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Manganese (Mn) exists as Mn(II), Mn(III), or Mn(IV) in soils, and the Mn oxidation state controls the roles of Mn in numerous environmental processes. However, the variations of Mn oxidation states with climate remain unknown. We determined the Mn oxidation states in highly weathered bulk volcanic soils (primary minerals free) across two rainfall gradients covering mean annual precipitation (MAP) of 0.25-5 m in the Hawaiian Islands. With increasing MAP, the soil redox conditions generally shifted from oxic to suboxic and to anoxic despite fluctuating at each site; concurrently, the proportions of Mn(IV) and Mn(II) decreased and increased, respectively. Mn(III) was low at both low and high MAP, but accumulated substantially, up to 80% of total Mn, in soils with prevalent suboxic conditions at intermediate MAP. Mn(III) was likely hosted in Mn(III,IV) and iron(III) oxides or complexed with organic matter, and its distribution among these hosts varied with soil redox potentials and soil pH. Soil redox conditions and rainfall-driven leaching jointly controlled exchangeable Mn(II) in soils, with its concentration peaking at intermediate MAP. The Mn redox chemistry was at disequilibrium, with the oxidation states correlating with long-term average soil redox potentials better than with soil pH. The soil redox conditions likely fluctuated between oxic and anoxic conditions more frequently at intermediate than at low and high MAP, creating biogeochemical hot spots where Mn, Fe, and other redox-sensitive elements may be actively cycled.

As rock-derived primary minerals weather to form soil, they create reactive, poorly crystalline minerals that bind and store organic carbon. By implication, the abundance of primary minerals in soil might influence the abundance of poorly crystalline minerals, and hence soil organic carbon storage. However, the link between primary mineral weathering, poorly crystalline minerals, and soil carbon has not been fully tested, particularly at large spatial scales. To close this knowledge gap, we designed a model that links primary mineral weathering rates to the geographic distribution of poorly crystalline minerals across the USA, and then used this model to evaluate the effect of rock weathering on soil organic carbon. We found that poorly crystalline minerals are most abundant and most strongly correlated with organic carbon in geographically limited zones that sustain enhanced weathering rates, where humid climate and abundant primary minerals co-occur. This finding confirms that rock weathering alters soil mineralogy to enhance soil organic carbon storage at continental scales, but also indicates that the influence of active weathering on soil carbon storage is limited by low weathering rates across vast areas.

Phosphorus (P) availability in soils controls critical functions and properties of terrestrial ecosystems. Vertical distribution patterns of P concentration and speciation in soil profiles provide historical records of how pedogenic processes redistribute and transform P and thus change its availability in soils, which, however, remain poorly understood. We determined the patterns in three forest soil profiles of contrasting climate, using fine sampling intervals, P K-edge X-ray absorption near edge (XANES) spectroscopy and chemical extractions. The major features of the patterns persist under the contrasting climate. The total P concentration decreases from A to B horizons, reaches a minimum in the B horizons, and then increases towards the upper C horizons, but with little variations with depth in the lower C horizons. Both calcium-bound inorganic P (Ca–Pi) and organic P (Po) decrease and Fe- and Al-bound Pi [(Fe + Al)–Pi] increases in proportion downward in the A horizons because dust inputs and accumulation of organic matter both decline with increasing depth. Ca–Pi is negligible and (Fe + Al)–Pi is dominant in the B horizons due to strong weathering. There is a strong downward increase in Ca–Pi proportion and decrease in (Fe + Al)–Pi proportion from the lower B to the upper C horizons. New Ca–Pi seems to form in the upper C horizons where downward leaching Ca2+ and phosphate accumulate due to the low water permeability of the soils. In the lower C horizons, Ca–Pi increases and (Fe + Al)–Pi decreases with increasing depth due to decreasing chemical weathering. Regarding P bioavailability, the proportion of occluded P (Pocc) shows an increasing and decreasing trend with increasing depth, being the highest in the B horizons; however, there are no consistent trends for non-occluded P (Pn-occ). While the P vertical patterns can be understood by considering the relative importance of different pedogenic processes, climate affects the intensities of these processes and thus the details of the patterns. When depth-integrated, warmer/wetter climate results in decreases in the proportions of both Ca–Pi and Pn-occ but increases in the P loss and the proportions of Po, (Fe + Al)–Pi, and Pocc. Regardless of soil depth and climate, the Pi speciation, i.e., the relative proportions of Ca–Pi and (Fe + Al)–Pi over total Pi, correlates well with soil pH and weathering degree (Chemical Index of Alteration, CIA), and the Po concentration correlates with pedogenic Fe and Al and organic carbon concentration. The correlations suggest that the Pi speciation is primarily controlled by soil geochemistry/mineralogy, and the Po concentration by both soil geochemistry/mineralogy and biological activities. Pocc correlates with CIA, and thus is mainly controlled by soil mineralogy; but Pn-occ correlates weakly with soil properties, probably due to its susceptibility to combined influences of dust inputs, leaching, biological activities, and adsorption on minerals. The above quantitative relationships may help predict P speciation and availability in diverse soils. We further show that soil profiles, and climate and CIA gradients are useful tools for studying P transformations, particularly for the Pi pool, during pedogenesis. This study provides an integration and synthesis of controls of climatic and edaphic variables on P dynamics in forest soils.

Watershed degradation due to invasion threatens downstream water flows and associated ecosystem services. While this topic has been studied across landscapes that have undergone invasive-driven state changes (e.g., native forest to invaded grassland), it is less well understood in ecosystems experiencing within-system invasion (e.g. native forest to invaded forest). To address this subject, we conducted an integrated ecological and ecohydrological study in tropical forests impacted by invasive plants and animals. We measured soil infiltration capacity in multiple fenced (i.e., ungulate-free)/unfenced and native/invaded forest site pairs along moisture and substrate age gradients across Hawaii to explore the effects of invasion on hydrological processes within tropical forests. We also characterized forest composition, structure and soil characteristics at these sites to assess the direct and vegetation-mediated impacts of invasive species on infiltration capacity. Our models show that invasive ungulates negatively affect soil infiltration capacity consistently across the wide moisture and substrate age gradients considered. Additionally, several soil characteristics known to be affected by invasive ungulates were associated with local infiltration rates, indicating that the long-term secondary effects of high ungulate densities in tropical forests may be stronger than effects observed in this study. The effect of invasive plants on infiltration was complex and likely to depend on their physiognomy within existing forest community structure. These results provide clear evidence for managers that invasive ungulate control efforts can improve ecohydrological function of mesic and wet forest systems critical to protecting downstream and nearshore resources and maintaining groundwater recharge.

