samples under three conditions: untreated, SOM removal, and Fe oxides removal. The amount of SSA covered by SOM (SSASOM-covered) was lower by 61% and 37% in cultivated eroding and depositional topsoils, respectively, compared with the corresponding grassland. Depositional soils had higher SSASOM-covered than eroding positions. In the cultivated hillslopes, aromatic and phenolic C species were more abundant in depositional soils than in the eroding topsoils, indicating that deposition and burial of eroded or in-situ plant-derived phenolic C protected them from further transformation. Our results, therefore, highlight the importance of anthropogenic activities in the interaction of SOM and minerals, including C speciation changes, which may exert a considerable influence on SOM retention in soils.

Interactions between organic carbon (OC) and minerals represent a critical mechanism for stabilizing organic matter in soils. Because both mineral weathering and plant productivity are negatively affected by soil erosion, mineral-associated organic carbon (MOC) chemistry is also expected to vary with erosion intensity. Here we show that MOC chemistry, determined by carbon X-ray absorption near-edge fine structure spectroscopy (XANES), exhibits little difference across a large (10-fold) gradient in erosion-derived soil turnover times. Mineral-associated OC chemistry further fails to explain the variation in radiocarbon-based MOC turnover times. Our results suggest that soil OC longevity is largely independent of organic matter chemistry in steep mountainous landscapes where soil development is constrained by erosion.

Steep soil-mantled hillslopes are thought to be important sources of sediments and organic carbon (OC) to rivers. Minerals in these sediments may protect OC from decomposition, yet the significance of such interactions in steep upland soils remains poorly constrained particularly in relation to erosion rates. We examined a tributary basin draining to the Middle Folk Feather River in California, where knickpoint migration has created a series of hillslopes with erosion rates varying over an order of magnitude (35 to 250 mm kyr−1). This setting provides a unique opportunity to study soil OC pools and their erosional exports as a function of changes in erosion rates. Soil OC inventories were 37% lower at rapidly eroding sites relative to slowly eroding sites. This difference was driven by coarse rock contents as rapidly eroding soils had more rock fragments, limiting their capacities to store OC. Although clay contents in soils were negatively correlated with erosion rates, the total mineral specific surface area remained relatively invariant. Based on secondary phyllosilicate minerals present in the studied soils and our field observations of saprocks, we suggest that this discrepancy may have originated from different clay mineralogy (types and abundance) associated with different degrees of deep subsurface chemical weathering. Across the erosion gradient, the radiocarbon age of mineral associated organic matter (MOC) in saprock varied by a factor 2 (from 1045 to 2110 14C years), while soil turnover times estimated from soil thickness and erosion rates varied from 17 to 5.4 kyr. At the site eroding at the fastest rate, the soil turnover time approaches the 14C age of MOC, suggesting erosion can potentially limit the timescale over which MOC is replaced. We found that organic matter generally covered

The interactive effects of climate and topography on the composition, distribution and storage of soil organic carbon (SOC) remain unclear. This is particularly true for pyrogenic carbon (PyC) which is considered long-lasting in soil environments. With a goal to characterize how PyC responds to climate-dependent erosion and deposition processes, two hillslopes from SE Australia, differing in climate (MAP: 549 vs. 815 mm) and vegetation but with similar topography were selected for this study. We examined the colluvial soils that include A and B horizons on eroding (convex) and depositional (convergent) areas within each hillslope. Mid-infrared measurements with a partial least squares regression model were adopted to quantify the size and distribution of SOC fractions (i.e., particulate organic carbon (POC) fraction, humus organic carbon (HOC) and resistant organic carbon (ROC; representing PyC fraction) along the two hillslopes. These SOC fractions were examined with respect to climate, topographic position and soil depth. Total SOC was dominated by HOC (44 to 88%) followed by PyC (7 to 37%) and POC (0 to 37%). Effects of topography on inventories of PyC were more strongly expressed in the drier hillslope (eroding vs. depositional: 0.39 vs. 1.73 kg m−2) than the wetter hillslope (eroding vs. depositional: 1.14 vs. 2.46 kg m−2), which also illustrates that climatic effects on soil PyC differ between eroding and depositional hillslope areas. In contrast, relative contributions of PyC to SOC showed less systematic variation with topography (6.9–36.8% in eroding soils vs. 6.9–37.3% in depositional soils) and climate (6.9–36.8% at drier site vs. 6.9–37.3% at wetter site), which together with other laboratory and field data suggest that erosion may not preferentially transport PyC over other organic carbon forms or that erosional PyC flux may not be large enough to alter SOC concentration and distribution into fractions. Our results highlight the importance of considering topography in understanding SOC pools in general and PyC specifically under different climates.

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 ( 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.