We used a simple “toy” model to aid in the evaluation of the controls of biogeochemical patterns along a climate gradient. The model includes simplified treatments of water balance (precipitation minus Potential Evapotranspiration), leaching, weathering of cation- and P-bearing minerals, N cycling and loss, biomass production, and biological N fixation. We use δ15N as a central integrator of biogeochemical processes, because δ15N integrates multiple pathways of N input, output, and transformation in ecosystems. The model simulated the location and magnitude of a peak in δ15N on a gradient on Kohala Volcano, Hawai‘i which peaked ~ + 14 ‰ in sites receiving ~ 3.5 cm/month average precipitation (− 1300 mm/year water balance); the model also captured a peak in total P in surface soil at intermediate levels of precipitation and water balance, and other biogeochemical features on the gradient. We then applied the model to understanding the patterns of and mechanisms underlying nutrient limitation to net primary production (NPP) and plant biomass on the gradient, testing for the existence and extent of N and P limitation by simulated additions of N and/or P in the model. Both a simulated symbiotic biological N fixer and a simulated non-fixer were limited by P supply across the gradient; the non-fixer was independently limited by N supply in wetter sites. By running the toy model with and without the influence of temperature, we demonstrated that water is the most important factor shaping biogeochemical patterns on this gradient.

Magnesium (Mg) isotopes fractionate during rock/mineral weathering and leaching, secondary mineral neoformation, adsorption/desorption, and plant-related Mg recycling, but the mechanisms and extent of fractionation are not well understood. Here, we report the fate of Mg and its isotopes during basalt weathering and soil development in the Hawaiian Islands by sampling soils of varying age (0.3, 20, 150, 1400, and 4100 ka) in undisturbed humid rainforests. Magnesium concentrations in bulk soils are variable with depth and age, ranging from 0.07 to 8.79 wt.%, and significant Mg depletions (up to 99%) relative to parent basalts are visible after 20 ka. Bulk soils display a large age-dependent range of δ26Mg values ranging from −0.60 to +0.26‰. A sequential leaching scheme showed that labile Mg is depleted whereas residual Mg is enriched in isotopically heavy Mg. The two youngest soils (0.3 ka) display δ26Mg value similar to basalt for both labile or residual Mg, indicating either that basalt weathering causes little Mg isotope fractionation or that δ26Mg value is overwhelmed by the primary minerals during 0.3 ka. However, in the older soils (≥20 ka), the δ26Mg values of both labile and residual Mg vary non-linearly as a function of time, with an increase in the difference between them. These variations are explained by both plant-related Mg recycling and progressive mineral transformations, evolving from short-range-order (SRO) minerals (allophane and ferrihydrite) to more crystalline products (goethite, gibbsite and kaolin minerals). Indeed, plant-related Mg recycling causes the enrichment of light Mg isotopes in the labile Mg, while secondary phases incorporate more and more heavy Mg isotopes with time. These results reconcile experimental and field studies and highlight a weathering control of Mg isotopes delivered to the oceans.

Understanding the influence of terrestrial soil-atmosphere-biosphere exchanges on Li geochemical behaviors is vital before using Li isotopes as a weathering tracer. We investigated Li geochemistry of the humid and arid regolith profiles formed on the Pololu lavas, the Kohala Mountain, Hawaii. The shallow regolith (0–1 m depth) retains Li (τLi,Nb > 0) and displays peak Li accumulation in biologically-active, near-surface soil layers (humid, τLi,Nb = 10.9; arid, τLi,Nb = 2.8), with heavy Li isotopic compositions (humid, 4.7–9.9‰; arid, 4.0–13.9‰) with respect to the basalt signal (2.2‰). The deep regolith (>1 m) demonstrates δ7Li (ave. 3.3‰) comparable to the composition of the underlying parent basalt (2.2‰). Decoupling of Li abundance and isotopic composition in the shallow regolith from those of the deep regolith implies different regolith controls on Li chemistry at vertical locations. The Li geochemistry in the shallow regolith has been substantially influenced by: (i) atmospheric deposition, (ii) plant cycling, and (iii) secondary mineral formation. In addition to weathering alteration, our data show that dust addition mainly influences δ7Li in the humid regolith, and marine aerosol largely affects δ7Li in the arid regolith. In the humid regolith, downward seepage migration allows for deeper and more advanced weathering, whereas occasional wetting events followed by rapid drying likely dominate in the arid regolith. Thus, biological cycling and the deposition of Asian dust and volcanic ash from the more recent Hawi eruptions are responsible for upward Li enrichment in soils and increases of soil δ7Li composition in the shallow regolith. This is particularly important in the humid site with dense plant coverage and heavy rainfall. By contrast, the deep regolith of the humid and arid sites is mostly affected by pore fluid percolation and accumulation, thus inheriting heavy Li isotope signals from pore fluids. Our study emphasizes climate-regulated and long-neglected biological controls on terrestrial Li cycling during chemical weathering.

Wetting of dry soil triggers a pulse of microbial respiration that has been attributed to two broad mechanisms: (1) recycling of microbial cellular carbon (C), and (2) consumption of extracellular organic C made available to microbes by wetting. We evaluated these two mechanisms by measuring cumulative CO2 release, changes in the size and chemical composition of microbial biomass, and water-extractable organic carbon (WEOC) concentrations following artificial wetting of soil sampled from two depths at each of seven sites across California spanning a range of geologic parent materials. In samples collected from surface soil (0–10 cm depth), we found that cumulative CO2 release after wetting in the laboratory was most strongly correlated with microbial biomass. In these samples, the relative abundance of trehalose—a putative microbial osmolyte—decreased from 25% (SD = 12) to 16% (SD = 7) of the chloroform-labile fraction of the microbial biomass after wetting. This suggested a role for osmolyte consumption in generating the respiration pulse. In subsoil (40–50 cm depth, or sampled at contact with rock), however, the cumulative CO2 release after wetting was unrelated to microbial biomass and more strongly related to WEOC. The concentrations of selected microbial biomass constituents (e.g. trehalose and amino acids) in WEOC were negligible (

Radiocarbon ages and thermal stability measurements can be used to estimate the stability of soil organic carbon (OC). Soil OC is a complex reservoir that contains a range of compounds with different sources, reactivities, and residence times. This heterogeneity can shift bulk radiocarbon values and impact assessment of OC stability and turnover in soils. Four soil horizons (Oa, Bhs, Bs, Bg) were sampled from highly weathered 350 ka Pololu basaltic volcanics on the Island of Hawaii and analyzed by Ramped PyrOX (RPO) in both the pyrolysis (PY) and oxidation (OX) modes to separate a complex mixture of OC into thermally defined fractions. Fractions were characterized for carbon stable isotope and radiocarbon composition. PY and OX modes yielded similar results. Bulk radiocarbon measurements were modern in the Oa horizon (Fm = 1.013) and got progressively older with depth: the Bg horizon had an Fm value of 0.73. Activation energy distributions (p(E)) calculated using the ‘rampedpyrox’ model yielded consistent mean E values of 140 kJ mol−1 below the Oa horizon. The ‘rampedpyrox’ model outputs showed a mostly bimodal distribution in the p(E) below the Oa, with a primary peak at 135 kJ mol−1 and a secondary peak at 148 kJ mol−1, while the Oa was dominated by a single, higher E peak at 157 kJ mol−1. We suggest that mineral-carbon interaction, either through mineral surface-OC or metal-OC interactions, is the stabilization mechanism contributing to the observed mean E of 140 kJ mol−1 below the Oa horizon. In the Oa horizon, within individual RPO analyses, radiocarbon ages in the individual thermal fractions were indistinguishable (p > 0.1). The flat age distributions indicate there is no relationship between age and thermal stability (E) in the upper horizon (> 25 cm). Deeper in the soil profile higher µEf values were associated with older radiocarbon ages, with slopes progressively steepening with depth. In the deepest (Bg) horizon, there was the largest, yet modest change in Fm of 0.06 (626 radiocarbon years), indicating that older OC is slightly more thermally stable.

Glaciers in central Asia that developed under a range of climatic conditions from arid to humid provide an excellent opportunity to test glacial responses to changes in climate. To do this, we mapped and dated glacial deposits at 11 sites spread over five mountain ranges in central Asia: the Altai, Tian Shan, Altyn Tagh, Qilian Shan, and Kunlun. The glacial chronologies for these sites were determined from new 10Be and 26Al exposure ages for the mapped moraines, in addition to 10Be ages available in the literature. Paleo–equilibrium-line altitudes were estimated for past glacier extents from the dated moraines. The equilibrium-line altitudes (ELAs) were also estimated for existing glaciers to characterize the spatial pattern in modern climate across the study region. Differences between the modern and paleo-ELAs (ΔELAs) were used to explore the climatic reasons for variations in the glacier sensitivities and responses to past changes in climate. The results show that the glaciers in more humid regions advanced to their maximum during marine oxygen-isotope stage (MIS) 3–2 with ΔELAs of ~1100–600 m. However, glaciers in the arid interior of central Asia, in the rain shadows of the Karakorum and Pamir ranges and in the Gobi Desert ranges, reached their maximum between MIS 6 and 4, and glacier extents during the subsequent colder/drier MIS 3–2 were significantly smaller or did not extend beyond their cirques. Comparisons of our results and the sensitivity analysis of modern glaciers suggest that depression of air temperature was the primary driver of glacier advances in central Asia but that precipitation played a major role in shaping the spatial and temporal heterogeneity of glacier advances. Precipitation was especially important in hyperarid conditions. Therefore, inferences about paleoclimate parameters from past glacial extents must be made after careful consideration of the climatic setting in which the glaciers are found, as well as their sensitivity to climatic factors.

Pedogenic thresholds describe where soil properties or processes change in an abrupt/nonlinear fashion in response to small changes in environmental forcing. Contrastingly, soil process domains refer to the space between thresholds where soil properties are either unchanged, or change gradually, across a broad range of environmental forcing. Here, we test quantitatively for the presence of thresholds in patterns of soil properties across a climatic gradient on soils developed from about 20-ky-old basaltic substrate on the Island of Hawai’i. From multiple soil properties, we quantitatively identified a threshold at about 750 mm/y of water balance (precipitation minus potential evapotranspiration), delineating the upper water balance boundary of soil fertility in these soils. From the threshold in the ratio of exchangeable Ca to total Ca, we identified the lower water balance boundary of soil fertility in these soils at − 1000 mm/y; however, this threshold was qualitatively described as it lies near the limit of the climate gradient data where the statistical approach cannot be applied. These two results represent the first time that pedogenic thresholds have been identified using statistically rigorous methods and the limitations of said methods, respectively. Comparing the 20-ky soils to soils that developed on basaltic substrates of 1.2 ky, 7.5 ky, 150 ky, and 4100 ky in a time–climate matrix, we found that our quantitative analysis supports previous qualitatively identified thresholds in the soils developed from older substrates. We also identified the 20 ky as the transition from kinetic to supply limitation for plant nutrients in soil in this system.

Sequential chemical extraction has been widely used to study soil phosphorus (P) dynamics and inform nutrient management, but its efficacy for assigning P into biologically meaningful pools remains unknown. Here, we evaluated the accuracy of the modified Hedley extraction scheme using P K-edge X-ray absorption near-edge structure (XANES) spectroscopy for nine carbonate-free soil samples with diverse chemical and mineralogical properties resulting from different degrees of soil development. For most samples, the extraction markedly overestimated the pool size of calcium-bound P (Ca-P, extracted by 1 M HCl) due to (1) P redistribution during the alkaline extractions (0.5 M NaHCO3 and then 0.1 M NaOH), creating new Ca-P via formation of Ca phosphates between NaOH-desorbed phosphate and exchangeable Ca2+ and/or (2) dissolution of poorly crystalline Fe and Al oxides by 1 M HCl, releasing P occluded by these oxides into solution. The first mechanism may occur in soils rich in well-crystallized minerals and exchangeable Ca2+ regardless of the presence or absence of CaCO3, whereas the second mechanism likely operates in soils rich in poorly crystalline Fe and Al minerals. The overestimation of Ca-P simultaneously caused underestimation of the pools extracted by the alkaline solutions. Our findings identify key edaphic parameters that remarkably influenced the extractions, which will strengthen our understanding of soil P dynamics using this widely accepted procedure.

The strong elevation gradient of the Himalaya allows for investigation of altitude and orographic impacts on surface water δ18O and δD stable isotope values. This study differentiates the time- and altitude-variable contributions of source waters to the Arun River in eastern Nepal. It provides isotope data along a 5000-m gradient collected from tributaries as well as groundwater, snow, and glacial-sourced surface waters and time-series data from April to October 2016. We find nonlinear trends in δ18O and δD lapse rates with high-elevation lapse rates (4000–6000 masl) 5–7 times more negative than low-elevation lapse rates (1000–3000 masl). A distinct seasonal signal in δ18O and δD lapse rates indicates time-variable source-water contributions from glacial and snow meltwater as well as precipitation transitions between the Indian Summer Monsoon and Winter Westerly Disturbances. Deuterium excess correlates with the extent of snowpack and tracks melt events during the Indian Summer Monsoon season. Our analysis identifies the influence of snow and glacial melt waters on river composition during low-flow conditions before the monsoon (April/May 2016) followed by a 5-week transition to the Indian Summer Monsoon-sourced rainfall around mid-June 2016. In the post-monsoon season, we find continued influence from glacial melt waters as well as ISM-sourced groundwater.

It has been postulated that the amount of soil organic carbon (SOC) associated with soil minerals exhibits a threshold relationship in response to effective soil moisture (estimated as precipitation less evapotranspiration). To better characterize the role of moisture in influencing mechanisms of SOC storage during pedogenesis, we compare soils from two different chronosequence sites: the Santa Cruz and Mattole River marine terraces that together form a soil age-by-climate gradient (i.e., climo-chronosequence). Our results demonstrate how variation in the effective soil moisture may drive soil development along divergent pedogenic trajectories, resulting in variations in the form and depth distribution of secondary weathering products. In particular, the residual metals Fe and Al are directly related to the type of secondary minerals that accumulate during weathering, and these variations are coupled to differences in the storage and long-term preservation of SOC both within and between soils. Over time, these differences in soil development may lead to ‘pedogenic thresholds’ that further differentiate soil characteristics and influence SOC dynamics. In this case, the pedogenic threshold takes the form of clay-rich argillic horizons that once formed, inhibit aqueous transport, decouple shallow and deep soil environments, and potentially limit SOC inputs and increase microbial recycling in deep soils. Our data suggest argillic horizon development is favorable in the drier Santa Cruz soils, where kaolinite is the dominant secondary weathering product. In contrast, greater available moisture in soils of the Mattole chronosequence drive a different weathering trajectory characterized by the accumulation of more amorphous secondary minerals. As a result, the Mattole soils and do not exhibit argillic horizon development but are instead characterized by greater accumulation of SOC across all depths sampled. Overall, our results illustrate how the interaction of climate (i.e., moisture) and time may shape the trajectory of soil development and the dynamics of SOC storage and preservation.