We test the hypothesis that erosion driven soil movement on hillslopes results in an increase in new organomineral associations and overall organic matter storage in colluvial deposits within a forested hillslope. We measured mineral specific surface area (SSA), organic carbon (OC), meteoric radioisotopes (210Pb, 137Cs, 10Be), soil physical properties, C/N, δ15N, δ13C, and ∆14C in bulk soil and density fractions in a hillslope transect of soil pits. The quantity of OC per unit of mineral surface area (OC/SA) and OC inventories increased by a factor of 2–3 in depositional sites as result of soil mixing due to erosional movement as confirmed by 210Pb, 137Cs, and 10Be profiles and inventories. Soil mixing systematically decreased C/N and enriched stable isotopes of δ13C and δ15N, revealing that formation of organomineral associations instead of microbial processing was responsible for depth trends in organic matter composition. Our findings indicate that the processes that associate organic matter and minerals are fundamentally linked with organic matter composition, and OC/SA, C/N, δ13C, and δ15N provide proxies for organic matter stabilization by soil minerals.

We present a statistical model of soil and rock weathering in deep profiles to expand the capacity to assess weathering to heterogeneous bedrock types, which are common at the Earth's surface. We developed the Weathering Trends (WT) model by extending the fractional mass change calculation (tau) of the geochemical mass balance model in two important ways. First, WT log transforms the elemental ratio data, to discern the log-linear patterns that naturally develop from thermodynamic and kinetic laws of chemistry. Second, WT statistically fits log-transformed element concentration ratio data – log(cj/ci), the only depth-varying term in tau – as a function of depth to determine characteristic depths of transitions in weathering processes, along with confidence intervals. With no prior assumptions, WT estimates average parent material composition, average composition of the upper weathered zone and mean fractional mass change of each element over the entire weathering profile. WT displays the mean shape of weathering profiles of log-transformed geochemical data bounded by calculated confidence intervals. We share the WT model code as an open-source R package (https://github.com/fisherba/WeatheringTrends). The WT model was designed to interpret two 21 m cores from the Laurels Schist bedrock in the Christina River Basin Critical Zone Observatory in the Pennsylvania Piedmont, where our morphological and elemental data provided inconclusive estimates of bedrock depth. The WT model differentiated between rock variability and weathering to delineate the maximum extent of weathering at 12.3 m (CI 95% [9.2, 21.3]) in Ridge Well 1 and 7.2 m (CI 95% [4.3, 13.0]) in Interfluve Well 2. The water table was 5–8 m below fresh rock at Ridge Well 1, but at the same depth as fresh rock at the lower elevation interfluve. We assess statistical approaches to identify the best immobile element for use in WT and tau calculations. Copyright © 2017 John Wiley & Sons, Ltd.

The empirical quantification of rates of material movement in cryoturbated soils has lagged behind the physical and chemical characterisation of these materials. We applied a novel suite of element ...

Physical and chemical interactions between soil organic matter (OM) and minerals is one of the primary mechanisms for stabilizing OM in terrestrial ecosystems. Focusing on OM association with mineral surfaces, this study sought to examine mineral-associated OM from the perspectives of both mineral surface characteristics and organic matter chemistry. The research was conducted at paired-sites under North American Mid-Atlantic Coastal forest and crop production with shared environmental factors. Using carbon (C) and nitrogen (N) 1s micro- X-ray absorption near-edge fine structure (XANES) spectroscopy, we investigated the amounts and types of mineral-associated OM. Mineral specific surface area (SSA) of bulk soil was determined for three conditions: untreated, post OM removal and post iron (Fe) (oxyhydr)oxides removal. Amounts of mineral-associated OM were smaller in the agricultural soil, where greater SSA sourced from clay-sized phyllosilicates and Fe (oxyhydr)oxide minerals did not result in greater OM coverage of the mineral surface area. Although agricultural surface soil showed less abundance of phenolic C, speciation of mineral-associated OM did not differ between comparable horizons. Our results suggest that despite the plow-derived mixing of soil, which increased SSA and secondary minerals available to interact physically and chemically with OM in the plowed layer, the formation of mineral-associated OM in agricultural soil is ultimately limited by available OM.

Non-native, invasive earthworms are altering soils throughout the world. Ecological cascades emanating from these changes stem from earthworm-caused changes in detritus processing occurring at a mid-point in the trophic pyramid, rather than the more familiar bottom-up or top-down cascades. They include fundamental changes (microcascades) in soil morphology, bulk density, nutrient leaching, and a shift to warmer, drier soil surfaces with loss of organic horizons. In North American temperate and boreal forests, microcascades cause effects of concern to society (macrocascades), including changes in CO(2) sequestration, disturbance regimes, soil quality, water quality, forest productivity, plant communities, and wildlife habitat, and facilitation of other invasive species. Interactions among these changes create cascade complexes that interact with climate change and other environmental changes. The diversity of cascade effects, combined with the vast area invaded by earthworms, lead to regionally important changes in ecological functioning.