Plants play a critical role in the cycling of potassium (K) and the fractionation of its isotopes. However, little is known about K stable isotopic compositions in natural soil–plant systems and possible fractionation during intra-plant transport and root-soil uptake of K. This study focuses on K isotopic fractionation within a humid and an arid soil–plant system sampled on the windward and leeward sides of Kohala Mountain, Hawai‘i. We determined the K isotopic compositions of (1) K redistribution during intra-plant circulation and (2) uptake at the root-soil interface. For intra-plant circulation of K, there is a high affinity of isotopically lighter K to organic complexes as K-pectate, and K-pectate is particularly enriched in roots and fresh leaves. For K uptake at root-soil interface, isotopically lighter K is preferentially taken by roots from soil bioavailable pools following a low-affinity (passive) transport path. Soil K budget in two sites reflects strong source mixing effects with limited plant imprints. This work provides exploratory data on the biogeochemical fractionation of K isotopes in the soil–plant system.

Trees, the most successful biological power plants on earth, build and plumb the critical zone (CZ) in ways that we do not yet understand. To encourage exploration of the character and implications of interactions between trees and soil in the CZ, we propose nine hypotheses that can be tested at diverse settings. The hypotheses are roughly divided into those about the architecture (building) and those about the water (plumbing) in the CZ, but the two functions are intertwined. Depending upon one's disciplinary background, many of the nine hypotheses listed below may appear obviously true or obviously false. (1) Tree roots can only physically penetrate and biogeochemically comminute the immobile substrate underlying mobile soil where that underlying substrate is fractured or pre-weathered. (2) In settings where the thickness of weathered material, H, is large, trees primarily shape the CZ through biogeochemical reactions within the rooting zone. (3) In forested uplands, the thickness of mobile soil, h, can evolve toward a steady state because of feedbacks related to root disruption and tree throw. (4) In settings where h ≪ H and the rates of uplift and erosion are low, the uptake of phosphorus into trees is buffered by the fine-grained fraction of the soil, and the ultimate source of this phosphorus is dust. (5) In settings of limited water availability, trees maintain the highest length density of functional roots at depths where water can be extracted over most of the growing season with the least amount of energy expenditure. (6) Trees grow the majority of their roots in the zone where the most growth-limiting resource is abundant, but they also grow roots at other depths to forage for other resources and to hydraulically redistribute those resources to depths where they can be taken up more efficiently. (7) Trees rely on matrix water in the unsaturated zone that at times may have an isotopic composition distinct from the gravity-drained water that transits from the hillslope to groundwater and streamflow. (8) Mycorrhizal fungi can use matrix water directly, but trees can only use this water by accessing it indirectly through the fungi. (9) Even trees growing well above the valley floor of a catchment can directly affect stream chemistry where changes in permeability near the rooting zone promote intermittent zones of water saturation and downslope flow of water to the stream. By testing these nine hypotheses, we will generate important new cross-disciplinary insights that advance CZ science.

Studies in the leeward Kohala field system on Hawai‘i Island have considered the processes and timing of agricultural development associated with sociopolitical transformations and the production of agricultural surpluses. Using extensive soil sampling, we explore the use of relatively mobile and immobile soil parameters within the agricultural landscape to interpret the development and maintenance of the system over time. Results show that in the context of the leeward Kohala field system, the immobile element niobium can be used to interpret the location of ancient ground surfaces and that phosphorus is stable enough to be used to understand anthropogenic influences. In contrast, total and exchangeable calcium are too mobile to indicate purely anthropogenic processes. Data suggest that there was more intensive depletion of soil phosphorus on agricultural alignments than in field areas between the alignments, potentially explained by traditional cropping methods associated with the system. Spatial analyses within the field system identifies an area with high naturally occurring soil nutrient levels.

Nitrogen (N) availability influences patterns of terrestrial productivity and global carbon cycling, imparting strong but poorly resolved feedbacks on Earth's climate system. Central questions concern the timescale of N cycle response to elevated CO2 concentration in the atmosphere and whether availability of this limiting nutrient increases or decreases with climate change. Nitrogen isotopic composition of bulk plant leaves provides information on large-scale patterns of N availability in the modern environment. Here we examine the utility of chlorins, degradation products of chlorophyll, hypothesized to persist in soil subsequent to plant decay, as proxies for reconstructing past plant δ15N. Specifically, we test the hypothesis that δ15N of plant leaves (δ15Nleaf) is recorded in δ15N of pheophytin a (δ15Npheo) along the leaf–litter–soil continuum across an array of ecosystem climate conditions and plant functional types (C3, C4, legumes, and woody plants). The δ15N of live foliage and bulk soil display marked declines with increasing rainfall, consistent with past studies in Hawaii and patterns worldwide. We find measurable chlorin concentrations along soil–depth profiles at all sites, with pheophytin a present in amounts required for isotopic analysis (>10 nmol). δ15Npheo in leaves, litter, and soil track δ15Nleaf of plant leaves. We find potential for δ15Npheo records from soil to provide proxy information on δ15Nleaf.

PUBLICATIONS Journal of Geophysical Research: Earth Surface RESEARCH ARTICLE 10.1002/2016JF003864 Key Points: • Soil chemistry and weathering vary nonlinearly across a large rainfall gradient (400–4700 mm/yr) in NZ • Climate control evidenced in detailed soil chemistry, Fe and Al mobilities, and cation leaching • Moisture availability can act as a “switch” to enable rapid chemical weathering in young soils Supporting Information: • Supporting Information S1 • Data Set S1 Correspondence to: J. L. Dixon, jean.dixon@montana.edu Citation: Dixon, J. L., O. A. Chadwick, and P. M. Vitousek (2016), Climate-driven thresholds for chemical weathering in postglacial soils of New Zealand, J. Geophys. Res. Earth Surf., 121, 1619–1634, doi:10.1002/2016JF003864. Received 18 FEB 2016 Accepted 12 AUG 2016 Accepted article online 17 AUG 2016 Published online 16 SEP 2016 Climate-driven thresholds for chemical weathering in postglacial soils of New Zealand Jean L. Dixon 1,2 , Oliver A. Chadwick 2 , and Peter M. Vitousek 3 Department of Earth Sciences and the Institute on Ecosystems, Montana State University, Bozeman, Montana, USA, Department of Geography, University of California, Santa Barbara, California, USA, 3 Department of Biology, Stanford University, Stanford, California, USA Abstract Chemical weathering in soils dissolves and alters minerals, mobilizes metals, liberates nutrients to terrestrial and aquatic ecosystems, and may modulate Earth’s climate over geologic time scales. Climate-weathering relationships are often considered fundamental controls on the evolution of Earth’s surface and biogeochemical cycles. However, surprisingly little consensus has emerged on if and how climate controls chemical weathering, and models and data from published literature often give contrasting correlations and predictions for how weathering rates and climate variables such as temperature or moisture are related. Here we combine insights gained from the different approaches, methods, and theory of the soil science, biogeochemistry, and geomorphology communities to tackle the fundamental question of how rainfall influences soil chemical properties. We explore climate-driven variations in weathering and soil development in young, postglacial soils of New Zealand, measuring soil elemental geochemistry along a large precipitation gradient (400–4700 mm/yr) across the Waitaki basin on Te Waipounamu, the South Island. Our data show a strong climate imprint on chemical weathering in these young soils. This climate control is evidenced by rapid nonlinear changes along the gradient in total and exchangeable cations in soils and in the increased movement and redistribution of metals with rainfall. The nonlinear behavior provides insight into why climate-weathering relationships may be elusive in some landscapes. These weathering thresholds also have significant implications for how climate may influence landscape evolution and the release of rock-derived nutrients to ecosystems, as landscapes that transition to wetter climates across this threshold may weather and deplete rapidly. 1. Introduction 1.1. Climate’s Elusive Control on Chemical Weathering Soils lie at the interface of air, water, life, and rock, and the weathering dynamics that transform minerals and water in soils are shaped by diverse processes. Climate has long been recognized to be one of the major dri- vers of these weathering processes [Jenny, 1941]. Temperature controls the kinetics of chemical reactions, and water has a role in nearly every chemical weathering reaction that directly results in mass loss from a rock or mineral. Therefore, warmer and wetter conditions should lead to higher weathering rate and intensity in soils. However, a coherent understanding of how climate controls soil chemical weathering remains elusive. Several reasons emerge for the lack of consensus across studies. 1.2. Competing Variables ©2016. The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distri- bution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. DIXON ET AL. First, while the conceptual framework for climate control on weathering rates is validated by laboratory experiments quantifying dissolution rates under different temperature or water-flow conditions [White et al., 1999; White and Brantley, 2003], field-based studies reveal significant complexity among climate- weathering linkages [Brantley, 2003; Drever et al., 1994; White and Brantley, 2003], which may be explained by time-dependent factors such as changing mineral surface area, pore water concentrations, and secondary precipitates. Similarly, climate’s control on soil weathering can be modified by the complex influence of other competing variables such as lithology, erosion rates, and/or dust deposition [e.g., Ferrier et al., 2012; Riebe et al., 2004]. Furthermore, field-based weathering rates are often measured in locations where multiple variables (including climate variables such as temperature and water availability) exert competing controls on mineral weathering and the fate of released ions [Chadwick and Chorover, 2001]. These competing climatic controls may be deconvolved using careful sampling designs and accounting for multiple variables [e.g., Dixon et al., 2009a; Rasmussen et al., 2011; White and Blum, 1995]; however, derived relationships and models may be site specific or have limited applicability. CLIMATE-DRIVEN WEATHERING THRESHOLDS

RESEARCH ARTICLE The effects of check dams and other erosion control structures on the restoration of Andean bofedal ecosystems Brett D. Hartman 1,2 , Bodo Bookhagen 1,3 , Oliver A. Chadwick 4 Restoring degraded lands in rural environments that are heavily managed to meet subsistence needs is a challenge due to high rates of disturbance and resource extraction. This study investigates the efficacy of erosion control structures (ECSs) as restoration tools in the context of a watershed rehabilitation and wet meadow (bofedal) restoration program in the Bolivian Andes. In an effort to enhance water security and increase grazing stability, Aymara indigenous communities built over 15,000 check dams, 9,100 terraces, 5,300 infiltration ditches, and 35 pasture improvement trials. Communities built ECSs at different rates, and we compared vegetation change in the highest restoration management intensity, lowest restoration management intensity, and nonproject control communities. We used line transects to measure changes in vegetation cover and standing water in gullies with check dams and without check dams, and related these ground measurements to a time series (1986–2009) of normalized difference vegetation index derived from Landsat TM5 images. Evidence suggests that check dams increase bofedal vegetation and standing water at a local scale, and lead to increased greenness at a basin scale when combined with other ECSs. Watershed rehabilitation enhances ecosystem services significant to local communities (grazing stability, water security), which creates important synergies when conducting land restoration in rural development settings. Key words: Aymara, human-environment system, indigenous people, land restoration, NDVI, wet meadow Implications for Practice • Check dams increase bofedal vegetation and standing water at a local scale but can also lead to landscape-level effects that extend beyond the surface area covered by check dams. • The effects of large-scale and long-term restoration efforts need to be evaluated in the context of environmental change resulting from regional shifts in climate and land use. • Check dams and other erosion control structures can increase grazing stability and water security for local com- munities. When land restoration is aligned with the provi- sion of ecosystem services, indigenous people are capable of achieving extensive areas of land restoration even under continued agriculture and grazing management. Introduction Significant portions of the world’s tropics have been degraded by human use, with land degradation concentrated in dryland montane areas managed by the rural poor (Bridges & Oldeman 1999; Lambin et al. 2003; Bai et al. 2008). Local and indigenous people can be effective at ecosystem restoration, provided there is sufficient social coordination and mobilization (e.g. Walters 2000; Long et al. 2003; Mingyi et al. 2003; Stringer et al. 2007; Blay et al. 2008). However, restoration efforts in rural environ- ments that are heavily managed to meet subsistence needs are November 2016 Restoration Ecology Vol. 24, No. 6, pp. 761–772 often complicated by high levels of disturbance from agricul- ture, grazing, fire, and biomass harvest (Brown & Lugo 1994; Lamb et al. 2005). To improve restoration success in rural devel- opment settings, there is a need to better understand restoration dynamics where land use pressure is high and management objectives include restoring ecosystem services important to local communities (e.g. grazing stability and water security). A geographic region where intensive management by rural poor populations has led to environmental degradation is the Central Andes of South America (Ellenberg 1979; Sarmiento & Frolich 2002). The Central Andes are dominated by dry, tropical montane Puna grasslands composed of bunchgrasses, rosette-forming herbs, and dwarf shrubs in upland positions, and bofedal vegetation composed of rosette-forming herbs and cushion-forming species in seeps, springs, wet meadows, and floodplains (Squeo et al. 2006). Large portions of the Central Author contributions: BDH, BB OAC designed research; BDH performed research; BDH, BB analyzed data; BDH, BB, OAC wrote the manuscript. 1 Department of Geography, University of California, Santa Barbara, CA 93106, U.S.A. 2 Address correspondence to B. D. Hartman, email hartman@geog.ucsb.edu 3 Institute of Earth and Environmental Science, University of Potsdam, Karl-Liebknecht-Street 24-25, 14476 Potsdam-Golm, Germany 4 Department of Geography and Environmental Studies, University of California, Santa Barbara, CA 93106, U.S.A. © 2016 Society for Ecological Restoration doi: 10.1111/rec.12402 Supporting information at: http://onlinelibrary.wiley.com/doi/10.1111/rec.12402/suppinfo

Successful land restoration in impoverished rural environments may require adoption of new resource management strategies; however, feedbacks between local knowledge and introduced restoration technologies have rarely been articulated. We used interview scenarios to analyze the role of local knowledge in land restoration at a large-scale, long-term watershed rehabilitation and wet meadow restoration program in the highland Andes. Indigenous communities built over 30,000 check dams, terraces and infiltration ditches, and the density of erosion control structures and visible restoration varied greatly across participant communities. We developed a survey reaching across the highest restoration management intensity, lowest restoration management intensity, and non-project (control) communities. We interviewed 49 respondents using 14 scenarios based on photos depicting biophysical phenomena related to land degradation and restoration. The scenarios generated 5,828 statements that were coded into 964 distinct concepts. As expected, respondents that built more erosion control structures had more detailed knowledge of check dam construction and ecosystem development following physical interventions. More significantly, there was a shift in the conceptualization of and attitudes toward land degradation and restoration. Respondents who built more erosion control structures were more likely to: attribute wetland hydrology to groundwater recharge rather than myth constructs about seeps and springs; attribute land degradation to human rather than mythological causes; and have more proactive attitudes regarding land restoration. Evidence suggests that when addressing severe land degradation or restoring ecosystem processes not readily observable by indigenous people, such as groundwater flow and wetland recharge, restoration success will depend on combining local and scientific knowledge.