Phosphorus (P) is an essential nutrient for life. Deficits in soil P reduce primary production and alter biodiversity. A soil P paradigm based on studies of soils that form on flat topography, where erosion rates are minimal, indicates P is supplied to soil mainly as apatite from the underlying parent material and over time is lost via weathering or transformed into labile and less-bioavailable secondary forms. However, little is systematically known about P transformation and bioavailability on eroding hillslopes, which make up the majority of Earth's surface. By linking soil residence time to P fractions in soils and parent material, we show that the traditional concept of P transformation as a function of time has limited applicability to hillslope soils of the western Southern Alps (New Zealand) and Northern Sierra Nevada (USA). Instead, the P inventory of eroding soils at these sites is dominated by secondary P forms across a range of soil residence times, an observation consistent with previously published soil P data. The findings for hillslope soils contrast with those from minimally eroding soils used in chronosequence studies, where the soil P paradigm originated, because chronosequences are often located on landforms where parent materials are less chemically altered and therefore richer in apatite P compared to soils on hillslopes, which are generally underlain by pre-weathered parent material (e.g., saprolite). The geomorphic history of the soil parent material is the likely cause of soil P inventory differences for eroding hillslope soils versus geomorphically stable chronosequence soils. Additionally, plants and dust seem to play an important role in vertically redistributing P in hillslope soils. Given the dominance of secondary soil P in hillslope soils, limits to ecosystem development caused by an undersupply of bio-available P may be more relevant to hillslopes than previously thought.

The delineation and mapping of eroded phases of existing soil series has been an important activity throughout the history of soil survey activities in the United States, with implications for land management, crop production and the estimation of historical sediment losses and fluxes. An analysis of the SSURGO database shows that 462,979 km2 of eroded phase soils (16% of which were classified as severely eroded) are mapped in the conterminous United States, with 9% of 2013 cultivated lands occurring on eroded phase soils. Eroded phases of 2265 soil series in 9 soil orders (excluding Gelisols, Oxisols and Histosols) have been identified and mapped. Examining the distribution of eroded phase soils within survey-independent large-scale physiographic (EPA Level III Ecoregion) boundaries reveals consistent patterns in land-use histories and eroded soils, and that the pattern and distribution of eroded phase soils at regional to continental scales are responsive to the five soil forming factors. The proportion of ecoregion land area mapped as eroded phase was significantly affected by topography, with eroded phase soils peaking in ecoregions with topographic ruggedness indices (TRI, a normalized elevation difference index) between 1 and 2. Among TRI groups, the proportion of total ecoregion land area mapped as eroded phase was significantly related to the historical maximum cultivation intensity, while among a subset of ecoregions with significant histories of cultivation, eroded phase proportion was related to the rainfall-runoff erosivity (RUSLE R-Factor). Of the 2265 named soil series with mapped eroded phases, 73% had family particle size classes of fine, fine-loamy and fine-silty which corresponds with the peak in the RUSLE erodibility factor (K) with respect to particle size. Lastly, ecoregions with histories of significant cultivation for more than 100 years had greater proportions of their land area mapped as eroded phase. These results suggest that despite morphological constraints and subjective factors in the delineation of eroded phases, these soils should be viewed as unique pedological entities that hold lasting value for understanding the effects of accelerated erosion on soil morphology, crop production, and ecosystem services.

It is now well established that European earthworms are re-shaping formerly glaciated forests in North America with dramatic ecological consequences. However, few have considered the potential invasiveness of this species assemblage in the European arctic. Here we argue that some earthworm species (Lumbricus rubellus, Lumbricus terrestris and Aporrectodea sp.) with great geomorphological impact (geoengineering species) are non-native and invasive in the Fennoscandian arctic birch forests, where they have been introduced by agrarian settlers and most recently through recreational fishing and gardening. Our exploratory surveys indicate no obvious historical dispersal mechanism that can explain early arrival of these earthworms into the Fennoscandian arctic: that is, these species do not appear to establish naturally along coastlines mimicking conditions following deglaciation in Fennoscandia, nor were they spread by early native (Sami) cultures. The importance of anthropogenic sources and the invasive characteristics of L. rubellus and Aporrectodea sp. in the arctic is evident from their radiation outwards from abandoned farms and modern cabin lawns into adjacent arctic birch forests. They appear to outcompete previously established litter-dwelling earthworm species (i.e. Dendrobaena octaedra) that likely colonized the Fennoscandian landscape rapidly following deglaciation via hydrochory and/or dispersal by early Sami settlements. The high geoengineering earthworm biomasses, their recognized ecological impact in other formerly glaciated environments, and their persistence once established leads us to suggest that geoengineering earthworms may pose a potent threat to some of the most remote and protected arctic environments in northern Europe.

Agricultural activities alter elemental budgets of soils and thus their long-term geochemical development and suitability for food production. This study examined the utility of a geochemical mass balance approach that has been frequently used for understanding geochemical aspect of soil formation, but has not previously been applied to agricultural settings. Protected forest served as a reference to quantify the cumulative fluxes of Ca, P, K, and Pb at a nearby tilled crop land. This comparison was made at two sites with contrasting erosional environments: relatively flat Coastal Plain in Delaware vs. hilly Piedmont in Pennsylvania. Mass balance calculations suggested that liming not only replenished the Ca lost prior to agricultural practice but also added substantial surplus at both sites. At the relatively slowly eroding Coastal Plain site, the agricultural soil exhibited enrichment of P and less depletion of K, while both elements were depleted in the forest soil. At the rapidly eroding Piedmont site, erosion inhibited P enrichment. In similar, agricultural Pb contamination appeared to have resulted in Pb enrichment in the relatively slowly eroding Coastal Plain agricultural soil, while not in the rapidly eroding Piedmont soils. We conclude that agricultural practices transform soils into a new geochemical state where current levels of Ca, P, and Pb exceed those provided by the local soil minerals, but such impacts are significantly offset by soil erosion.

The ridgelines of mountain ranges are a source of geomorphic information unadulterated by the arrival of sediment from upslope. Studies along ridgecrests, therefore, can help identify and isolate the controls on important regolith properties such as thickness and texture. A 1.5 km section of ridgeline in the Sierra Nevada (CA) with a tenfold decrease in erosion rate (inferred from ridgetop convexity) provided an opportunity to conduct a high-resolution survey of regolith properties and investigate their controls. We found that regolith along the most quickly eroding section of the ridge was the rockiest and had the lowest clay concentrations. Furthermore, a general increase in regolith thickness with a slowing of erosion rate was accompanied by an increase in biomass, changes in vegetation community, broader ridgeline profiles, and an apparent increase in total available moisture. The greatest source of variation in regolith thickness at the 10–100 m scale, however, was the local topography along the ridgeline, with the deepest regolith in the saddles and the thinnest on the knobs. Because regolith in the saddles had higher surface soil moisture than the knobs, we conclude that the hydrological conditions primarily driven by local topography (i.e. rapid vs. slow drainage and water-storage potential) provide the fundamental controls on regolith thickness through feedbacks incorporating physical weathering by the biota and chemical weathering. Moreover, because the ridgeline saddles are the uppermost extensions of first-order valleys, we propose that the fluvial network affects regolith properties in the furthest reaches of the watershed

The characteristics of the sediment transported by rivers (e.g. sediment flux, grain size distribution – GSD) dictate whether rivers aggrade or erode their substrate. They also condition the architecture and properties of sedimentary successions in basins. In this study, we investigate the relationship between landscape steepness and the grain size of hillslope and fluvial sediments. The study area is located within the Feather River basin in northern California, and studied basins are underlain exclusively by tonalite lithology. Erosion rates in the study area vary over an order of magnitude, from > 250 mm ka−1 in the Feather River canyon to < 15 mm ka−1 on an adjacent low-relief plateau. We find that the coarseness of hillslope sediment increases with increasing hillslope steepness and erosion rates. We hypothesise that, in our soil samples, the measured 10-fold increase in D50 and doubling of the amount of fragments larger than 1 mm when slope increases from 0.38 to 0.83 m m−1 is due to a decrease in the residence time of rock fragments, causing particles to be exposed for shorter periods of time to processes that can reduce grain size. For slopes in excess of 0.7 m m−1 , landslides and scree cones supply much coarser sediment to rivers, with D50 and D84 more than one order of magnitude larger than in soils. In the tributary basins of the Feather River, a prominent break in slope developed in response to the rapid incision of the Feather River. Downstream of the break in slope, fluvial sediment grain size increases, due to an increase in flow competence (mostly driven by channel steepening) as well as a change in sediment source and in sediment dynamics: on the plateau upstream of the break in slope, rivers transport easily mobilised fine-grained sediment derived exclusively from soils. Downstream of the break in slope, mass wasting processes supply a wide range of grain sizes that rivers entrain selectively, depending on the competence of their flow. Our results also suggest that, in this study site, hillslopes respond rapidly to an increase in the rate of base-level lowering compared to\ud rivers.

Hardwood forests of the Great Lakes Region have evolved without earthworms since the Last Glacial Maximum, but are now being invaded by exotic earthworms introduced through agriculture, fishing, and logging. These exotic earthworms are known to increase soil mixing, affect soil carbon storage, and dramatically alter soil morphology. Here we show, using an active earthworm invasion chronosequence in a hardwood forest in northern Minnesota, that such disturbances by exotic earthworms profoundly affect inorganic nutrient cycles in soils. Soil nutrient elemental concentrations (Ca, Mg, K, and P) were normalized to biogeochemically inert Zr to quantify their losses and gains. This geochemical normalization revealed that elements were highly enriched in the A horizon of pre-invasion soils, suggesting tight biological recycling of the nutrients. In the early stage of invasion, epi-endogeic earthworm species appeared to have been responsible for further enriching the elements in the A horizon possibly by incorporating leaf organic matter (OM). The arrival of geophagous soil mixing endogeic earthworms, however, was associated with near complete losses of these enrichments, which was related to the loss of OM in soils. Our study highlights that elemental concentrations may not be sufficient to quantify biogeochemical effects of earthworms. The geochemical normalization approach, which has been widely used to study soil formation, may help when determining how invasive soil organisms affect soil elemental cycles. More generally, this approach has potential for much wider use in studies of belowground nutrient dynamics. The results support the existing ecological literature demonstrating that invasive earthworms may ultimately reduce productivity in formerly glaciated forests under climate change.