Nutrient biolifting is an important pedogenic process in which plant roots obtain inorganic nutrients such as phosphorus (P) and calcium (Ca) from minerals at depth and concentrate those nutrients at the surface. Here we use soil chemistry and stable isotopes of the alkaline earth elements Ca, strontium (Sr) and barium (Ba) to test the hypothesis that biolifting of P has been an important pedogenic process across a soil climosequence developed on volcanic deposits at Kohala Mountain, Hawaii. The geochemical linkage between these elements is revealed as generally positive site-specific relationships in soil mass gains and losses, particularly for P, Ba and Ca, using the ratio of immobile elements titanium and niobium (Ti/Nb) to link individual soil samples to a restricted compositional range of the chemically and isotopically diverse volcanic parent materials. At sites where P is enriched in surface soils relative to abundances in deeper soils, the isotope compositions of exchangeable Ca, Sr and Ba in the shallowest soil horizons ( 10 cm depth) at those sites is consistently heavier than the volcanic parent materials. The isotope compositions of exchangeable Ca and Sr trend toward heavier compositions with depth more gradually, reflecting increasing leakiness from these soils in the order Ba

Topography strongly regulates soil formation at the hillslope scale through its effects on sediment redistribution and biological activities. Spatially explicit land surface parameters (LSPs) such as slope and curvature hold potential for modeling the resulting soil carbon (C) and nitrogen (N) distributions, but their representation of deep soil profiles remains largely unexplored. In this study we examine relationships between deep soil profile C and N stocks and LSPs derived from a fine-resolution digital elevation model (DEM) on prototypical rolling hillslope catenas. Consistent with other studies we found that soil thickness was the primary controller of soil organic C and N stocks and was best predicted by mean curvature. Specifically, subsoil thickness, instead of A horizon thickness, explained variability of soil C and N on hillslopes. In addition, our results suggest that, along ridge to toeslope catenas, the processes mediating soil C and N distribution varied from convex to concave positions. Convex ridge positions appeared to favor processes that enrich soil profiles with high C and N concentrations despite their drier position, while concave hollow and toeslope positions favored cumulic processes, despite their conceptually moister conditions in which enrichment processes would be favored. Our data also point to slope aspect as a weak but potentially geomorphically important covariate in modeling soil thickness and C and N stocks using LSPs. Overall, LSPs of curvature and aspect explained 51% of the variability in soil thickness, while curvature and aspect explained 50% of the variability in soil organic C stocks. Our results suggest that diffusive sediment transportation likely exerts a first-order control on soil thickness and soil organic C and N stocks in many semi-arid landscapes. Our data also highlight the importance of subsoil in mapping soil C and N stocks and other soil properties. Quantitative modeling of soil C and N as in our study supports examination of additional ecosystem properties at fine spatial scales.

The strong elevation gradient of the Himalaya allows investigation of altitude and orographic impacts on precipitation isotope values as captured in river samples. This study provides new high-elevation data along a 5000 m gradient collected from rain, snow, and glacial-sourced surface waters and time-series data from April to October 2016 to differentiate the time-variable contributions of source waters to the Arun River. We find nonlinear trends in δ18O and δD lapse rates driven by samples collected at high elevations and a distinct seasonal signal indicative of moisture source influences the surface-water isotope values. Deuterium excess is correlated to snowpack and used to track melt events during the monsoon. Our analysis identifies contributions from snowpack to river discharge before the monsoon onset followed by a 5-week transition to Indian Summer Monsoon-sourced rainfall around mid-June 2016.

Collaborations between biologists and geologists are key to understanding and projecting how landscapes function and change over time. Such collaborations are stimulated by on-going scientific developments, advances in instrumentation and technology, and the growing recognition that environmental problems necessitate interdisciplinary investigation. Here, we show how the biogeosciences are well placed to answer more completely the core questions that motivate the world's invaluable environmental research networks: specifically, the venerable Long Term Ecological Research networks (LTERs), the newer surveillance facilities of the Earth Observatory Networks (EONs including the USA's NEON), and the geosciences' interdisciplinary network of Critical Zone Observatories (CZOs). Because LTER and EON programs have been supported largely by ecological and biological communities and CZOs largely by the geological community, we assert that a concerted biogeoscience approach across these invaluable networks can benefit both their scientific productivity and usefulness to the wider public.

Rain-fed, intensive field systems based on sweet potato and to a lesser extent dryland taro were essential to the political economies of emergent archaic states in late pre-contact Hawai'i. The productivity of these dryland field systems was dependent upon soil nutrient pools that are constrained primarily by geological substrate age and rainfall. We investigated soil nutrient properties across the Kaupō fan, locus of the largest intensive field system on eastern Maui Island. Thirty-six soil samples were obtained along three rough transects, and analyzed for exchangeable bases (calcium (Ca), magnesium (Mg), potassium (K), sodium (Na), base saturation, pH, and rock content). Although some variation in soil properties is evident across the Kaupō fan, in aggregate the samples exhibit high levels of base saturation and exchangeable Ca, indicating good potential for sustained, intensive cultivation. When compared with the Leeward Kohala Field System on Hawai'i Island, the Kaupō field system was likely to have been even more productive based on the relative soil nutrient pools. The high productive capacity of the Kaupō field system is likely to have played a key role in the decision of the early 18th century Maui King Kekaulike to move the island's royal center to Kaupō.

Plants are known to scavenge nutrients such as phosphorus (P) from rock subtrates and concentrate those nutrients at the surface in order to satisfy long-term nutritional requirements. Using the stable isotope systematics of calcium (Ca), strontium (Sr) and barium (Ba) as proxy tracers, here we test the hypothesis that soils along a Hawaiian climosequence have “biolifted” P and other nutrients such as Ca from depth and concentrated those nutrients at the surface. Relative to isotope compositions in the volcanic soil parent materials, exchangeable Ca, Sr and Ba in the shallowest soils at sites having evidence for P biolifting are light confirming the role for plant uptake of these elements while exchangeable Ca, Sr and Ba of deeper soils at those sites are similar or heavy suggesting a complementary pool. In this pedogenic system Ca and Sr are more susceptible to leaching than Ba, thus Ba isotopes provide the most resilient evidence for biolifting across the climate gradient and a useful proxy for P biodynamics.