Exotic earthworms are invading forests in North America where native earthworms have been absent since the last glaciation. These earthworms bioturbate soils and may enhance physical interactions between minerals and organic matter (OM), thus affecting mineral sorption of carbon (C) which may affect C cycling. We quantitatively show how OM-mineral sorption and soil C inventories respond to exotic earthworms along an earthworm invasion chronosequence in a sugar maple forest in northern Minnesota. We hypothesized that mineral surface area in A horizons would increase as burrowing earthworms incorporated clay minerals from the B horizons and that enhanced contacts between OM and minerals would increase the OM sorption on mineral surfaces and mineral-associated C inventories in A horizons. Contrary to our hypotheses, mineral surface areas within A horizons were lowered because earthworm burrows only extended into the silt-rich loess that separated the A and clay-rich B horizons. Furthermore, where endogeic earthworms were present, a smaller fraction of mineral surface area was covered with OM. OM sorption on minerals in the A horizons shifted from a limitation of mineral surface availability to a limitation of OM availability within a decade after the arrival of endogeic earthworms. C-mineral sorption depends on earthworm consumption of OM as well as the composition and vertical distribution of minerals. This finding may thus explain the contradictory results reported in earlier investigations. Our results highlight the rapid and drastic effects of exotic earthworms on key ecosystem processes in deciduous forests in post-glacial settings.

Cryoturbation in high-latitude soils is crucial for the long-term cycling of elements, but the rates of soil motion are poorly constrained. Here, we test whether the rate of frost creep, soil erosion and vertical soil mixing in frost boils can be estimated using short-lived radionuclides (Cs-137 and Pb-210). We find a small-scale variation in Cs-137 and Pb-210 inventories in the lower levels of the eroding regions of frost boils in comparison to the expected depositional sites; hence, the distribution of the radionuclides appears to reflect a lateral transport of atmospheric fallout from the centre of the boil (inner domain) towards the surrounding soil (outer domain). C-14 dating of the soil indicates that fallout of Cs-137 was mobile in the soil and that Pb-210 moved with the soil matrix. A soil creep model and a surface soil erosion model are derived and applied to the lateral and vertical distributions of Pb-210 in the frost boil. Both models predict the expected trajectories of soil motion and provide rates of creep, erosion and mixing at a mm yr(-1) to cm yr(-1) scale. The distribution of Pb-210 provides new insights about the processes and rates of soil mass movement in frost boils, if sound mass-balance models are applied. Copyright (C) 2014 John Wiley & Sons, Ltd.

[1] Many geomorphic studies assume that bedrock geology is not a first-order control on landscape form in order to isolate drivers of geomorphic change (e.g., climate or tectonics). Yet underlying geology may influence the efficacy of soil production and sediment transport on hillslopes. We performed quantitative analysis of LiDAR digital terrain models to examine the topographic form of hillslopes in two distinct lithologies in the Feather River catchment in northern California, a granodiorite pluton and metamorphosed volcanics. The two sites, separated by

Introduction Hillslopes are the most common landforms on the earth's surface, are fundamental drivers of the distribution of soil properties and landscape-scale geomorphic processes and, together with surface waters, form watersheds that are amenable to constraining and balancing the fluxes of water, solutes, and sediments (Bormann, 1994). For these reasons, hillslopes have long been the subject of study for soil scientists, biogeochemists, and geomorphologists. Although hillslopes are virtually ubiquitous as a landform, most current concepts of terrestrial ecosystem development are built upon studies of soils formed on flat and stable geomorphic surfaces. This bias toward geomorphically inactive land surfaces has been further reinforced by the agronomic legacy involved in the development of the modern field of soil science (Amundson and Yaalon, 1995; Yallon and Berkowicz, 1997). This presents an opportunity to assess the appropriateness of our current concepts of soil formation for understanding the past, present, and future of hillslope soils and hilly ecosystems. The nexus between hillslopes, anthropogenic systems, and food security continues to intensify and this trend, though complex in its biophysical and socioeconomic contexts (Kiage, 2013), is only growing with populations that are still in upward motion (Conway and Wilson, 2012). A major challenge to researchers remains in understanding exactly how biophysical factors such as climate and soil properties converge under increasing land-use pressure in causing excessive soil degradation (Kiage, 2013). Hillslope soil processes are thus critical for deepening our understanding of the developments of both natural and anthropogenic ecosystems. This chapter examines the interaction of physical, chemical, and biological processes in the formation of hillslope soils and the ecosystems they support, and emphasizes the fact that these interactions are strongly intertwined on hillslope landforms, where it is difficult to isolate one soil-forming process and examine it independently of the others. Nonetheless, constraining a set of pedogenic processes involving mass and volume fluxes on hillslopes in order to develop a unified theoretical framework for the evaluation of hillslope soil development has become achievable and necessary. In this chapter, we begin by focusing on the physical processes responsible for shaping hillslope soils – temporarily placing chemical and biological processes into black boxes in the early part of the chapter.

How and how fast do hillslope soils form as the landscape's morphology changes over time? Here results are shown from an ongoing study that simultaneously examines the morphologic and geochemical evolution of soil mantled hillslopes that have been exposed to distinctively different denudation history. In Northern Sierra Nevada, California, the authors are investigating a tributary basin to the Middle Fork Feather River. A major incision signal from the river is well marked in a knickpoint within the tributary basin which stretches from its mouth to the Feather River at an elevation of similar to 700 m to the plateau at an elevation of similar to 1500 m. Hillslopes are significantly steeper below the knickpoint. The area's total denudation rates are currently being constrained using cosmogenic radio nuclides, but a previous study suggested an order of magnitude difference in total denudation rates below and above the knickpoint. When compared with topographic attributes calculated from LIDAR data, physical erosion rates can be modeled as a linear function of ridge top curvature. Surprisingly, over the wide range of total denudation rates, soil thicknesses do not vary significantly until a threshold point where soil mantled landscapes abruptly shift to bedrock dominated landscapes. Bioturbation by tree falls appear to buffer soil thickness over the wide range of physical soil erosion rates. From three hillslopes with different physical erosion rates, the concentrations of Zr, which were considered conserved during dissolution and leaching, were determined and used as a proxy for the degree of mass losses via chemical denudation. There is a general trend that colluvial soils along the hillslopes with lower physical erosion rates are enriched in fine size fractions, Zr, and pedogenic crystalline Fe oxides. Likewise, the saprolites show greater degrees of chemical denudation at the sites above the knickpoint, presumably because of the saprolites' longer turnover time in the slowly eroding landscapes. In the two steep hillslopes below the knickpoint, no significant or systematic topgraphic trends were found for soil geochemistry. However, soils show increasing Zr enrichment in the downslope direction in the hillslope above the knickpoint, which suggests a critical denudation rate beyond which soils' turnover time is too short to develop a geochemical catena. As detailed CRN-based soil production rates and catchment scale denudation rates are acquired, the data will be combined with a mass balance model to calculate the rates of chemical denudation and weathering in soils and saprolites along the denudation gradient.

The interactions of organic matter and minerals contribute to the capacity of soils to store C. Such interactions may be controlled by the processes that determine the availability of organic matter and minerals, and their physical contacts. One of these processes is bioturbation, and earthworms are the best known organisms that physically mix soils. Earthworms are not native species to areas previously glaciated, and the introduction of earthworms to these regions has been associated with often dramatic changes in soil structure and geochemical cycles. The authors are studying C mineral interaction along an approximately 200 m long earthworm invasion transect in a hardwood forest in northern Minnesota. This transect extends from the soils where earthworms are absent to soils that have been invaded by earthworms for nearly 30–40 years. Pre-invaded soils have an approximately 5 cm thick litter layer, thin (5 cm) A horizon, silt rich E horizon, and clay-rich Bt horizons. The A and E horizons formed from aeolian deposits, while the clay-rich Bt horizons probably developed from underlying glacial till. With the advent of earthworm invasion, the litter layer disappears and the A horizons thicken at the expense of the O and E horizons. In addition, organic C contents in the A horizons significantly increase with the arrival of earthworms. Simultaneously, measured mineral specific surface areas suggest that minerals’ capacities to complex the organic matter appear to be greater in soils with active earthworm populations. Based on the data from two end member soils along the transect, mineral specific surface areas in the A and E horizons are larger in the earthworm invaded soil than in the pre-invasion soil. Additionally, within < 5 a of earthworm invasions, A horizon materials are turned from single grain to a strong medium granular structure. While A horizon organic matter content and organic C-mineral complexation increase after earthworm invasion, they are also more vigorously mixed. This growing data set, when ultimately combined with ongoing measurements of (1) the population dynamics of earthworms along the invasion transect, (2) C-mineral association (via surface adsorption and physical collusion in mineral aggregates) and (3) dissolved organic C will show how and how much soil capacity to store C is affected by burrowing organisms, which are often the keystone species of given ecosystems.

Minnesota forested soils have evolved without the presence of earthworms since the last glacial retreat. When exotic earthworms arrive, enhanced soil bioturbation often results in dramatic morphological and chemical changes in soils with negative implications for the forests’ sustainability. However, the impacts of earthworm invasion on geochemical processes in soils are not well understood. This study attempts to quantify the role of earthworm invasion in mineral chemical weathering and nutrient dynamics along an earthworm invasion chronosequence in a sugar maple forest in Northern Minnesota. Depth and rates of soil mixing can be tracked with atmospherically derived short lived radioisotopes 210 Pb and 137 Cs. Their radioactivities increase in the lower A horizon at the expense of the peak activities near the soil surface, which indicate that soil mixing rate and its depth reach have been enhanced by earthworms. Enhanced soil mixing by earthworms is consistent with the ways that the vertical profiles of elemental and mineralogical compositions were affected by earthworm invasion. Biologically cycled Ca and P have peak concentrations near the soil surface prior to earthworm invasion. However, these peak abundances significantly declined in the earthworm invaded soils presumably due to enhanced soil mixing. It is clear that enhanced soil mixing due to earthworms also profoundly altered the vertical distribution of most mineral species within A horizons. Though the mechanisms are not clear yet, earthworm invasion appears to have contributed to net losses of clay mineral species and opal from the A horizons. As much as earthworms vertically relocated minerals and elements, they also intensify the contacts between organic matter and cations as shown in the increased amount of Ca and Fe in organically complexed and in exchangeable pools. With future studies on soil mixing rates and elemental leaching, this study will quantitatively and mechanically address the role of earthworms in geochemical evolution of soils and forests’ nutrient dynamics.