RESEARCH ARTICLE The effects of check dams and other erosion control structures on the restoration of Andean bofedal ecosystems Brett D. Hartman 1,2 , Bodo Bookhagen 1,3 , Oliver A. Chadwick 4 Restoring degraded lands in rural environments that are heavily managed to meet subsistence needs is a challenge due to high rates of disturbance and resource extraction. This study investigates the efficacy of erosion control structures (ECSs) as restoration tools in the context of a watershed rehabilitation and wet meadow (bofedal) restoration program in the Bolivian Andes. In an effort to enhance water security and increase grazing stability, Aymara indigenous communities built over 15,000 check dams, 9,100 terraces, 5,300 infiltration ditches, and 35 pasture improvement trials. Communities built ECSs at different rates, and we compared vegetation change in the highest restoration management intensity, lowest restoration management intensity, and nonproject control communities. We used line transects to measure changes in vegetation cover and standing water in gullies with check dams and without check dams, and related these ground measurements to a time series (1986–2009) of normalized difference vegetation index derived from Landsat TM5 images. Evidence suggests that check dams increase bofedal vegetation and standing water at a local scale, and lead to increased greenness at a basin scale when combined with other ECSs. Watershed rehabilitation enhances ecosystem services significant to local communities (grazing stability, water security), which creates important synergies when conducting land restoration in rural development settings. Key words: Aymara, human-environment system, indigenous people, land restoration, NDVI, wet meadow Implications for Practice • Check dams increase bofedal vegetation and standing water at a local scale but can also lead to landscape-level effects that extend beyond the surface area covered by check dams. • The effects of large-scale and long-term restoration efforts need to be evaluated in the context of environmental change resulting from regional shifts in climate and land use. • Check dams and other erosion control structures can increase grazing stability and water security for local com- munities. When land restoration is aligned with the provi- sion of ecosystem services, indigenous people are capable of achieving extensive areas of land restoration even under continued agriculture and grazing management. Introduction Significant portions of the world’s tropics have been degraded by human use, with land degradation concentrated in dryland montane areas managed by the rural poor (Bridges & Oldeman 1999; Lambin et al. 2003; Bai et al. 2008). Local and indigenous people can be effective at ecosystem restoration, provided there is sufficient social coordination and mobilization (e.g. Walters 2000; Long et al. 2003; Mingyi et al. 2003; Stringer et al. 2007; Blay et al. 2008). However, restoration efforts in rural environ- ments that are heavily managed to meet subsistence needs are November 2016 Restoration Ecology Vol. 24, No. 6, pp. 761–772 often complicated by high levels of disturbance from agricul- ture, grazing, fire, and biomass harvest (Brown & Lugo 1994; Lamb et al. 2005). To improve restoration success in rural devel- opment settings, there is a need to better understand restoration dynamics where land use pressure is high and management objectives include restoring ecosystem services important to local communities (e.g. grazing stability and water security). A geographic region where intensive management by rural poor populations has led to environmental degradation is the Central Andes of South America (Ellenberg 1979; Sarmiento & Frolich 2002). The Central Andes are dominated by dry, tropical montane Puna grasslands composed of bunchgrasses, rosette-forming herbs, and dwarf shrubs in upland positions, and bofedal vegetation composed of rosette-forming herbs and cushion-forming species in seeps, springs, wet meadows, and floodplains (Squeo et al. 2006). Large portions of the Central Author contributions: BDH, BB OAC designed research; BDH performed research; BDH, BB analyzed data; BDH, BB, OAC wrote the manuscript. 1 Department of Geography, University of California, Santa Barbara, CA 93106, U.S.A. 2 Address correspondence to B. D. Hartman, email hartman@geog.ucsb.edu 3 Institute of Earth and Environmental Science, University of Potsdam, Karl-Liebknecht-Street 24-25, 14476 Potsdam-Golm, Germany 4 Department of Geography and Environmental Studies, University of California, Santa Barbara, CA 93106, U.S.A. © 2016 Society for Ecological Restoration doi: 10.1111/rec.12402 Supporting information at: http://onlinelibrary.wiley.com/doi/10.1111/rec.12402/suppinfo

Using field observations and geochemical and digital terrain analyses, we describe the structure and thickness of the regolith across a climosequence on the island of Hawai'i to gain insight into the relative roles of precipitation and the near-surface hydrologic structure in determining weathering patterns. In the wet portion of the climosequence, where the long-term water balance is positive, the regolith thickness reaches an observed maximum of ~40m and appears limited by the geomorphic base-level of the landscape. However, even within this thick regolith, distinct units of varying weathering intensity occur; the vertical ordering of which largely reflects differences in the initial permeability structure of the basalt flows rather than a systematic decrease in weathering intensity downwards from the ground surface. In the dry portion of the climosequence, where the long-term water balance is negative, the regolith thickness is confined to ~1m, is highly dependent on the inferred permeability structure of the basalt flows, and is independent of geomorphic base-level. Weathering intensity also varies according to permeability structure and decreases in this thin regolith with distance beneath the ground surface. The abrupt change in regolith depth and character that coincides with the transition from net-positive to net-negative long-term water balance implies that small changes in precipitation rates around a neutral water balance result in large changes in the distribution and depth of weathering. Together our observations indicate that the distribution and depth of weathering in basalts (and probably other lithologies) might be best understood by considering how precipitation interacts with the complicated near-surface permeability structure over regolith-forming timescales to weather rock in the vadose zone. © 2013 John Wiley & Sons, Ltd.

The spacing of hills and valleys reflects the competition between disturbance-driven (or diffusive) transport on hillslopes and concentrative (or advective) transport in valleys, although the underlying lithologic, tectonic, and climatic controls have not been untangled. Here, we measure geochemical and geomorphic properties of catchments in Kruger National Park, South Africa, where granitic lithology and erosion rates are invariant, enabling us to evaluate how varying mean annual precipitation(MAP = 470 mm, 550 mm, and 730 mm) impacts hill-valley spacing or landscape dissection. Catchment-averaged erosion rates, based on 10Be concentrations in river sands, are low (3-6 m/m.y.) and vary minimally across the three sites. Our lidar-derived slope-area analyses reveal that hillslopes in the dry site are gentle (3%) and short, such that the terrain is low relief and appears highly dissected. With increasing rainfall, hillslopes lengthen and increase in gradient (6%-8%), resulting in less-dissected, higher-relief catchments. The chemical depletion fraction of hilltop regoliths increases with rainfall, from 0.3 to 0.7, reflecting a climate-driven increase in chemical relative to physical erosion. Soil catenas also vary systematically with climate as we observe relatively uniform soil properties in the dry site that contrast with leached sandy crests and upper slopes coupled with downslope clay accumulation zones in the intermediate and wet sites. The geomorphic texture of this slow-eroding, granitic landscape appears to be set by climate-driven feedbacks among chemical weathering, regolith fabric differentiation, hydrological routing, and sediment transport that enhance the vigor of hillslope sediment transport relative to valley-forming processes for wetter climates. © 2013 Geological Society of America.