Streams, rivers, lakes, and other inland waters are important agents in the coupling of biogeochemical cycles between continents, atmosphere, and oceans. The depiction of these roles in global-scale assessments of carbon (C) and other bioactive elements remains limited, yet recent findings suggest that C discharged to the oceans is only a fraction of that entering rivers from terrestrial ecosystems via soil respiration, leaching, chemical weathering, and physical erosion. Most of this C influx is returned to the atmosphere from inland waters as carbon dioxide (CO2) or buried in sedimentary deposits within impoundments, lakes, floodplains, and other wetlands. Carbon and mineral cycles are coupled by both erosion–deposition processes and chemical weathering, with the latter producing dissolved inorganic C and carbonate buffering capacity that strongly modulate downstream pH, biological production of calcium-carbonate shells, and CO2 outgassing in rivers, estuaries, and coastal zones. Human activities substantially affect all of these processes.

article i nfo Hillslopes have been intensively studied by both geomorphologists and soil scientists. Whereas geomorphologists have focused on the physical soil production and transport on hillslopes, soil scientists have been concerned with the topographic variation of soil geochemical properties. We combined these differing approaches and quantified soil chemical weathering rates along a grass covered hillslope in Coastal California. The hillslope is comprised of both erosional and depositional sections. In the upper eroding section, soil production is balanced by physical erosion and chemical weathering. The hillslope then transitions to a depositional slope where soil accumulates due to a historical reduction of channel incision at the hillslope's base. Measurements of hillslope morphology and soil thickness were combined with the elemental composition of the soil and saprolite, and interpreted through a process-based model that accounts for both chemical weathering and sediment transport. Chemical weathering of the minerals as they moved downslope via sediment transport imparted spatial variation in the geochemical properties of the soil. Inverse modeling of the field and laboratory data revealed that the long-term soil chemical weathering rates peak at 5gm −2 yr −1 at the downslope end of the eroding section and decrease to 1.5gm −2 yr −1 within the depositional section. In the eroding section, soil chemical weathering rates appear to be primarily controlled by the rate of mineral supply via colluvial input from upslope. In the depositional slope, geochemical equilibrium between soil water and minerals appeared to limit the chemical weathering rate. Soil chemical weathering was responsible for removing 6% of the soil production in the eroding section and 5% of colluvial influx in the depositional slope. These were among the lowest weathering rates reported for actively eroding watersheds, which was attributed to the parent material with low amount of weatherable minerals and intense coating of the primary minerals by secondary clay and iron oxides. We showed that both the morphologic disequilibrium of the hillslope and the spatial heterogeneity of soil properties are due to spatial variations in the physical and chemical processes that removed mass from the soil.

Differences in chemical weathering extent and character are expected to exist across topographic escarpments due to spatial gradients of climatic and/or tectonic forcing. The passive margin escarpment of south-eastern Australia has a debated but generally accepted model of propagation in which it retreated (within 40 Ma) to near its current position following rifting between Australia and New Zealand 85–100 Ma before present. We focus on this escarpment to quantify chemical weathering rates and processes and how they may provide insight into scarp evolution and retreat. We compare chemical weathering extents and rates above and below the escarpment using a mass balance approach coupling major and trace element analyses with previous measurements of denudation rates using cosmogenic nuclides (10Be and 26Al). We find a slight gradient in saprolite chemical weathering rate as a percentage of total weathering rate across the escarpment. The lowlands area, encompassing the region extending from the base of the escarpment to the coast, experiences a greater extent of chemical weathering than the highland region above the escarpment. Percents of denudation attributable to saprolite weathering average 57 ± 6% and 47 ± 7% at low and high sites respectively. Furthermore, the chemical index of alteration (CIA), a ratio of immobile to mobile oxides in granitic material that increases with weathering extent, have corresponding average values of 73·7 ± 3·9 and 65·5 ± 3·4, indicating lower extents of weathering above the escarpment. Finally, we quantify variations in the rates and extent of chemical weathering at the hillslope scale across the escarpment to suggest new insight into how climate differences and hillslope topography help drive landscape evolution, potentially overprinting longer term tectonic forcing. Copyright © 2009 John Wiley & Sons, Ltd.

We present a mathematical model that integrates geochemical and geomorphic processes responsible for soils' elemental and mineralogical evolution on diverse landforms: eroding to depositional to level grounds. This new model combines a hillslope sediment mass balance and a soil geochemical mass balance. We model soils as the sum of a physically disturbed zone (PDZ) and the underlying physically undisturbed but chemically altered zone (CAZ). The model considers processes including the production of PDZ, weathering front propagation, mineral-specific dissolution, colloidal translocation, and colluvial transport. Thus this study serves to (1) review preexisting models of pedogenic processes and to (2) offer a framework to integrate the geochemical and geomorphic processes active in soils. We further discuss the implications of our model in (1) scaling chemical weathering reactions from mineral grains to pedons to soil catenas, (2) understanding the emergence of geochemical soil catenas, and (3) coupling physical and chemical weathering processes.

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.

Chemical weathering affects hillslope form through dissolution and mineral transformations that lower the surface. In addition, mineral transformations affect the rheology, hydrology, and nutrient cycling of soils, all of which alter the geomorphic processes sculpting landscapes. Soil rheology is altered by the weakening of rocks from chemical weathering and by changes in soil cohesion from the formation of clays, a major product of weathering reactions. Clay formation can also decrease a soil's hydraulic conductivity and infiltration capacity, thereby altering the partitioning of surface and subsurface flow and potentially increasing rates of hillslope erosion by overland flow. Changes in the hydrological characteristics and nutrient cycling of a soil can affect the plants and animals that disturb and displace it. The complex interactions of these processes suggest that chemical weathering can either accentuate or dampen landscape dissection and relief generation.