Question: How do canopy disturbance and soil properties structure vascular plant community species composition and resilience to encroachment by exotic species in a tropical montane wet forest? Location: Hawai'i Experimental Tropical Forest (HETF), a tropical montane wet forest, on Mauna Kea, Hawai'i Island, Hawai'i, USA. Methods: Previous studies employing airborne LiDAR were used to define three zones across an elevation gradient from 900 to 1500m. Within each zone, a ~1000-m block transect was selected to cross two different volcanic substrates: one derived from surface lava and one derived from thick ash deposits. Non-metric multidimensional scaling (NMS) scores of vegetation data were related to independently-derived environmental NMS scores and spatial location with generalized additive models (GAM). Results: Vascular plant species composition in all elevation zones consists of three NMS axes, which are best modelled by one of three possible environmental NMS axes or by location. The first NMS axis of species composition in the lowest elevation zone (40% variance explained (VE)) is a function of location on volcanic substrates (61% deviance explained (DE)). The second lowest elevation axis (27% VE) is a function of unexplained spatial heterogeneity (31% DE). The third lowest elevation NMS axis (24% VE) is a function of the spatial mosaic of canopy disturbance (16% DE). In the middle elevation zone, species composition most strongly relates to the interaction between volcanic substrate and the condition of the soil surface for all three NMS axes (41%, 27%, 24% VE; 70%, 16%, 24% DE). The primary axis of species composition in the highest elevation zone (41% VE) corresponds with substrate and soil condition (55% DE) but the second and third axes of species composition (27% and 25% VE) relate to canopy dieback disturbance (36%, 14% DE). Counts of exotic species and 0-2m height class native tree species respond to the type of volcanic substrate and soil surface condition in all three elevation zones. Lava-derived substrates have a higher incidence of exotic species and less native tree regeneration; whereas ash-derived substrates have higher numbers of native tree species regenerating and many fewer exotic species. Discussion: The tropical montane forests on Mauna Kea reflect a native-dominated plant community response to disturbance on both lava- and ash-derived volcanic substrates, and a higher propensity for exotic species to occur on the lava-derived substrate. Native plant communities on ash-derived soils may have higher resilience to exotic invasion than communities on lava-derived substrates. Our results indicate resource managers should explicitly account for variation in soils and substrate type when prioritizing, implementing and monitoring management interventions to foster native plant assemblages and control the spread of exotic and invasive species. © 2013 International Association for Vegetation Science.

A catena is a series of distinct but co-evolving soils arrayed along a slope. On low-slope, slowly eroding catenas the redistribution of mass occurs predominantly as plasma, the dissolved and suspended constituents in soil water. We applied mass balance methods to track how redistribution via plasma contributed to physical and geochemical differentiation of nine slowly eroding (~ 5 mm ky − 1 ) granitic catenas. The catenas were arrayed in a 3 × 3 climate by relief matrix and located in Kruger National Park, South Africa. Most of the catenas contained at least one illuviated soil profile that had undergone more volumetric expansion and less mass loss, and these soils were located in the lower halves of the slopes. By comparison, the majority of slope positions were eluviated. Soils from the wetter climates (550 and 730 mm precipitation yr − 1 ) generally had undergone greater collapse and lost more mass, while soils in the drier climate (470 mm yr − 1 ) had undergone expansion and lost less mass. Effects of differences in catena relief were less clear. Within each climate zone, soil horizon mass loss and strain were correlated, as were losses of most major elements, illustrating the predominant influence of primary mineral weathering. Nevertheless, mass loss and volumetric collapse did not become extreme because of the skeleton of resistant primary mineral grains inherited from the granite. Colloidal clay redistribution, as traced by the ratio of Ti to Zr in soil, suggested clay losses via suspension from catena eluvial zones. Thus illuviation of colloidal clays into downslope soils may be crucial to catena development by restricting subsurface flow there. Our analysis provides quantitative support for the conceptual understanding of catenas in cratonic landscapes and provides an endmember reference point in understanding the development of slowly eroding soil landscapes.

How did traditional farming transform the natural environment in the Hawaiian Islands? This question is one that has largely been addressed for rainfed farming of crops like sweet potato (Ipomoea batatas), but evidence is lacking for irrigated farming of the critical staple crop taro (Colocasia esculenta). We describe the results of soil nutrient and sedimentological analyses of deeply-stratified pondfield deposits representing a 600-year-long record of irrigated taro farming in the North Kohala District, Hawai‘i Island. Soil is categorized by particle size to determine modes of transport and deposition, and concurrent soil nutrient analyses were conducted to infer shifts in the source of sediments and changes associated with taro harvesting. The advent of farming is clearly detectable in sedimentology, the presence of charcoal found within sediments, and soil chemistry. However, diminished nutrient concentrations can be attributed largely to deposition of a mixture of upstream sediments. Overall, there is no clear evidence for nutrient draw-down by taro harvesting, but we cannot yet rule it out as a factor. This study demonstrates the inherent difficulty of correlating changes in soil nutrients evident in irrigated pondfields with the long term history of soil nutrient cycling.

Soil chronosequences on marine terraces along the Pacific Coast of California and Oregon show evidence of podzolization, though soils ultimately evolve to Ultisols. It is not clear if this pathway of soil evolution can be extended to the humid, inland Oregon Coast Range. We analyzed soil properties for a fluvial terrace chronosequence sampled along the Siuslaw River (Oregon, USA) about 50 km from the Pacific coast. The seven terraces ranged in age from

The four sources of rhyolitic obsidian on Rapa Nui (Maunga Orito, Moti Iti, Rano Kau I [Te Manavai], Rano Kau II) were differentially utilized throughout prehistory (ca. AD 1200–1860). In order to document the nature of quarry exploitation and obsidian distribution, energy dispersive X-ray fluorescence was used to characterize the elemental composition of each source. The classification of 331 archaeological samples from inland habitation sites, ritual centers, and crematoria by discriminant analysis revealed selective patterns of obsidian usage. Obsidian from Rano Kau II was rarely used while Maunga Orito and Rano Kau I glasses were present in nearly equal proportions at inland domestic habitation sites. Motu Iti obsidian was a rare occurrence at interior locations but more frequent at coastally located ritual centers, crematoria, and caves. Surface architecture in the form of an elite house at Maunga Orito, restricted source usage, and the sacred nature of the offshore islets, suggested an elit...

Pedogenic thresholds, where multiple soil properties vary substantially and coherently in a narrow portion of a broad environmental gradient, are well-described on basaltic soils in Hawaii. One such threshold occurs along climate gradients where primary minerals virtually disappear, base saturation decreases sharply, and aluminum is mobilized within a narrow range of increasing rainfall. A recent study that evaluated thresholds along a climate gradient of non-basalt-derived soils on the South Island of New Zealand found that while base saturation declined steeply in a narrow range of rainfall on that gradient, the change was not coherent across soil properties; a substantial fraction of the Ca present in primary minerals (40–60 %) remained through the highest-rainfall sites ((Dixon et al. in J Geophys Res, doi: 10.1002/2016JF003864, 2016). We developed a simple model to explore potential mechanisms driving differences between basalt-derived and non-basalt soils. Incorporating a broader spectrum of mineral weathering rates (including some primary minerals that are highly recalcitrant to weathering) into simulated non-basalt than simulated basalt-derived soils (and accounting for the lower rates of evapotranspiration in New Zealand) was sufficient to simulate observed differences between these substrates. Further, we used the simple model to evaluate the consequences of rainfall variation in the short- (time step to time step) and long-term (a change in rainfall after 50,000 time steps). Results of these analyses demonstrated that year-to-year variation in rainfall could play an important role in controlling changes in the position of the pedogenic threshold during soil development.