Erosion rates dictate the morphology of landscapes, and therefore quantifying them is a critical part of many geomorphic studies. Methods to directly measure erosion rates are expensive and time consuming, whereas topographic analysis facilitates prediction of erosion rates rapidly and over large spatial extents. If hillslope sediment flux is nonlinearly dependent on slope then the curvature of hilltops will be linearly proportional to erosion rates. In this contribution we develop new techniques to extract hilltop networks and sample their adjacent hillslopes in order to test the utility of hilltop curvature for estimating erosion rates using high-resolution (1 m) digital elevation data. Published and new cosmogenic radionuclide analyses in the Feather River basin, California, suggest that erosion rates vary by over an order of magnitude (10 to 250 mm kyr(-1)). Hilltop curvature increases with erosion rates, allowing calibration of the hillslope sediment transport coefficient, which controls the relationship between gradient and sediment flux. Having constraints on sediment transport efficiency allows estimation of erosion rates throughout the landscape by mapping the spatial distribution of hilltop curvature. Additionally, we show that hilltop curvature continues to increase with rising erosion rates after gradient-limited hillslopes have emerged. Hence hilltop curvature can potentially reflect higher erosion rates than can be predicted by hillslope gradient, providing soil production on hilltops can keep pace with erosion. Finally, hilltop curvature can be used to estimate erosion rates in landscapes undergoing a transient adjustment to changing boundary conditions if the response timescale of hillslopes is short relative to channels.

In natural ecosystems, bioturbation is an essential component of soil formation, whereas tillage drives soil mixing in agricultural soils. Yet soil mixing is commonly neglected in modeling soil org ...

Critical Zone (CZ) research investigates the chemical, physical, and biological processes that modulate the Earth's surface. Here, we advance 12 hypotheses that must be tested to improve our understanding of the CZ: (1) Solar-to-chemical conversion of energy by plants regulates flows of carbon, water, and nutrients through plant-microbe soil networks, thereby controlling the location and extent of biological weathering. (2) Biological stoichiometry drives changes in mineral stoichiometry and distribution through weathering. (3) On landscapes experiencing little erosion, biology drives weathering during initial succession, whereas weathering drives biology over the long term. (4) In eroding landscapes, weathering-front advance at depth is coupled to surface denudation via biotic processes. (5) Biology shapes the topography of the Critical Zone. (6) The impact of climate forcing on denudation rates in natural systems can be predicted from models incorporating biogeochemical reaction rates and geomorphological transport laws. (7) Rising global temperatures will increase carbon losses from the Critical Zone. (8) Rising atmospheric P(CO2) will increase rates and extents of mineral weathering in soils. (9) Riverine solute fluxes will respond to changes in climate primarily due to changes in water fluxes and secondarily through changes in biologically mediated weathering. (10) Land use change will impact Critical Zone processes and exports more than climate change. (11) In many severely altered settings, restoration of hydrological processes is possible in decades or less, whereas restoration of biodiversity and biogeochemical processes requires longer timescales. (12) Biogeochemical properties impart thresholds or tipping points beyond which rapid and irreversible losses of ecosystem health, function, and services can occur.

Contaminants as Tracers for Studying Dynamics of Soil Formation: Mining an Ocean of Opportunities.

Linking mineral weathering rates measured in the laboratory to those measured at the landscape scale is problematic. In laboratory studies, collections of minerals are exposed to the same weathering environment over a fixed amount of time. In natural soils, minerals enter, are mixed within, and leave the soil via erosion and dissolution/leaching over the course of soil formation. The key to correctly comparing mineral weathering studies from laboratory experiments and field soils is to consistently define time. To do so, we have used reservoir theory. Residence time of a mineral, as defined by reservoir theory, describes the time length between the moment that a mineral enters (via soil production) and leaves (via erosion and dissolution/leaching) the soil. Age of a mineral in a soil describes how long the mineral has been present in the soil. Turnover time describes the time needed to deplete a species of minerals in the soil by sediment efflux from the soil. These measures of time are found to be sensitive to not only sediment flux, which controls the mineral fluxes in and out of a soil, but also internal soil mixing that controls the probability that a mineral survives erosion. When these measures of time are combined with published data suggesting that a mineral's dissolution reaction rate decreases during the course of weathering, we find that internal soil mixing, by partially controlling the age distribution of minerals within a soil, might significantly alter the soil's mass loss rate via chemical weathering.

A positive relationship between the mean annual precipitation (MAP) and soil organic carbon (SOC) is found in most surveys covering the subarctic and boreal region. In this paper we assess mechanis ...

Thawing of permafrost and a subsequent accelerated loss of mercury from the soil constitute a possible threat to the quality of high-latitude surface waters. In this paper we estimate the export of ...

Contaminants as Tracers for Studying Dynamics of Soil Formation: Mining an Ocean of Opportunities.